AWS S3 with Rails problems with caching and reloading

Question: I have a bit of code like so which works fine if the file in question doesn't already exist.
<code>if AWS::S3::S3Object.exists? file_name, bucket.name + path_to_images
  puts "file exists (deleting)"
  AWS::S3::S3Object.delete file_name, bucket.name + path_to_images, :force => true
end

AWS::S3::S3Object.store file_name,
   File.read(file_path),
   bucket.name + path_to_images,
   :content_type => 'image/png',
   :access => :public_read

`rm #{file_path}`
</code>
The problem I'm having is if the file does exist, I want to overwrite it with a new copy... now I'm not sure if its a problem with overwriting it, so I tried deleting the file first if it already exists.  That didn't seem to work either.  So I assume it's either not deleting it, or it's cached too.
When displaying the image it of course trails with ?123232 a random number, I even tried deleting the browsers cache just for kicks.
I'm sure there's something easy I'm missing, and probably a more succinct way to do this anyway.
Thanks
UPDATE: I think the problem must have something to do with cloudfront or regular s3 caching, because eventually... it does update.  But only after a day or so?  And it is not my browser caching it, so it's probably this.  Anyone know how to tell it to dump the cache?
Answer: I've never experienced any issues with s3 caching or anything like that.  More than likely, I'd still consider this a local cache issue as the caching could be taking place in multiple places.  To confirm, I'd recommend viewing the actual files in your amazon bucket and seeing what you get after an update (I think amazon has this built into their s3 account site now, otherwise I've had good luck with cloudberry http://cloudberrylab.com/).  Alternatively, you could just try accessing it from a different computer after you've updated the file.
Answer: What about

File.open(file_path)

instead of File.read. This should do the trick :)
Petr
Comment: That didn't seem to do anything.  ;-(


How did the SWIFT ban on Russia affect the Russian economy?

Question: It should have had a negative effect, but in what ways did it hurt Russian economy, exactly?
Comment: Here is what they predicted: [How disastrous would disconnection from SWIFT be for Russia?](https://carnegie.ru/commentary/84634) (2021)
Answer: Not all Russian banks were actually banned from Swift just most commercial ones.
Swift is the Society for Worldwide Interbank Financial Telecommunication. It is basically a highway for financial messages to go through. Every time you buy or sell/buy something using your debit/credit card the bank has to send a message to the bank of the person who is buying from/selling to you.
In the past bank transfers were expensive and took long time because it took long time to send messages between different banks. Swift simplifies all this by creating common framework (with common way of numbering accounts identifying banks etc) which makes sending and processing financial messages (and hence transactions) much more simpler.
So being banned from Swift increases costs for Russian banks that have to either develop their new system or use alternatives (like the Chinese Swift equivalent). However, the western banks are unlikely to be willing to process payments if they are not done via Swift or if so only at extra expense so this effectively means that Russian people are cut of sending money abroad or buying things abroad (of course alternatives such as Bitcoin exist).
However, EU didn't sanctioned all banks because they still buy hydrocarbons from Russia.
As to what effect this has on Russia economy that's unclear because there is no rigorous research on this yet and this sort of action is unprecedented. This sanctions clearly hurt the trade between Russia and EU and US, but that was already hurt by other sanctions. Moreover, it does not affect the trade in hydrocarbons. On the other hand it creates extra costs for Russian banks/financial system.


How to make Dapper resilient for SqlAzure?

Question: I recently found out Entity Framework has a very easy way to make connections resilient in SQL Azure. Is there a recommended way to accomplish the same in Dapper?
Comment: I want all Execute and Query calls I make in Dapper to survive minor cloud problems caused by Azure. Following this link http://blogs.msdn.com/b/dotnet/archive/2013/10/17/net-framework-4-5-1-rtm-gt-start-coding.aspx it means I could just upgrade my framework to 4.5.1 and ADO.Net connections to SQL Azure environmnet will automatically be resilient?
Comment: No @JakobLithner that won't be enough.  That link only discusses connection disconnects.  It doesn't mention transient errors which is the main concern.
Comment: Dapper is just a wrapper around ADO.NET; what *exactly* do you mean by "make ... resilient" here? what do you want it to do differently?
Answer: The fastest way to protect against connection issues in C# against Azure is the  Microsoft Transient Fault Handling Block.
For example the below code would retry upto 3 times with 1 second intervals in-between when attempting to open a connection to a Windows Azure SQL Database:
var retryStrategy = new FixedInterval(3, TimeSpan.FromSeconds(1));

var retryPolicy =
  new RetryPolicy<SqlDatabaseTransientErrorDetectionStrategy>(retryStrategy);

retryPolicy.ExecuteAction(() => myConnection.Open());

