Sugar forming tiny bubbles in microwaved coffee?

Question: When I heat up coffee in the microwave and then pour in sugar, a layer of tiny dense bubbles forms at the top of the glass and stays there for the duration of the drinking, diminishing slightly over time. When I prepare the coffee by other means of heating this does not happen. What causes those tiny bubbles?
Comment: Normally, a microwave is capable of superheating water. It is then above 100°C, but still liquid, because it lacks nucleation sites. Crystals like sugar provide such sites, so this would have been my first guess. But "stays there for the duration of the drinking" is strange, I hope you don't drink your coffe while it's above 100°C. Do you think this might be it? http://en.wikipedia.org/wiki/Superheating
Comment: @rumtscho I seriously doubt it because, although I am not the best judge of temperature, it seems to happen even when the coffee is not that hot (i.e. below boiling point).
Comment: I doubted it too (else I would have written it as an answer), but it sounded so similar, I felt it was important to get it mentioned on the way to a differential diagnosis. I have no more ideas right now, but maybe more info can help somebody else: Does this happen with coffee only, or also with other hot liquids? What are the bubbles like? Air-filled, fat-filled, something else? A real foam crown like on bear, or only a few of them?
Comment: Re other beverages, I could have sworn it happened on tea as well, but I edited the tea out of the question because I don't have the means of testing it right now.
Comment: Is there something in your mug besides coffee? Milk/cream? Or maybe even a tiny bit of residue (soap or a previous drink)?
Comment: @Jefromi It happens consistently and with clean, empty glasses. Could it be that there is consistently [close to invisible] soap residue left on them every time I do dishes? Would that explain it?
Comment: I am offering similar experience to WAF. When I dropped the coffee powder into a cup of superheated water (using microwave), a layer of creamy foam was formed. It may be psychological but my black coffee seems to taste better with the foam. I have difficulty replicating the same foam using other method. I hope someone has an answer.
Answer: Upon addition of sugar to the superheated coffee, the formation of bubbles (phase transformation) occurs because the fine particles of sugar provide sites for the heterogeneous nucleation of gas from the liquid.
When the introduction of a fine inoculant (such as sugar particles) results in sudden fizzing of a liquid, it is an indication that there has been minor superheating of that substance.
Superheating tends to occur more in microwaves than on stovetops because people use metal saucepans/kettles on the stove, while generally using glass or glazed ceramic containers when heating a volume of water in a microwave oven.
When water is heated in a glass or glazed ceramic container rather than a metal one, the very hard surface of the container means that there are few scratches on it to act as sites for the heterogeneous nucleation of gas. Fewer heterogeneous nucleation sites means less heat loss through the transformation of liquid to gas.
This university site also makes a good point about the tendency for stovetop heating to cause localised superheating in the vicinity of the container walls... However, I think that this would tend to cause more boiling due to localised heating to the point that homogenous nucleation can occur (with the homogeneously nucleated bubbles then acting as further heterogenous nucleations sites) – not because of stirring of the water.
With regard to why your bubbles remained present for the duration of drinking, I would hypothesise that this perhaps has something to do with the oils in the crema of the coffee, and/or reaction of the dissolved sugar to form something that increases the surface tension of the water. (Not sure what you would call the opposite of a surfactant effect).
A final thing I would like to mention is that (as indicated on the UNSW page), microwaving liquids has the potential to result in violent reaction, or even explosion, of the liquid — in other words, you are risking serious burn injuries by taking a shortcut to heat it up.
Adding a powder (like sugar or instant coffee) to superheated water is particularly bad because in doing so, you are introducing millions of nucleation sites at once. That said, you should also be aware that since gas bubbles themselves promote heterogeneous nucleation, simply placing a spoon in the cup can be enough to cause an 'explosion'.
If you really need to microwave a liquid, (e.g., if you are cooking something), consider heating it in a microwave-safe plastic container that has been washed a few times (and is therefore abraded on the inside). You should also stop the microwave to check the temperature of the liquid at regular intervals, rather than nuking the fluid for an excessively long time.
Comment: To eliminate the risk you can place a wooden skewer in the vessel to provide nucleation sites.
Answer: It can be water overheat. When you pour in sugar water starts boiling slightly. Maybe you can achieve such result while cooking on a stove.
Such effect also can be seen when you put salt (or sugar) into almost boiling water. It seems that process starts to run strongly.
Answer: This doesn't seem to be anything to do with the temperature, or at least not the temperature alone, because in my experience it only happens after the coffee has had a chance to sit (hot or cold) for more than about eight-ten hours. If you were to superheat (microwave) a very fresh cup in the same style container (coffee mug) then dump in a teaspoon of sugar, nothing happens - I've tested this because it has long bugged and perplexed me. I believe it must be some kind of oil into acid degradation (probably chlorogenic acid, mostly) to the point where a high enough concentration then reacts to the introduction of glucose leaving a transitional compound of excess lipids which encase particulates of both the sugar and coffee contaminants, which compound then would naturally collect a bit of the gas escaping the neutralization reaction as well, because of the viscosity difference between the lipids and the water. Put all that together - foamy residue. I'm not at all sure this is true, but it is my best hypothesis. My only other evidence supporting this is that if you allow the residue itself to adhere to your coffee cup, then rehydrate it slightly later, it feels like an oily/fatty substance when rubbed away.


Careers 2.0 Refine search - please add offer H1B check box

Question: Would it be possible to add a checkbox indicated that companies offer H1B visa sponsorship to Careers 2.0, in addition to those indicating telecommute jobs and jobs offering relocation?
Comment: I think this is a very good idea. Many people have a hard time finding places that are willing to do this sponsorship.
Answer: While I think it's great if an employer adds this kind of info in their ad, I do not think visa sponsorship should be codified into a "yes/no" checkbox.

Everybody and their dog wants into the US on a H-1B visa. Few organizations are used to sponsoring visas and know how to handle the process - usually, they are huge businesses like Google and Microsoft. If they started marking their job offers with a "we do H1B" flag, they would likely start drawing more responses from unsuitable candidates.
Sponsoring a work visa is a huge effort on the employer's side as well as the employee's. Companies are often afraid of the insight they think they have to give to the INS about their everyday business dealings. (My immigration handbook encourages applicants to tell their prospective employers it's not that bad really.) They might be inclined to rethink their position if that perfect, super-awesome dream applicant walks in - but not for a mediocre normal one. Forcing employers to state a definite "yes" or "no" would unnecessarily limit everybody's options.
Comment: Yes but it is a good way in the first place to say "We could sponsor H1B if you are a good candidate" or we simply cannot.
On Ycombinator sites - companies simply write on Jobs thread are they willing to support H1B - http://news.ycombinator.com/item?id=4184755 e.g: San Francisco, CA. Full time. H1B okay.
Comment: @gizmo I agree it's a good thing to provide the information in the job listing if you know you can or can not sponsor. But I wouldn't make it a check box for the reasons mentioned above


Line integral along the edge of an isoparametrically mapped quadrilateral

Question: I need to integrate a function along the edge of a quadrilateral (boundary integral). For example, the function is $f(x,y)=x^3+y^3$, the quadrilateral coordinates are $(0,0),(2,-1),(3,2),(1,3)$ and the concerned edge is the line joining $(2,-1)$ and $(3,2)$. So, the required integral is
$$I=\int_{(2,-1)}^{(3,2)}x^3+y^3\ dl\ \ ;\ \ dl=\sqrt{dx^2+dy^2}$$
In another procedure I mapped the quadrilateral into a standard square in $s,t $ axes.
 By using Lagrange interpolation,
$$X(s,t)=\ \frac{3(s + 1)(t + 1)}{4} - \frac{(s + 1)(t - 1)}{2} - \frac{(s - 1)(t + 1)}{4}$$
$$Y(s,t)=\ \frac{(s + 1)(t - 1)}{4} - \frac{3(s - 1)(t + 1)}{4}+ \frac{(s + 1)(t + 1)}{2}$$
Since integration is from $t=-1$ to $t=1$, $dl=J dt$ where $J=\sqrt{\left(\frac{\partial X}{\partial t}\right)^2+\left(\frac{\partial Y}{\partial t}\right)^2}$. So the required integral is,
$$I=\int_{-1}^1X^3+Y^3\ Jdt$$
Unfortunately, the second method gives the wrong result. I got 66.4078 while the actual result is 55.3399.
Where did I make mistake here?
Comment: Did you compute $(X^3+Y^3)J$?
Comment: @chris, Yes. I have integrated it. Is there any conceptual error in procedure?
Comment: Your first integral goes from (2,-1) to (3,2). The exact solution is $35\sqrt{10}/2$. If that edge corresponds to $s=1$ and $-1 \leq t \leq 1$, your mapping would go from (2,-1) to (3,-2), so it's probably not correct. Besides, I would expect the determinant of the mapping's Jacobian, not the 2-norm.
Comment: @chris How would I get a better result numerically from mapping?
Comment: @chris, Could you explain how does my mapping go from (2,-1) to (3,-2)
Answer: I prefer a bit different notation, but the workflow is certainly similar.
Denote the rectangular vertices, as follows:
$$
\mathbf{v}_{00}=\left(\matrix{0\\0}\right),\quad \mathbf{v}_{10}=\left(\matrix{2\\-1}\right),\quad
\mathbf{v}_{11}=\left(\matrix{3\\2}\right),\quad
\mathbf{v}_{01}=\left(\matrix{1\\3}\right)
$$
Now, the position vector $\mathbf{r}$ inside (or on the boundary) of the quadrilateral can be expressed in terms of barycentric coordinates $(\xi,\eta)$:
$$
\mathbf{r}(\xi,\eta) = (1-\xi)(1-\eta)\mathbf{v}_{00} + \xi(1-\eta)\mathbf{v}_{10} +
(1-\xi)\eta\mathbf{v}_{01} + \xi\eta\mathbf{v}_{11}, \quad \xi,\eta\in[0,1]
$$
Now, the integral for the edge between $\mathbf{v}_{10}$ and $\mathbf{v}_{11}$, will correspond to $\xi=1$ and $\eta \in[0,1]$. Now, in a slighlty sloppy mathematical notation:
$$
\int\limits_{\mathbf{v}_{10}}^{\mathbf{v}_{11}}f(\mathbf{v})d\mathbf{v} = \int\limits_0^1d\eta f\big(\mathbf{r}(\xi=1,\eta)\big)J
$$
where $f(\mathbf{v}) = (\mathbf{v}\cdot\hat{x})^3 +  (\mathbf{v}\cdot\hat{y})^3$, which is just a fancy way to write $f(x,y) = x^3+y^3$ using position-vectors. For this integral, the Jacobian $J$ would be the length of the line between $\mathbf{v}_{10}$ and $\mathbf{v}_{11}$. So, $J=\sqrt{10}$
Now, we can just compute the integral and obtain that
$$
J\int\limits_0^1d\eta f\big(\mathbf{r}(\xi=1,\eta)\big)=\sqrt{10}\frac{35}{2}\approx 55.3399
$$
which is the analytical result.
I also checked your Lagrange interpolant and Jacobian. Seems right to me. I guess the error is somewhere in the piping between your functions, main code and interpolants.
Please, see below screenshot of Mathcad screen for reference that implements both the "method" described above, as well as your Lagrange interpolant (only for the integral over the requested edge).


IMovie error -50

Question: I am working in IMovie 7.1.4. When I try to export the movie I get this error message...
"The project could not be prepared for publishing because an error occurred. (Error code = -50)"
Answer: Error code = -50 would usually suggest a write error - whether permissions or an actual disk error I'm finding it hard to ascertain.
I would start by checking & fixing Permissions & Verify & if necessary Repair Disk from Disk Utility.

IMovie error -50


Is there a way to download all posts from a blog on Tumblr (not just images)?

Question: I have Chrome on Windows 11. I've never downloaded blog posts from Tumblr but found a couple of blogs with interesting text posts and wanted to download them for reading when I'm off the internet. Is there a way someone who knows little about computers can do this? I tried "save as" to save it as a webpage and my Chrome keeps crashing (I get "Your file couldn't be accessed"). I tried printscreen and it saved empty pages.
Answer: I did a bit of research and found two potential, albeit imperfect, solutions. Given the context of what you are trying to do, the 1st solution is preferrable. You'll have  the contents of the blog but none of the formatting. I'll explain why the formatting cannot be saved at the end.
1st Solution:
There's an open source tool called TumblThree which can download an archived version of a given blog's content:

Download application from: https://github.com/TumblThreeApp/TumblThree/releases

Open the application by clicking the TumblThree.exe file with the logo on it.

Go to Settings->Connection->Authenticate, login to a Tumblr account. Hit save to exit settings window.

On the main window, copy/paste the blog URL and hit the "Add Blog" button (ex: I used citriccomics.tumblr.com for testing).

The blog should show up in the download list and begin downloading. You can right click -> show files to open up the folder that the blog assets are downloaded to.

2nd Solution:
Tumbler offers gated API access with an endpoint available to obtain blog post data, documentation here:
https://www.tumblr.com/docs/en/api/v2#posts--retrieve-published-posts
Tumblr requires API requests (i.e. "give me the data from these blog posts") to be made using an API or oAuth key. That's tech speak for a password they give you. And from what I can tell they are pretty strict with handing API keys out. There's a good reason for that - they charge money past a certain number of requests. And that's because answering requests, well, costs money. Selling API access is part of what keeps the company alive. So while blog data can be obtained from the API, it's a bit more involved than the 1st solution.
Why is the Formatting Lost?:
You had trouble print-screening blog posts because it's one of many anti-scraping features Tumblr implements. Anti-scraping is standard practice across news websites. They do this to protect their IP and prevent people from circumventing the pay-for-access API revenue model by just scraping the data for free. Let me show you a sneak peak of arguably the biggest anti-scraping feature:
I went to the following blog: https://www.tumblr.com/todayontumblr
On this webpage, I open up the inspector on the first blog post to see what the HTML formatting looks like:

Uhhhh.... okay, so it's inside a div container, inside a div container, inside a... That's a lot of divs, and a lot of class names which look like random gibberish, all for one blog post, in a list of blog posts. Just how many divs are there on this page (currently)?

978 div elements, but definitely not 978 blog posts. So what gives?
This is all intentional. The seemingly random amount of div containers, the gibberish class names - it makes it very difficult (if not impossible?) for web scrapers to consistently parse the formatting of the contents and thus be able to save a perfect copy onto your device for offline viewing. Which means to access Tumblr in all of it's glory - content, formatting and all - you either have to access the API or get the webpages from Tumblr's servers. There's probably a way to restructure the data from solution 1 into a reasonably close HTML formatting locally, but much easier said than done.
tl;dr - you can't save perfect offline copies of Tumblr webpages because that cuts their data trackers out of the picture and hurts their bottom line.


[JavaScript]Is there a way to use Cloudinary without terminal

Question: my development environment(chat bot app) doesnt offer terminal.(uses Rhino)
but what I can do is putting module(.js) files in it.
Would there be a way to use Cloudinary upload and etc codes?
It would be nice if there is a .js format of Cloudinary module.
Comment: There is https://www.npmjs.com/package/cloudinary
Answer: I'm Danny and I work on Cloudinary's Developer Support team.
We've got a Node SDK, as well as a jQuery one and a vanilla Javascript one too. If you're just looking to allow users to upload content via your site, the upload widget may be a better solution for you.
Would you mind posting a little about your use case? If you let us know the specific problem you're trying to solve, we should be able to help. Feel free to reply here, or if you're providing specific details about your account, you may prefer to raise a ticket via our support center.
Looking forward to hearing back from you.


Correlation Between Input and Output Variable

Question: Let's say I want to model a problem with 5 input variables and 1 output variable. I have measured the correlation of each input with the output. 3 of the 5 inputs have correlation less than 0.1, and the remaining 2 inputs have correlation greater than 0.7. Should I use all input variables in my model, or only the two with high correlation?
Comment: I edited your question for grammar/clarity. Please check that it still reflects what you wanted to ask
Answer: Play with it a bit. Run some scatterplots to get a feel for your data.
In all likelihood, you'll find that since those two are so highly correlated with the output variable, they're also correlated with each other, possibly even more strongly. If so, putting both of them in the model will result in some multicollinearity, risking both of them being declared insignificant, and skewing the coefficient estimates.
Try running all subsets regression, since you're working with so few variables, and then pick the best performing combination!


PM Interview Case Study approach Question

Question: My background is primarily in enterprise services/sales engineering with a focus on AI/ML however I am trying get into Product Management and as part of an interview process, I am have a take home challenge that looks like
"We want to include feature X into our product Y, how would you 1 - Describe this to the dev team 2 - the business stakeholders"
As an additional background Feature X and Product Y are AI/ML enabled.
A - Given my limited background in PM, I assume, the best way to address #1 is a PRD and generally falls into the Product Design aspects. #2 is more about Product Strategy and consists of value proposition for specific persona/challenge, differentiation etc.
B - Based on my research so far, feature X already exists in their offering addressing some key KPIs for a specific type of user persona that Product Y is seemingly designed for
Given the very short description of the task and in light of B above, what clarifying questions can I ask or assumptions I can make?
For example,
Is feature Y in the case (Y') an improvement over what you have? Does feature Y' optimize something else other than the KPIs the existing feature Y optimizes?
Should I assume the question is about how would I go about building it if it didn't exist?
Any other thoughts?
Thanks a lot!
Comment: Welcome o pmse. "what clarifying questions can I ask or assumptions I can make" seems very broad. You may want to edit to focus on a specific goal, to avoid close votes.
Comment: Is this [good subjective](https://stackoverflow.blog/2010/09/29/good-subjective-bad-subjective/)?
Answer: Don't ask clarifying questions; make assumptions and state them explicitly
Welcome to pmse!
You can even cover two scenarios with two different assumptions, if needed. Also, assume that the feature doesn't exist now.
There are two key aspects you are being tested on:

Dev team and business people don't speak the same language. As the Product Manager it is your job to speak business language to the business stakeholders and technical language to the dev team.
While talking to the dev team, someone coming from an engineering background tends to wade into the 'how' of building a feature. As the Product Manager you should stick to 'what' you want to build and 'why'. And leave the 'how' part entirely to the dev team.


Install libjpeg.so.8 to CentOS 6.6

Question: I need install libjpeg.so.8 to CentOS 6.6 but i do not understand how to do...
I have installed libjpeg-turbo libjpeg-turbo-devel but libjpeg.so.8 there is not
Answer: It depends on why you want that particular version.  CentOS 6 has /usr/lib64/libjpeg.so.62
Some applications may require a specific old version.  A forum comment pdf2swf and libjpeg.so.8 problem stated that a solution was found by adding the rpmforge repository and getting the library from that location (using this page).
Answer: Just had this specific problem, wanted to install "Genymotion" software on Fedora 23.  This is proprietary software and you can't just recompile it against a different version of this lib, so I compiled libjpeg 8.0 from source. You can grab it here (64bit only).
Remember that using binaries from random people on Internet is probably a bad idea. ;)

Install libjpeg.so.8 to CentOS 6.6


Benefits of rank in clans?

Question: I am playing Clash of Clans and I just recently joined a clan. There was a message that followed telling me that the leader of the clan had promoted me to elder. Is there any benefits of being in a different rank (for example, is there a better reward for a co-leader than an elder or a member?)? Thank you in advance.
Answer:
They say a picture is worth 1,000 words, but in this case I just need 30 characters to make a post.
Comment: Co-Leaders can promote Member to Co-Leader.
Answer: Being an elder means you have basic privileges over unranked members such that you can invite people from outside and kick unranked members.They however cannot demote people.
Co-leader has the same privileges except that they can kick both unranked members and elders and have the power to demote lower ranks until they are a member (the lowest rank).
They can also promote unranked members and elders up to the co-leader rank.
The leader has the same privileges but can promote/demote/kick out anyone within the clan.
Additionally co-leaders and the leader can change the clan message that is displayed at the front clan page as well as sending clan announcements to the other members within the clan.
so in a sense, ranks give a certain individual a portion of the leader's power. These powers/privileges increase with rank.
As for rewards given to higher members? there is none.
Its all a measure of responsibility within the clan... in a fun way
Answer: There are four ranks:

Member-The current rank you have which means you have no advantage yet your treated equally.
Elder means you can kick players that are not ranked and you can invite other players to you clan.
Co-Leader-Can promote members to elder and co-leaders. They can demote elders to members. They can kick elders and members. They also have the advantage of editing the clan description like the war frequency, badge, description, minimum trophies and clan location. They can also invite other players. They also have a special feature to send out a message to all the clan mates for example a warning for being inactive.
Leaders-Is the highest rank and has all those advantages mentioned and also the extra advantage of demoting a co leader. They have to demote them selves and promote someone else if they want to leave their clan.
Comment: What about war search / selecting participants? Is that available to co-leader or leader only?

Benefits of rank in clans?


Import brush set

Question: I had a problem in importing  brushes in blender 2.79, because in the user preferences there is no import brush add-on. So, Now what to do to solve this. Did blender 2.79 support this. I think,it should ,But how, I don't know.
Comment: How does "sapling-addon" tag help to describe your question? To import brushes from another file use Append or Link options and choose Brush category
Comment: @Mr Zak That was probably the only tag the user could "Afford". It's ridiculous that you cant post without entering at least one tag, but the majority of the tags are off limits to new users...the ones asking most of the questions.
Comment: @InternetWarriors they could use "import" tag perfectly fine. New users are limited in creating new tags which does have quite serious reasons, while they can use existing tags and at least "import" was available (because it was created back in year 2013).
Answer: Activate the Brush addon from User Preferences -> Addon
If u have older version download the addon from here
Launch Blender, then open the User Preferences. Go to the Addons tab.
Go to the Import-Export Addons and enable the script.

select the import BrushSet from the file menu

select the folder with your textures and press import

Edit Mode
<code>Unwrap your mesh and add a new texture to paint on.
</code>
Texture Paint Mode
Under texture tab, press the field which will pop up the texture selection.

all your textures should now be loaded there and you can start painting.
Comment: This addon doesn't appear in the list of addons packaged with Blender on my end, 2.79a on Mac.
Comment: @Craig D Jones I believe you can find that add on here. It does not appear in the windows version either. https://github.com/sftd/blender-addons-contrib/blob/master/io_import_BrushSet.py
Answer: You can get the addon from the original author's github
and there is also another addon that is similar in function

Import brush set


How to get related list Id

Question: In SF1, sforce.one has a method navigateToRelatedList() that takes a related list Id as a parameter. How do I get this?
Answer: This is very strange that it is not documented anywhere.
I took time to research. Here is what I found:
For Standard Related Lists like Opportunities or Contacts related lists on Account id of the related list are:
RelatedOpportunityList
RelatedContactList
RelatedCaseList
For Related lists of custom objects it's the id of the Lookup or Master-detail field to the parent object.
So if we have Custom_Object__c with Lookup field Account__c to account we need to go to this field and copy id of that field.
But there is an easier way to find the id:
if you open source of the page you will see id of the div element for related list has id like this:  parentId + '_' + relatedListId.
Couple examples(highlighted related list id):
custom object:

Standard(opportunity related list on account):
Answer: The easiest way is to open the related list in the browser and get the value from the URL.
For example, if you go to an Account and click on "Contacts" so the related list is in full view, the URL will look like this:
https:///lightning/r/001RM0000049DMSYA2/related/Contacts/view
The value between related and view is the relatedListId, so in this case it's Contacts.

How to get related list Id


What type of cable is a cat 7 : UTP or STP ?

Question: I'm a ICT engineering student studying networking. I was wondering if someone could tell me what type of a cable Cat7 is ?
Comment: ANSI never defined "cat 7", so the question is nonsense. Do you mean ISO Class F?
Comment: Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer.
Answer: Cat 7 is neither UTP or STP.  It is typically either F/FTP or S/FTP.
Comment: @cpt_fink, actually FTP generally uses foil on the cable, not the pairs.  By the "newer" ISO/IEC spec, which hasn't caught on in some places, this would more accurately be F/UTP.  F/FTP specificies a foil shield on both the cable and on each pair.  S/FTP would be a braided shield on the cable and foil on each pair.
Comment: FTP is `foil-shielded twisted pair`, where each pair is shielded separately from the other pairs to lower cross-talk.  Adding to save someone else the google time...


DDD: Getting aggregate roots for other aggregates

Question: I've been studying DDD for the past 2 weeks, and one of the things that really stuck out to me was how aggregate roots can contain other aggregate roots. Aggregate roots are retrieved from the repository, but if a root contains another root, does the repository have a reference to the other repository and asks it to build the subroot?
Comment: Is it Domain-Driven Design ? Did you look at the composite design pattern ?
Answer: @Paco: You are wrong. Repositories are not only for storing objects. If you have read Eric Evan's DDD book, you'd know that Repositories are like an object-oriented in-memory representation of data. You can use a Repository object the same way as you use a Collection. You can use indexer to get or set objects, you can use the Add() method to add a new object, you can use the Remove() method to remove an object, etc.
The Repository then uses the infrastructure to read/write data from/to the database. It can use a DataMapper for simplifying the retrieval of data from your relational database and mapping to your Entities.
Answer: The repository doesn't build but it stores. When you using ddd, you might want to get familiar with basic persistence patterns like unit of work, identity map, lazy load, object relation mapper, query object, (dynamic) proxy. (these patterns have nothing to do with ddd, but are very useful to know). The repository is just a facade to hide the implementation of the patterns mentioned before and to abstract dataaccess in a domain driven way. Most people don't manually write their persistence infrastructure manually nowadays, especially when using ddd, you might like to look at an orm.
The actual reference with the code that converts the database records into an object will be in the datamapper. There will be references between mappingclasses in the mappingclasses itself, or created by something like a mapperfactory.
<code>public interface IDataMapper<T>
{
   T Map(IDataReader reader);
}
</code>
You don't have to implement this code yourself, just use a tool that does it for you and try to understand how parts of the code in the tool (orm) works. Pure ddd without any orm is almost impossible without a good set of tools that save you from writing a lot of code.
Answer: From my opinion it depends - I have the same scenario and they way I deal with this is via the ORM I'm using - nHibernate.
I have mappings for all my entities of which several are aggregate roots and one of these has several other aggregate roots as member variables. The repository for this composite aggregate root does not require references to other aggregate root repositories because nHibernate knows how to get all the data required (via the mappings).
HTH
AWC


What is polarisation, spin, helicity, chirality and parity?

Question: Polarisation, spin, helicity, chirality and parity keep confusing me. They seem to be related, but exactly how they are related is unclear to me. Can someone maybe give a short overview about what these quantities mean and how they are related? What are the values for e.g. basic particles like electron or photon?
Comment: Related: https://physics.stackexchange.com/q/1111/2451 and links therein.
Answer:
Spin is determined from the representation of the Lorentz group the quantum field transforms in. The projective finite-dimensional representations of the Lorentz group are labeled by two half-integers $(s_1,s_2)$. The spin of a field is the sum $s = s_1+s_2$. For example, a scalar transforms in $(0,0)$, a vector field in $(\frac{1}{2},\frac{1}{2})$, a Dirac spinor in $(\frac{1}{2},0)\oplus(0,\frac{1}{2})$, and those have spin $0$, $1$ and $\frac{1}{2}$, respectively.
Helicity is the projection of the spin vector upon momentum. Formally, if $S^{\mu\nu}$ are the generators of the Lorentz group in a given representations, then helicity is given by the operator
$$ h = \epsilon_{ijk} p^i S^{jk}$$
where $p$ is the momentum operator. For massive particles, it is not Lorentz-invariant, but for massless particles, which lack a rest frame, it is a Lorentz invariant notion.
Chirality only makes sense for Dirac spinors and similar objects whose representation decomposes into smaller representations, or where exactly one of the two $s_i$ labelling their spin is zero. The Dirac spinor representation $(\frac{1}{2},0)\oplus(0,\frac{1}{2})$ decomposes into the left-handed $(\frac{1}{2},0)$ and the right-handed $(0,\frac{1}{2})$, which are also called the (left/right)-handed Weyl spinors. For massless fermions, the evolution equations that couple the right-handed part of a massive Dirac spinor to its left-handed part decouple, meaning a massless Dirac spinor is equivalently a theory to two uncoupled Weyl spinors. The projection operator into the chiral subspaces of the Dirac spinor representation is
$$ \mathbb{P}_\pm = \frac{1}{2}(1\pm\gamma^5)$$
for $\gamma^5$ the usual product of 4D gamma matrices. It is to be noted that, for massless fermions, the chiral subspaces are precisely the eigenspaces of helicity.
Parity is the unitary operator $P$ upon the quantum space of states that is associated to the classical symmetry $\vec x\mapsto -\vec x$. It is equivalently one of the generators of the $\mathbb{Z}/2\mathbb{Z}$ when presenting the Lorentz group as the semi-direct product
$$ \mathrm{SO}_0(1,3)\ltimes \left(\mathbb{Z}/2\mathbb{Z}\oplus\mathbb{Z}/2\mathbb{Z}\right)$$
of its component $\mathrm{SO}_0(1,3)$ connected to the identity with the group $\mathbb{Z}/2\mathbb{Z}\oplus\mathbb{Z}/2\mathbb{Z} = \{1,P,T,PT\}$
Comment: What about polarization?
Comment: @asmaier: I'm not sure polarizations has a well-defined meaning that's distinct from these. E.g. the two polarization states of a photon arise from its two helicity states, and I haven't really heard polarization for anything else
Comment: @Craig Ah, this is for the solutions of the (massless) Dirac equation in momentum space: The solution is a Dirac-spinor valued function $u(p)$, and solutions with $hu(p) = \pm\frac{1}{2}u(p)$ are precisely those that take values only in one of the chiral eigenspaces.
Comment: Late follow up: how can the eigenspaces of Chirality and Helicity be the same in the massless case, when one operator is purely a matrix and one is a derivative? Or am I misunderstanding that momentum operator?
Answer: Polarization is, generically, a normalized vector in spin-space, i.e. the spin-state of a particle (When I say spin, I actually mean total angular momentum, spin+orbital angular momentum). Usually we take "fundamental" polarizations to be an orthonormal basis in spin-space.

For scalars (spin-0), there is only a trivial polarization because the spin-space is 1-dimensional.
For spin-1/2 particle you can have a normalized linear-combination of spin-up or spin-down, $c_1|\uparrow\rangle+c_2|\downarrow\rangle$. Actually this isn't Lorentz covariant notation, but never mind that.
For spin-1 massive you have $3$ spin degrees-of-freedom. The usual choice of basis polarizations here are $\epsilon^\mu_{\pm}=(0,1,\pm i,0)$ and $\epsilon^\mu_{3}=k^{\mu}/m$.
For spin-1 massless (which means helicity-1, since the concept of 'spin' only strictly applies to massive particles) you have $2$ spin degrees-of-freedom, which can be taken to be right/left handed polarizations $\epsilon^\mu_{\pm}=(0,1,\pm i,0)$, or any linear combination of them.
etc.


What are the reasons some thoughts cannot be simplified, reduced to a simpler set or phrases?

Question: What are the reasons some thoughts cannot be simplified, reduced to a simpler set or phrases? When reading Heidegger or Hegel, one wonders why those authors couldn't simplify their tangled web of phrases, is there any specific thing in language, philosophy that explains why certain things that can't prevent a person from simplifying an idea to a simpler set of phrases. I noticed even things in advanced mathematics and physics can be simplified to a certain extent, but certain ideas cannot really be, why?
Comment: When you see an Existentialist, run! Post-structuralist, jump out a window! It's all over!
Comment: Are there not many thoughts that can't be expressed with words, even if one could use a huge number of them? It's not a huge leap from there to thoughts that have minimum number of words needed to express them due to their complexity and "distance" from concepts that can be expressed in few words.
Comment: I'm reminded of an exam question where you have to evaluate rival arguments: "All words have dictionary entries, so can be defined." "Definitions presuppose other words, so not all words can be defined."
Answer: Three (or so) thoughts:

Philosophers like Heidegger, Hegel or the French post-structuralists introduce a new vocabulary of technical termini. These new words, say "Zuhandenes" ("Ready-to-hand") in Heidegger, are implicitly defined by page-long explanations. At least some of these technical termini are usually irreducible.
The explanations which define these technical termini implicitly could most likely be given somewhat differently without changeing their meaning. Not all philosophers are the most talented teachers of their own philosophy.
That said, what is a difficult text to one reader may be fairly easy to another. Horkheimer and Adorno's "Dialectic of Enlightenment" is as impossible to understand for a beginner of philosophy as is Analysis III for beginner of mathematics. Both are easy reads to the advanced learner.

So, there are new irreducible words and there is necessarily text which defines these words as part of the theoretical web it builds. This text often could have been written better. But what is difficult or easy depends on ones state of knowledge.
The situation is not totally different in mathematics, for instance. What a set is, is implicitly (partly) defined by the axioms of ZFC. These axioms are (some of) the "text". And they are not that easy to understand, either.
Comment: Any sufficiently advanced terminology is indistinguishable from gobbledygook.
Comment: It is also worth mentioning that language patterns change over time. There are plenty of constructions that modern readers find confusing but were readily understood by contemporary readers. Also, we moderns sometimes have clearer or more concise language _because_ of the work that was done in philosophy -- language that was not available to the very philosophers who wrote the pioneering essays we still study.
Answer: I can provide an intuitive answer to how thoughts can be simplified. From a general language, we can create out "sub languages". That is, a smaller language which is capable of consistently dealing with the phenomena in a certain field.
So, for example, from the foundations of mathematics, one could come up with the "language" of differential forms. Once we understood that, then many of the multivariable theorems in arbitrary dimensions get unified into one single result.
For a simpler example, consider the sentence "three plus five is eight" and "eight is three plus five". Now, the information of these two statement is contained in the equality 3+5=8.
The reflexivity of the equality encodes the two statements and the symbols denote the objects inside the statement.
Now, in reverse, would it make sense to add any random words and define equality between sum of arbitary words? Maybe but it would beyond the scope of this sub language we created.
Comment: In programming, the idea of defining Domain Specific Languages is very useful.
Answer: What do you mean by simplified and simplified to whom? Like for the respective author a text is simple if it gets the meaning across with as little text and explanation as possible.
That in turn can mean that a lot of the mental labor to decode the text comes down to the reader having to figure it out.
Like in general you could send an information in plain text or you can encode it, meaning the other end runs the code on their hardware and produces the message. So idk you maybe just nod to another person on a railway and that nod sets a series of events into motion that can produce idk the works of Shakespeare or whatnot. It's incredibly simple for the sender because the algorithmic work comes down to the receiver.
So often times the sender expects the receiver to be familiar with a subject matter or have general knowledge or experience so that they can simplify their task of writing by making use of these experiences and mental images. While if you want to send a complex piece of information without ANY prior knowledge on the end of the receiver you'd have to take much wider turns and write up a lot more information to the point where you might have to summarize the full human history and every science at which point you would agree that you'd touched upon something that is impossible. So at some point you need to simplify your ideas and/or take things for granted or leave it to the receiver to figure it out for themselves.
Comment: Nod nod wink wink
Answer: For easier thoughts:
Short version: Convenience
Long Version:
I think if you wanted to have a chance at being rigorous (without actually choosing to be rigorous) you could acknowledge that its possible to assign a complexity to thoughts not unlike the notion of "Kolomogorov complextiy" for arbitrary strings in computer science. Instead we treat the thought as a string (how much do you have to write or say to feel like you got the thought out (which is a subject experience)) and then take the kolomogorov complexity of this resultant string.
Some thoughts just have HIGHER complexity to the point that the amount of explanation you would need in terms of simple ideas is so high that it becomes pointless to explain that thought without assuming some baseline complexity in your ability to express. In the Kolomogorov world, you need a very expressive programming language for writing your programs down for expressing that string without having a bad time.
We can look at a particular thought to see how this kind of thing manifests:
An example which comes to mind for example is Einstein's theory of general relativity. In its most DIRECT sense this is just a system of partial differential equations (I think 16 in all) which any one with basic knowledge of multivariable calculus should be able to understand (meaning look at and say "yea in principle I know what that is").
But writing down that formula is extremely tedious and requires a lot of explanation on how to write it down that you'll never find an author talk about the theory this way.
Instead they will talk about concepts like curvature (and the curvature tensor and christoffel symbols) and set up a whole bunch of theory and language and make you process a textbook or two's worth of knowledge BEFORE you can even make sense of the modern formulation
$$ R_{\mu \nu} - \frac{1}{2} R g_{\mu \nu} + \Lambda g_{\mu nu}  = \frac{8\pi G}{c^4} T_{\mu nu} $$
That equation is just ONE line, as opposed to 16 and it requires quite a bit of understanding in order to actually make sense of. The way you critique some philosophers with overcomplicated the matter I too feel about this equation sometimes but... I guess the onus is on us to write out in the verbose but "simple" way if no one else has bothered to do it first.
For Harder thoughts:
If we try to ask ourselves what is a "set" and go to the dictionary you would find it defined to be a "collection" and if we ask what a collection is you would find "grouping" and if you look for a grouping it would be a "set" etc...
Some ideas / thoughts are just ALL or nothing. If you try to break them down you are missing the point. We learn these by conditioning and habit, and its entirely possible for some advanced philosopher to discover a new such ALL or nothingism and then when trying to explain it frantically beat around with complex words because there simply isn't a way to break this idea down that isn't ALL or nothing. Learning the idea cannot be done abstractly by reading and thinking it but by living it.
Answer: I'm not sure if I really agree with this statement. Who says those statements can not be simplified further? I think if enough effort is put into it most statements can be simplified, possibly at the cost of requiring more time/space to describe. Many things that are taught now in schools/universities were once considered out of reach for the general public but after many decades of rehashing and simplifying it is now part of some curriculum.
Yet I do think that there is some kind of "minimal complexity" to concepts, like mentioned by Sidharth. But the complexity of a single statement can be traded for time/space. For example consider adding the two numbers 952+462 in decimal. If you write those above each other you can perform the addition using 3 rounds of adding the appropriate digits. If you convert to binary the addition looks like 1110111000 + 111001110. The benefit of working in binary is that each operation is the absolute simplest possible, there are only 3 possible ways to add digits: 0+0=0, 1+0=1 and 1+1=10. But the downside is that instead of 3 rounds of "hard" operations you now need 10 rounds of simple operations. Likewise, you could convert the split the contents of a hard-to-read book into a large number of simpler books until the content is readable by anyone with basic mental capacity. The amount of space you need to write a statement in dumb-dumb language could a measure of the actual complexity of that statement. This is similar to how Shannon Entropy measures the "true" information content of something.
Answer: The purpose of a language is to encode concepts or cognitive structures in a form that conveys those concepts or structures to others adequately. That last word — adequately — places a lower limit on how simple an expression can be. With physical objects and emotional states — sphere, chair, love, envy, etc — others have strong internal references of their own for those concepts, so using a single word or phrase will often be adequate to convey the sense. For abstract concepts — Dasein, capital, qualia, angst — others may lack both internal references to the idea and internal references to other concepts and structures are necessary to properly represent the idea. So philosophers often find themselves building not just an idea, but the entire worldview in which that idea can be adequately expressed.
To put it in context, this is what I think of as the 'cricket' problem. The first time Americans see a UK cricket match, they find it utterly incomprehensible, looking like some weird perversion of baseball. They have to be taught all of the things that Brits know from a young age — to enter into the British sports worldview, with all the practical and experiential cognitive structures Brits share — and only then can they understand cricket properly. Complex concepts cannot stand alone outside of the worldviews that encompass them; that's just not cricket.
Incidentally, maths are exactly the same. A phrase like 1+1=2 seems very simple, because we can relate it easily to everyday objects: one apple and another apple gives me two apples. But in the abstract, well... Bertrand Russell's "Principe Mathematica" doesn't work its way up to the concept "2" until (if I remember correctly) page 365. Appearances can be deceiving…


How to use the /testfor command to detect what a player says in chat

Question: How do I use the <code>/testfor</code> command in a command block to test for a specific word that a Minecraft player playing on the server says. For my example, if a player says:

[Player] - Hi

The command block then says:

[Command Block] - Hello.

Please help, just trying to create a cool command =) {If This Works Please Tell Me}
Answer: You just can't.
As per current, command blocks cannot read from the chat line.
Read this answer for more information.


QGIS raster GeoTIFF file covert to kml?

Question: I have a NDVI image, which I Geo-referenced in QGIS (raster).  Now I have to open the image in Google Earth - how do I convert the GeoTIFF raster file to a kml file in QGIS, so that I can open the image as a layer in Google Earth?
Answer: These days you can use Google Earth Pro for free:
Note: Google Earth Pro requires a license key. If you do not have a key, use your email address and the key GEPFREE to sign in.
Google Earth Pro should be able to load directly your GeoTIFF (and save as something else if you need).
Comment: Try as I might I can't get Google earth pro to read in GeoTIFFs properly.
Comment: @HeinduPlessis I just dragged and dropped  GeoTIFF to Google Earth Pro now and it loaded no problem. If the size is over relatively small GEP maximum it will ask if Scale/Crop/etc. My testing GeoTIFF was created in QGIS in Float32 format. Maybe can't do Float64? Also custom projection might be an issue, not sure really.
Answer: To display the image is Google Earth, your CRS has to be EPSG:4326. So re-project your image first if required.
Then, you can use gdal2tiles.py script which will create a KML SuperOverlay and you can view it in Google Earth.
http://www.gdal.org/gdal2tiles.html
Comment: Is your input rgb or single band? Try to convert to 3 band and see if that helps.
Comment: I've done the following:gdalwarp -of GTiff -t_srs EPSG:4326 test.tif test2 tif  then gdal2tiles -p geodetic -k test2.tif which did create a doc.kml file.  If I open the doc.kml file in Google Earth, it takes me to the correct location - but no image is displayed.  Any suggestions?

QGIS raster GeoTIFF file covert to kml?


Why doesn't the industry use my line?

Question: I started a game with busses because getting a train line started is very expensive, but having earned a couple of million, I've now build a train line.
However the quarry on my train line won't produce stone, probably because the line is not in use. The other end of the track doesn't directly use stone, but that shouldn't matter. Since the Construction materials plant is very far downhill, trying to connect the line to a station there is huge pain, so I'm going to build a bus route there. I've had to sell my train because it was just losing money.
Answer: If I understand the question correctly, you're planning to just dump the stone somewhere? That's not going to work. You can force unload the stone at a station and transport the last bit by truck, though.
Comment: Changing to unload only makes no difference. I checked to make sure it's a goods station, not a passenger station. I don't have a bus line yet, but what's the point of making a bus line when there's no goods there? Wow, it actually detects whether there's a complete line. With the bus line in place it does start producing stone.
Answer: The game does these things automatically after some time (one in-game month roughly). All your stations that are connected form a network and the game ensures that goods travel across it. You don't need to manually tell vehicles to dump goods at mid-points (like in old Transport Tycoon), the vehicles understand this and even sometimes pick alternative routes, if several are available.
You must however ensure the following:

Station 1 is connected to the quarry. Looks ok from the screenshot.
Station 2 is close enough to a truck station, with road between them.
There's a train going on a line between the stations, with correct wagon types.
There's a second truck station near the resource consumer, such as the construction materials plant, again with road between the station and the plant entrance.
The produced goods by the plant are transported to another consumer.

Just as in the real world, stone is transported with trucks, not by bus. So obviously a bus line will do no good here, except for transporting stray workers to the plant (you won't get rich from that).
The quarry won't produce stone if you don't pick it up, but it also won't produce stone if there is no demand for it. The construction material plant only has a demand for stone, if there is a demand in turn for construction materials (view "potential" in the details tab of the plant's info window). And so on - in the end you'll have the consumers in a town.
When planning a new line, always start on the consumer side. Who will use the end product? And can you afford to build the whole supply chain needed?

Why doesn't the industry use my line?


"Disabling" folder with WAMP / Joomla

Question: I'm curious about a WAMP + Joomla feature...
On my project folder and within /www/, I have several Joomla folders that are accessible (features inside are usable) but when directly opened from the Web browser show no contents (but no error message is shown)... This is a behaviour I would like to make other folders have but I don't seem to be able to get it!
Let me explain a but further, folder called "CLI" if opened in web browser as:
.../components
returns a blank screen... But some of my modules are using content within, so folder is not restricted but contents are not visible through Web brower.
Now I have another folder:
.../clients
Which I would need to behave the same way as many modules of my site need this, but I can't afford to leave it "open" for users (if someone enters ".../clients" on their web browser, I need the output to be blank, same as the .../components folder above). Currently, if someone enters .../clients on a web browser, they can see all files within, this is a CRITICAL security bug for me.
I've succeeded to disable them by doing DENY FROM ALL in httpd.conf. However, this is not valid as it absolutely restricts everything within the folder, causing my modules to crash.
Hope you get the point, if not, happy to clarify :)
MANY MANY THANKS for your support!!!
PS - If possible, answer in a dummy way as I'm new to all of this Apache stuff.
Comment: You should read about .htaccess file.

http://stackoverflow.com/questions/4610524/htaccess-to-restrict-access-to-folder
Answer: The way Joomla returns a blank screen is by adding a blank index.html file in. Browsers prioritise a index.html file to display over the index.php file etc. So literally put in a index.html file in the subfolder containing:
<code><html>
  <body bgcolor="#FFFFFF">
  </body>
</html>
</code>
Comment: @Lodder if you look in the components folder for example, you'll see it had the code I posted originally. However for reference this will work as well

"Disabling" folder with WAMP / Joomla


Glide Ratio of an unpowered "sub"

Question: Flying gliders can have glide ratios (ratio of horizontal distance vs vertical distance) around 50 or 60.  How would that change when the fluid density changes, in particular under water.  Could a craft designed for under water gliding achieve a similar glide ratio, or does the increased drag reduce it?  Would the reduction be linear to density?
Comment: Wouldn't the situation be radically different. A glider is in the category of heavier than air flight and must proceed at greater than stall speed. A submarine can have neutral buoyancy and therefore has no stall speed. For that reason it could glide at the most gentle downward trajectory very very slowly for a long long time.
Comment: @Transistor I would assume OP is referring to no ballast or "equivalent" ballast to an airplane and just hydrofoiling.
Comment: Do submarines need wings to provide lift? how do the sizes of the "wings" compare? How do the functions of a wing on a glider and on a sub compare?
Comment: And gliders can do more than 50 or 60 for a glide ratio using thermals - copied from birds: nature figured that out...
Comment: Consider the "glide ratio" of a hot-air balloon when it is just below neutral buoyancy.  That's really all you are asking about.
Comment: A glide ratio of a hot air balloon would be 0, as it has no way of providing a lateral force as it falls through the air.  A submarine (with no 'wings') would also be 0, but a 'sub' with some form of hydrofoils could fall through the water column while producing lift (to fall slower) and forward force.
Answer: If all things were somehow magically equal, density does not affect things because although density increases drag it also increases lift by the same proportion. So the glide ratio doesn't change.
But not all things are equal and what density does influence is the Reynolds number and this changes the glide ratio. The Reynolds number is the ratio of inertial forces in the fluid to viscous forces. Higher Reynolds number indicates the fluid behaves with more inertia and less viscosity (like water) and lower Reynolds  numbers indicate the fluid behaves with more viscosity and less inertia (like honey).
The Reynolds number also depends on speed. Increasing fluid density has the same effect as moving faster where both increase the Reynolds number. Increasing speed or density helps inertia overpower viscosity.
Another effect that influences Reynolds number is "size" or more specifically the path length along which the fluid is guided and flows across the surface of the object. This is the chord for airfoils.
In fact, water is so dense compared air such that a submarine experienced a higher Reynolds number than an airplane even though it is traveling much more slowly through it's fluid medium.
Similarly, as aircraft and flying animals get smaller, this also reduces the Reynolds number since the chord length of their airfoils decreases and their smaller size tends to translate to slower flight speeds.
For an airplane to experience the same Reynolds number as a submarine it would have to fly impractically slow (almost a standstill) or be impractically small (microbe size). For a submarine to experience the same Reynolds number as an airplane it would need to move impractically fast (torn apart by the drag and friction from travelling at hypersonic speeds or faster through the water while also vapourizing it) or be ENORMOUS (I suspect it would not fit into the ocean).
That's why there are hummingbirds are all small, or birds that glide like albatrosses or soar on thermals like vultures are all huge.
Insects, in particular, experience a Reynolds number in air so low that they are swimming through the air more than they are flying through it. It means they can't glide but the benefit is that hovering is easier.
So for your actual question:
If we're talking about glide ratios, the glide ratio of a submarine I would expect it to be higher than that of an airplane...if submarines were actually built with wings to produce lift capable of supporting its own weight.
Comment: Edited because I had opposite mixing things up.
Comment: There's also the ratio of aircraft density to air vs.   ratio of submarine density to water.  As I mentioned in the comment to the question, a submarine is more like a blimp or balloon in this respect.
Comment: Not how it's done. Take any air glider. Now take a second one ballasted so that it weighs the same in water as the air glider does in air. Now all the driving forces are balanced and the geometry is the same. Both are released and develop a constant sink rate. What is the Reynolds number difference? What makes the glide ratio different given that the induced drag can be parameterized in terms of lift? The *only* thing that changes is the vehicle's friction drag. This is usually analyzed as the coefficient of drag at the zero lift condition. So which has a higher coefficient of friction?
Comment: See here - https://en.wikipedia.org/wiki/Lift-to-drag_ratio
Comment: @PhilSweet I was answering with the (admittingly unsaid) notion that the speeds of the two test cases match the speeds of their normal counterparts. TBH, I did not intend to say that it *only* depends on the Reynolds number. It depends even on speed and density of the medium and the craft.

Glide Ratio of an unpowered "sub"


Sharepoint 2016 Powershell Script to export all fields to csv - including Created and Modified Fields

Question: I am attempting to dump all data from my Sharepoint Lists to csv.  I seem to have the code working at about 95% the way I want it, but I am missing the Created By, Created On, Modified By, and Modified On dates.  How do I capture those fields?
Here is my current code for one of my lists:
Add-PSSnapin "Microsoft.SharePoint.PowerShell"
$url = "http://internalweburl/subsite"
$listName = "List1"
$path ="c:\Exports\$listname.csv"

$web = Get-SPWeb $url
$list = $web.Lists.TryGetList($listName)
$fields = $list.ContentTypes | %{ $_.FieldLinks } | select Name, DisplayName
$items = @() #array to store objects representing list items
$list.items | %{
    $item = $_;
    $hash = @{}; #hash table to store field name-value pairs
    $fields | %{ $hash[$_.DisplayName] = $item[$_.Name]  };
    $items += new-object psobject -Property $hash }
$items | Export-Csv -Path $path

Without having to list each individual field, I can't seem to find a way to include those.  This would be a huge undertaking as we have many lists I need to export, and I don't have the time to go through each and document all the fields. Any help would be greatly appreciated.
Answer: $list = $web.Lists["Your List Name"];
$fields = $list.Fields;
$list.Fields | select InternalName  |  Export-Csv -path $path;
The above should work, right using SPList.Fields ? Refer the answer here link to answer
Comment: This just gives me a list of the column titles.  I need to export all the rows AND include the Modified/Created fields.


Does a controlled undead polymorphed into a living creature stay under the necromancer's control?

Question: I'm working on a dread necromancer and was thinking of fun things to do. I got the idea of having polymorph cast on undead I've raised. The question is, would the polymorphed undead still be under my control?
Comment: Welcome to the site! Can you please edit the question to include what you do expect to be the problem with casting polymorph on your controlled or summoned undead?
Comment: Given the question title,I'm almost certain the crux of this question is "do undead remain under a necromancer or evil cleric's control if they cease to be undead due to a polymorph effect" (which is definitely an interesting question), but as the title edit wasn't made by the author, it's hard to be sure.
Comment: Related: https://rpg.stackexchange.com/questions/87516/how-does-polymorph-any-object-work
Answer: Well, if you are talking ordinary Polymorph...
According to the SRD, the target for Polymorph is one

Willing living creature touched

Undead aren't living, so they aren't a valid target for ordinary Polymorph.
Baleful Polymorph is also out, because Undead possess

Immunity to any effect that requires a Fortitude save (unless the effect also works on objects or is harmless).

and Baleful Polymorph only targets creatures and requires a Fortitude save.
This leaves us with only Polymorph any Object as a valid spell to polymorph an undead.
This spell is a true change. It alters something entirely into something else, including overwriting their mental scores and features. As Polymorph any Object otherwise works like Polymorph, it has the following effect as well...

The subject's creature type and subtype (if any) change to match the new form.

This is a complete and total change of the target creature or object. They are not an 'undead shapeshifted to be a monkey.' They are actually a monkey for as long as the spell lasts. They get the monkey's mental stats, physical stats...and they act like a monkey, because they are a monkey.
At this point, we do not have a hard ruling that I can find, but I think it is a fair assumption that whatever you are dealing with isn't Undead anymore (it has lost the Undead type)...and thus cannot be controlled with Command Undead. So, I would rule that an Undead PaO'd into something not undead is broken free from the necromancer's control.
Comment: I think you're (*ahem*) dead on until the final paragraph. Ruling that effects constantly check the legality of their targets—and *end* if the target *becomes* illegal—is a *Magic: The Gathering* style rule that I've never found in *3.5e.* That is, a necromancer that casts *PAO* on a controlled skeleton so that the skeleton becomes a monkey probably *should* retain control of that monkey, but if his control is ever severed, he won't be able to regain control of that monkey unless he's *also* a monkeymancer.
Comment: That's totally fair.  The risk in making such a ruling is exactly the one you've put forth: ending effects early that don't say they should be ended early. Personally, I gravitate toward an effect only checking if the target's valid when the effect is initially used because otherwise, for instance, spells like *raise dead* cause weird Schrödinger creatures (*or* the DM makes a house rule saying that instantaneous effects are exempt, which is cool, too). (Also discussed briefly in [this answer](https://rpg.stackexchange.com/a/94699/8610).)
Comment: @HeyICanChan Well, the rules do not actually say _anything_ that I can find about maintaining control of something if it becomes something the spell wouldn't work on. Like, no rulings on if Dominate Person keeps working if they turn into a lemur. At that point, I can't say there is a solid ruling, which is why that last bit was finished with "I think." There is no RAW answer here, and I couldn't find an RAI response either...so I had to go with my opinion.
Answer: Probably.
...but it is extremely poorly defined.
The general rule at play here is that the control effect of animate dead is instantaneous. It is not an ongoing spell effect, and can't be suppressed by e.g. antimagic fields. Unless something acts to change the situation (like casting more animate deads), the generic state of things you create is that "The undead you create remain under your control indefinitely."
If you squint at it though, you could read the control as only applying to "undead you create." Just like polymorphing them to e.g. a humanoid removes the undead immunity to cold, you could argue that the former zombies are no longer "undead you created"-ed with animate dead. They were previously, and the effect probably stays ongoing, but an effect which explicitly affects one thing has no effects on something else, even if that effect can be applied.
If you took the second interpretation, you would probably regain control as soon as they turned back into undead.
Further, if you took the second interpretation, the undead wouldn't suddenly become a full-fledged humanoid (or whatever). It would retain its no intelligence ability score.

A creature with no Intelligence score is mindless, an automaton
  operating on simple instincts or programmed instructions. It has
  immunity to mind-affecting effects (charms, compulsions, phantasms,
  patterns, and morale effects) and automatically fails Intelligence
  checks.

So, with that interpretation, it starts behaving however an uncontrolled undead behaves. Which is also completely undefined.


Ubuntu 12.04 compiling a C program

Question: I have recently installed ubuntu 12.04 LTS 32 bit desktop version and I wrote a computer program where it uses the math.h head, and I keep getting an error saying that it is an undefined reference to 'pow'. I compliled it using gcc -Wall -lm filex.c -o filex.out.
I really like this new system but what can be going wrong? All forums I have looked at just say to compile using -lm which I am. I have compiled the same program in another computer and it works just fine. Can someone help?
Answer: I met a same problem, and found your question by search engine.
I solve the problem by moving the option "-lm" to the last part of the command.
Please try the following command:
gcc -Wall filex.c -o filex.out -lm

Ubuntu 12.04 compiling a C program


Chi-square test for Homogeneity problem (cell count less than 2)

Question: My table of chi-square test for homogeneity has 3x5 cells, 3 samples to a likert scale item. Most cells in the table have a frequency count of less than two, so it is recommended to discard the column of which contains a cell with frequency of less than two. How will this affect my hypothesis if I discard the entire column just because a cell has a value of less than 2 or 0?
Answer: 1) Likert scale items are ordered. Chi-square ignores the information in that ordering. You're tossing away a lot of your power against interesting alternatives.
You would probably benefit from investigating some of the potential alternatives to straight homogeneity when you have ordered categories.
2) the usual requirement (which is usually too stringent in any case) isn't for a minimum observed count, it's for a minimum expected count
3) don't discard your valuable data, you have so little of it:
a) (not great, but better): you can combine it with an adjacent category (another small one if possible)
b) even better still, you can simulate from the distribution of the chi-square with the same margins, and with a large enough simulation, get accurate p-values. This is available right in the regular chisq.test command in R, for example
(or you could do an 'exact' test, whether based off the chi-square statistic or the fisher test. )
Answer: Use fisher's exact test instead which gives a more accurate p value than the chi squared for low sample populations hyper-geometric distributions (finite distribution parameters).
This link below tell of HYPGEOMDIST which is a pretty powerful excel function that should get you started.
http://www.real-statistics.com/binomial-and-related-distributions/hypergeometric-distribution/


Router with bandwidth control to switch

Question: If i limit the bandwidth in a router like only 2Mbps and i connect a switch, does all devices connected to a switch will have 2Mbps bandwidth, or all devices will share in 2Mbps bandwidth?
Answer: With your setup every device would share the 2Mbit/s connection to the router, but the connection between the devices may be faster, for example you have an 100Mbit/s switch, then every connected device (except the router) could communicate with any other device connected to the switch with the full 100Mbit/s.
Be aware, that limiting the port to 2Mbit/s will not give actually an 2Mbit/s connection to the router, you have to subtract overhead, collissions etc.
Comment: thank you, i got the answer, its just my question is wrong because english is not my  native language, the switch is the one connected in router i mean, but as you said that every device connected in router will share in 2Mbps, then all devices that connected to switch which is connected to router will share in 2Mbps given by router

Router with bandwidth control to switch


How do I access the Lubuntu menu in Openbox?

Question: Is there a way to include the Lubuntu applications menu in an Openbox menu (~/.config/openbox/menu.xml) so I can use it within a pure Openbox session?
Comment: Does [this question](http://askubuntu.com/q/196614/273670) from AskUbuntu help?
Answer: lxpanel use .desktop files for menu and openbox use xml(menu.xml)
so you need a converter, for example:
http://obmenugen.sourceforge.net/

Generates a full menu for openbox using the information present in *.desktop files (with static content or dinamically generated).
Answer: What you need is a Pipe menu.
A pipe menu is a dynamic menu item that is created by a program every time the menu is opened.
There are multiple programs that can create a pipe menu of the installed applications. Here is a list of featured pipe menus from Openbox.org.
Since you use Lubuntu you would most likely be interested in openbox-menu. It uses the menu cache of LXDE to create the application menu. There is a Ubuntu package here. I don't know if it is in the official repository, you should check that.
To use a pipe menu you need to edit your Openbox menu file at ~/.config/openbox/menu.xml.
In your menu file you need to add a new menu item under your root-menu that looks like this
<menu id="desktop-app-menu"
      label="Applications"
      execute="<path-to-exec>/openbox-menu" />

You will need to replace <path-to-exec> which the path to openbox-menu.
You can get the path with
type openbox-menu

Then you will have a new menu item labeled Applications in your Openbox menu.
It you never edited your menu.xml take a look at the menu documentation from Openbox.


AD RMS using with Sharepoint 2010 and Form Based Authtication

Question: I am working on a project, the need is that the site is for extranet and by using form based authtication the users can access the portal, I am stack at a point where I have to implement the RMS services for externet user, means using form based authtication the user should have limited rights on office document. Anyone has an idea about this, RMS can work with Sharepoint form based authtication? or it just work with windows authtication?
Comment: There's an older thread on the MS forums which seems to suggest that IRM/RMS and FBA aren't really the intended use case. Your clients would need to set a registry key to point to your RMS server and you would need to set up FBA to use an AD membership provider.  http://social.technet.microsoft.com/Forums/en/sharepointadmin/thread/9f6612b2-ad02-47b8-835f-02ba3d93ca9a
Comment: Thanks to reply, the link is quite older, after that there are not any further solutions and changes on Microsoft side? or we might have any alternative solution for that?
Comment: Well, the issue is RMS is integrated with AD DS. You can also use Windows Live IDs, but if you are using a custom FBA provider that uses neither AD DS or WLID I don't believe there is going to be a way for RMS to authenticate user and authorize the RMS license.
Answer: "RMS can work with Sharepoint form based authtication?" Yes, RMS can work with SP FBA.  Please have a look at my comment in this link https://social.msdn.microsoft.com/forums/sharepoint/en-US/d652f16a-a5a9-4263-8c1c-61bf13265afe/ad-rms-with-sharepoint-form-based-authentication


Where does mongodb stand in the CAP theorem?

Question: Everywhere I look, I see that MongoDB is CP.
But when I dig in I see it is eventually consistent.
Is it CP when you use safe=true? If so, does that mean that when I write with safe=true, all replicas will be updated before getting the result?
Answer: MongoDB is strongly consistent by default - if you do a write and then do a read, assuming the write was successful you will always be able to read the result of the write you just read.  This is because MongoDB is a single-master system and all reads go to the primary by default.  If you optionally enable reading from the secondaries then MongoDB becomes eventually consistent where it's possible to read out-of-date results.
MongoDB also gets high-availability through automatic failover in replica sets: http://www.mongodb.org/display/DOCS/Replica+Sets
Comment: According to https://aphyr.com/posts/322-call-me-maybe-mongodb-stale-reads even if you read from the primary node in the replica set you may get stale or dirty data. So is MongoDB strong consistent??
Comment: Just for the record, MongoDB v3.4 passed the test designed by Kyle so yes, MongoDB is strongly consistent, even with ReplicaSet and Sharding : https://www.mongodb.com/mongodb-3.4-passes-jepsen-test
Comment: For the record, I no longer stand completely by my original comment here from 9 years ago.  The CAP theory is a poor way to reason about systems like this as it's a vast oversimplification of reality.  Consistency and availability in the presence of network partitions is a spectrum with lots of little trade offs, rather than a single binary.  While all the answers on this post are a bit of an oversimplification, including mine, JoCa's probably comes closest to a complete picture.
Comment: Awesome experiments by Kyle. It really hunts mongo down. I wonder if there are production systems, for example using MongoDB doing payment transactions? If it's just a personal website, who cares strong consistency then.
Comment: This answer might be a bit too simplistic, since MongoDB can sacrifice availability from time to time, based on configuration. JoCa's better explains the situations in which it behaves CA/CP/AP
Comment: So, when you say - "all reads go to primary by default" , heavy load on one node might cause latencies.. and also what if the primary goes down.. we are very vulnerable with this option correct ? Please correct me if wrong
Comment: @santhosh - if the primary goes down then a new one will be elected and your system will recover.  Spreading reads across the whole replica set to increase query capacity can be dangerous if you're not careful. Imagine a system with 3 nodes, each processing 1/3 of the total read volume. The system is getting close to the max number of reads that any one node can handle.  Now one of the nodes goes down for any reason (including planned maintenance!). That nodes reads then get sent to the remaining two nodes, which can push them over the limit of what they can handle and cause cascading failure.
Answer: I agree with Luccas post. You can't just say that MongoDB is CP/AP/CA, because it actually is a trade-off between C, A and P, depending on both database/driver configuration and type of disaster: here's a visual recap, and below a more detailed explanation.

Scenario
Main Focus
Description

No partition
CA
The system is available and provides strong consistency

partition, majority connected
AP
Not synchronized writes from the old primary are ignored

partition, majority not connected
CP
only read access is provided to avoid separated and inconsistent systems

Consistency:
MongoDB is strongly consistent when you use a single connection or the correct Write/Read Concern Level (Which will cost you execution speed). As soon as you don't meet those conditions (especially when you are reading from a secondary-replica) MongoDB becomes Eventually Consistent.
Availability:
MongoDB gets high availability through Replica-Sets. As soon as the primary goes down or gets unavailable else, then the secondaries will determine a new primary to become available again. There is an disadvantage to this: Every write that was performed by the old primary, but not synchronized to the secondaries will be rolled back and saved to a rollback-file, as soon as it reconnects to the set(the old primary is a secondary now). So in this case some consistency is sacrificed for the sake of availability.
Partition Tolerance:
Through the use of said Replica-Sets MongoDB also achieves the partition tolerance: As long as more than half of the servers of a Replica-Set is connected to each other, a new primary can be chosen. Why? To ensure two separated networks can not both choose a new primary. When not enough secondaries are connected to each other you can still read from them (but consistency is not ensured), but not write. The set is practically unavailable for the sake of consistency.
Comment: So if Im using the correct write/read concern level,it means all wrotes and reads go to the primary (if I understood correctly), so what exactly do the secondaries do? Just sit there on standby in case the primary goes down?
Comment: @tomer.z you may want to read [this section](https://docs.mongodb.com/manual/core/replica-set-members/#replica-set-secondary-members) of the manual: You can use secondaries for reading. If you are using "majority" Read-Level the read will be valid as soon as a majority of the members acknowledged the read. The same goes for the "majority" Write-Level. If you are using "majority" Concern-Level for both, then you have a consistent database. You may want read more about this in the [manual](https://docs.mongodb.com/manual/reference/read-concern/).
Answer: As a brilliant new article showed up and also some awesome experiments by Kyle in this field, you should be careful when labeling MongoDB, and other databases, as C or A.
Of course CAP helps to track down without much words what the database prevails about it, but people often forget that C in CAP means atomic  consistency (linearizability), for example. And this caused me lots of pain to understand when trying to classify. So, besides MongoDB give strong consistency, that doesn't mean that is C. In this way, if one make this classifications, I recommend to also give more depth in how it actually works to not leave doubts.
Answer: Yes, it is CP when using safe=true. This simply means, the data made it to the masters disk.
If you want to make sure it also arrived on some replica, look into the 'w=N' parameter where N is the number of replicas the data has to be saved on.
see this and this for more information.
Answer: MongoDB selects Consistency over Availability whenever there is a Partition. What it means is that when there's a partition(P) it chooses Consistency(C) over Availability(A).
To understand this, Let's understand how MongoDB does replica set works. A Replica Set has a single Primary node. The only "safe" way to commit data is to write to that node and then wait for that data to commit to a majority of nodes in the set. (you will see that flag for w=majority when sending writes)
Partition can occur in two scenarios as follows :

When Primary node goes down: system becomes unavailable until a new
primary is selected.
When Primary node looses connection from too many
Secondary nodes: system becomes unavailable. Other secondaries will try to
elect a new Primary and current primary will step down.

Basically, whenever a partition happens and MongoDB needs to decide what to do, it will choose Consistency over Availability. It will stop accepting writes to the system until it believes that it can safely complete those writes.
Comment: _"It will **stop** accepting **writes** to the system until it believes that it can safely complete those writes._" What about **reads**? Would it remain read-available during that time?
Comment: Yes it would remain read-available, if you've specified a read preference of 'primaryPreferred', 'secondaryPrefered', 'secondary' or 'closest'.
Answer: Mongodb never allows write to secondary. It allows optional reads from secondary but not writes. So if your primary goes down, you can't write till a secondary becomes primary again. That is how, you sacrifice High Availability in CAP theorem. By keeping your reads only from primary you can have strong consistency.
Answer: I'm not sure about P for Mongo. Imagine situation:

Your replica gets split into two partitions.
Writes continue to both sides as new masters were elected
Partition is resolved - all servers are now connected again
What happens is that new master is elected - the one that has highest oplog, but the data from the other master gets reverted to the common state before partition and it is dumped to a file for manual recovery
all secondaries catch up with the new master

The problem here is that the dump file size is limited and if you had a partition for a long time you can loose your data forever.
You can say that it's unlikely to happen - yes, unless in the cloud where it is more common than one may think.
This example is why I would be very careful before assigning any letter to any database. There's so many scenarios and implementations are not perfect.
If anyone knows if this scenario has been addressed in later releases of Mongo please comment! (I haven't been following everything that was happening for some time..)
Comment: MongoDB's election protocol is designed to have (at most) a single primary. A primary can only be elected (and sustained) by a strict majority of configured replica set voting members (n/2 +1). In the event of a network partition, only one partition (with the majority of voting members) can elect a primary; a prior primary in a minority partition will step down and become a secondary. This is the way replica sets have always worked. In the event a former primary has accepted writes that were not replicated, they will be rolled back (saved to disk) when that member rejoins the replica set.
Answer: Mongodb gives up availability. When we talk about availability in the context of the CAP theorem, it is about avoiding single points of failure that can go down. In mongodb. there is a primary router host. and if that goes down,there is gonna be some downtime in the time that it takes for it to elect a new replacement server to take its place. In practical, that is gonna happen very qucikly. we do have a couple of hot standbys sitting there ready to go. So as soon as the system detects that primary routing host went down, it is gonna switch over to a new one pretty much right away. Technically speaking it is still single point of failure. There is still a chance of downtime when that happens.

There is a config server, that is the primary and we have an app server, that is primary at any given time. even though we have multiple backups, there is gonna be a brief period of downtime if any of those servers go down. the system has to first detect that there was an outage and then remaining servers need to reelect a new primary host to take its place. that might take a few seconds and this is enough to say that mongodb is trading off the availability


PC makes noise while on and off

Question: my PC sometimes makes crackling noises while it's on and I was wondering if there was something wrong. It sounds like something is inside my PC, but I don't know for sure. I have a HP Pavilion that I just brought a month ago. Whenever I turn on or off my computer, it makes this buzzing sound that lasts a short time. When my PC is off, it makes a little noise and you can't hear it unless you get close. Is a normal Pc supposed to be absolutely silent? Also, I forgot to mention that it sometimes makes noises that sound like a car engine. The noise is not very loud.
Answer: You should not be hearing any buzzing. Normally the only thing you should be hearing from the PC are the fans (may sound like humming) or if you have a cd in the cd tray and load that you can hear it spinning.
I would check to see if there is anything loose inside.
How old is the computer? Are you using a mechanical hard drive or a SSD?
Comment: I brought it about a month ago. I don't know why my Pc is like this. I think it uses a mechanical hard drive.
Comment: My best guess it is a failing hard drive. There are not too many moving parts in the computer. Besides that maybe a more serious issue with the power supply "crackling"? If so it could be a fire hazard. @Jeanne
Comment: I'll probably return this Pc and get a new one. If it could be a fire hazard, then I have to get rid of this Pc immediately. Thank you. @DidYouPingMe

PC makes noise while on and off


Using third-party functions

Question: I have a third-party PHP script that I want to integrate and use in my site. The script is quite small and comes with its own functions and array indexes. How would I be able to call the functions and have them displayed in Views or CCK nodes?
Answer: If this is you own script, you could simply convert it into a module. That is easy, just create a module (tons of guides to be found in the internet) and put your functions in the .module file, then they will be available to all other code running on the site.
If this is a script/library provides by someone else, you could either place it in a custom module folder and include it with module_load_include() or (better!) declare it as a library with the help of the Libraries module. You will however need to create a custom module with some code to connect the functions of your script with drupal (Usually implementing the correct hooks and calling your functions from that) for both methods.
See also How do I go about putting standalone PHP files into Drupal 6? and How do I load a library defined with hook_library() on node/edit pages?.
Comment: Thanks for the answer. I found out that someone already created a custom drupal module for the script however its coded with drupal 6 now I am just gonig to port it to 7.
Comment: Sounds good, remember to post a patch for your port to the issue queue :)
Answer: The usual way is to create a custom module that include the PHP file when required.
If the custom module integrates with Views or CCK, it is not different; it can call module_load_include() to load the file.

Using third-party functions


Sine 30 or Sine 60

Question: I am looking for a second pair of eyes for verification. In the solutions manual provided by the textbook and professor, sin30 is used to calculate the answer. However, some of the students are arguing with the professor and everyone believes their answer is correct. I believe the textbook may be incorrect however, I decided to turn it over to the community for confirmation. This is a basic question to introduce the students to vectors mathematics. At this point in time, it appears seems that everyone's answer will be considered valid at this point. So I am asking the community who is unbiased what they think should be used to calculate the answer. I thank you for your time it is truly appreciated.
Comment: My usual intuition in problems like this is that when computing a torque about an axis, the moment arm is determined by the distance of the line of action from the axis. (The "line of action" is the line through the point where the force is applied, in the direction of the force.) It does not matter where on the line the force occurs, so the force could just as well be applied at the point of closest approach to the axis as at any other point on the line. In this case, if you extend the force vector in a line it intersects the perpendicular radius at a distance $r\cos30^\circ$ from the axis.
Answer: The solution is wrong. The definition of the magnitude of the cross product is $$|\vec a\times\vec b|=ab\sin\alpha,$$
where $\alpha$ is the angle between the vectors. But in your case the angle between the vectors is not $30^\circ$ but $120^\circ$, so the answer should be $$rF\sin 120^\circ=rF\cos\theta$$
Comment: This formulation suggests a possible cause for the error in the book. The author recalled that the formula involves a factor $\sin \alpha,$ and having labeled just exactly one angle in the entire diagram, the author absent-mindedly chose that angle's value for $\alpha.$

Sine 30 or Sine 60


Third Grade Question -- This makes no sense to me

Question: Third grade grandchild had this for homework. Can someone explain the intent here?
Comment: I would like to see questions 1-13. Perhaps they have more information or context about how the student has been taught addition and multiplication and what it means to "join groups."
Comment: It is unfathomable why someone believes that is a good homework question. Probably most mathematicians can't make any sense of it so how is a 3rd grader supposed to?
Comment: I agree with ruferd.  We cannot answer this, because we have not seen the previous material which may have explained what "join groups" means.
Comment: I suppose the correct answer is not that the direct product of two groups is also called the direct sum if the groups are Abelian?
Comment: Is this even correct English?
Comment: Probably part of the idea is we can't add unlike objects. For example, adding a car and a truck means what? Unless of course we take cars and trucks as examples of automobiles so then adding a car and a truck makes perfect sense. As far as multiplication goes, I leave it to the experts here. Personally, the idea of identifying multiplication and addition as necessarily tied to counting or grouping problems has the unfortunate side-effect of saying addition and multiplication are necessarily those counting processes. But, they're not. Arithmetic exists independent from these heuristics. Imho.
Comment: @ruferd See my answer below.
Comment: The red line of text explains everything
Comment: @BillDubuque A mathematician can't understand it because they haven't been told the extra information needed to interpret the question. Someone younger may well be in a better position, if they have that information.
Comment: @JessicaB Of course, but my remark applies to the question as presented above, where the essential context is missing. Note that the context (prior 2 pages) in an answer below was added a year after my comment.
Answer: Here are the two previous pages from those materials (a pre-publication version found with a Google search):

And here is the page containing the homework problem in question:

The intent now seems pretty clear. Students know that you can join groups by adding them, but in the case where the groups are equal in size (e.g. five bags, each with nine goldfish), you can also use multiplication to join the groups.
Comment: Thank you! The idea that we should be able to understand the question when stripped of its context - and the inference drawn by some that there must be something wrong with the question, and perhaps all of Common Core, because it requires the context of the lesson to understand it, has bothered me ever since this question was posted. I'm really glad to see what the actual context is.
Answer: I am not too familiar with the Common Core State Standards Initiative (whose standards I assume the question above is intended to follow), but according to this introduction to the standards for Grade 3,

Students develop an understanding of the meanings of multiplication and division of whole numbers through activities and problems involving equal-sized groups, arrays, and area models; multiplication is finding an unknown product, and division is finding an unknown factor in these situations. For equal-sized group situations, division can require finding the unknown number of groups or the unknown group size.

and

Students recognize area as an attribute of two-dimensional regions. They measure the area of a shape by finding the total number of same-size units of area required to cover the shape without gaps or overlaps, a square with sides of unit length being the standard unit for measuring area. Students understand that rectangular arrays can be decomposed into identical rows or into identical columns. By decomposing rectangles into rectangular arrays of squares, students connect area to multiplication, and justify using multiplication to determine the area of a rectangle.

To me, it seems that the emphasis is on "equal-sized groups," "same-size units of area," "identical rows," and "identical columns."  The child's teacher could have emphasized this in class.
If so, then perhaps one "valid" answer to this higher-order thinking question is "You can add objects together if they belong to the same group.  You can multiply groups of objects if they are of the same size."

For example, say that students are riding in $3$ buses:  one bus has $30$ students, another has $30$, and another has $32$.  How many students are there in total?
The answer is not $30+30+32+3$, that is, the number of buses is not added because buses are not students.
The answer is not $30\times 3$, because not all the buses have exactly $30$ students.
Comment: "Why one has to backtrack and decode this" - because one was not present in class.
Comment: @user10216038: Regarding "*What does it mean to Multiply Groups*", I'm pretty sure Joel intended to describe a situation such as the following: Exactly $5$ groups of marbles such that each group contains exactly $3$ marbles represents (when brought together to form one group of marbles, without omitting or including other marbles) a group of $5(3) = 15$ marbles. And given that the context of this is math education, I think the shorthand phrasing he used was fine. And I agree with ruferd that this question is probably taken out of context, although personally I don't like the question.
Comment: Why one has to backtrack and decode this nonsense to figure out what the creator of the question meant? Are third-grade kids supposed to do this themselves? It is a rhetorical question. And by the way, slapping the "Common Core" label on a question does not make it Common Core, because Common Core is a set of requirements, not a curriculum. But parents think this nonsense IS Common Core.
Comment: Between the feedback here and some independent research of "Common Core" I think I understand the intent, although I still couldn't answer the problem as written. This seems to be math redefined by non-math people. Instead of "equations" it's now a "division sentence" or a "multiplication sentence".   see:   www.syracusecityschools.com/tfiles/folder748/Pages%20from%20math-g3-m1-topicC.pdf   Thanks all!
Comment: Sorry I still don't understand. What does it mean to Multiply Groups? Is this hinting at Matrix theory?  What does it mean to Add to Join Groups? Is this new terminology for a UNION of SETS? I don't see an underlying principle here.
Comment: @user10216038, Dave L Renfro has the idea.  I've edited my answer to clarify.
Answer: This is an excellent example of a question which leaves both children and adults baffled. This is a truly meaningless question. Let's start with the fact that you add and multiply numbers whereas you join together sets (groups) of objects. The clause "multiply to join groups" may mean "use multiplication to find the total number of objects when joining together groups of items", in which case I guess that the expected answer is "only if all the groups are of equal size". But the fact that I am speculating about the intent of the question says it all.
Comment: This may be an ok question to discuss in class, but not as a written homework (unless it is an essay).
Comment: I think that asking a 3rd grader to consider a "meta-mathematical" question is harmful. Moreover, you **can** add 6 fruit and 7 cars to get 13 objects. That may be a great subject for us to dwell on, but it is abstract nonsense for a child.
Comment: The question is expecting the student to write a thought, not answer "yes" or "no." I suspect that your answer would be marked correct -- "You can use multiplication to join together multiple groups of the same size but not different sizes." I also suspect that other answers would be marked correct, like "you can add 6 fruit to 7 fruit to say there are 13 fruit, but you can't add 6 fruit and 7 cars to get 13 fruitcars."