FixedInterval is the back off policy, so it will try, wait 1 second, try again, etc until it's tried 3 times.
SqlDatabaseTransientErrorDetectionStrategy is simply does a check on the exception thrown, if it's a connection exception that should be retried, it will tell the RetryPolicy to execute the action again.  If it is not a connection exception, then the action will not be executed and the original exception will be thrown as normal.
As for when you should use it with Dapper; you can safely retry opening connections and read operations, however be aware or retrying write operations as there is a risk of duplicating inserts, trying to delete a row twice etc.
More detail here, this library can be found as a NuGet Package here which includes the detection strategies for Windows Azure.


Can a vow of celibacy for 2 months be taken back?

Question: I took a vow that I would not masturbate for 2 months. Only 6 days into this, I cannot deal with this any longer.Please note that I have Obsessive compulsive disorder so it is possible that I took my vow because of that. I am not sure of it myself
Comment: @Wikash_ The question is not about deities. This doesn't look a constructive comment.
Comment: This isn't at all personal advice. If it were the OP would take decision on his own or from other online sources. He's more interested about rules of vow.
Comment: Why would you want to become celibate? Almost all hindu gods are not celibate.
Comment: Don't worry about it. Try reading some books which will help you achieve your goal whatever you truly desire. Only follow it if it truly makes sense to you. Don't follow blindly which just makes you more frustrated, annoyed, and lost but nothing else. Try to seek what you really want. Know yourself first.
Answer: Yes. Any vow or promise that affects you or someone else negatively, should be broken unless you want to prove to yourself that you have strength to keep it.
If you don't, even when you're suffering, sooner or later you'll realize you did a bad thing. It wasn't necessary. There's no rule that you must keep doing something if you've taken a vow. Rules should be for solving problems or for some benefits rather than creating problems. I don't see anything wrong in it.
After all, what's a vow? It's something you believe in and you feel you must not break the promise related to it. But no one knows if it was right thing or wrong. Only experience will tell you that. If you feel you've gained enough experience in 6 days, break the promise!


Can you measure the degree of uniformity of a 2d shape?

Question: Is there a calculation that could take the points that make of the outline of a 2 dimensional shape and provide a numeric evaluation representative of the uniformity or symmetry of the shape.
Such as circle, square or even pointed star would have a 'high' score but a rectangle or odd pointed star would have a some what lower score and an completely bizarre random shape would have a very low score.
Comment: The people who attempt to apply mathematical principles to drawing US Congressional districts have come up with various measures of regularity/bizarreness, maybe you'd enjoy going through their literature.
Answer: This problem is addressed in Estimating Complexity of 2D Shapes (2005). The notion of "complexity" seems to agree at least qualitatively with what the OP calls "uniformity". The complexity measure $C$ of a 2D shape is quantified by the combination of three criteria – (a) entropy of the global distance distribution, (b) entropy of local angle distribution, (c) shape randomness. The figure below shows results for 6 shapes, discretized by a set of points. Small $C$ means low complexity.
Answer: The Kolmogorov complexity of the shape would provide a measure of uniformity.
E.g., the first two shapes in Carlo Beenaker's answer could be programmed in Logo as:

circle: repeat 30 [ fd 1 pu fd 9 pd rt 12 ]

triangle: repeat 3 [ repeat 10[ fd 1 pu fd 9 pd ] rt 120 ]

The circle program is shorter than the triangle program, and both are shorter than programs for the other shapes. So the Kolmogorov measure is in rough agreement with the metric in that other answer and the intuition of the question.
Answer: Assuming that your shape has a nice enough description in polar coordinates, the Fourier-series might help you. Specifically, assume that the center of gravity of your shape is in $0$ and it can be written in the form
$$S := \{  (r\cos(\theta),r\sin(\theta)): \theta \in [0,2\pi], 0\leq r \leq f(\theta)\}$$
where $f$ can be considered a $2\pi$-periodic function. Now the more symmetrical your shape is, the more Fourier-coefficients of $f$ vanish. For example, if you have mirror-symmetry, it will be a (possibly phase-shifted) cosine-series, or if you have $n$-fold rotational symmetry, all non-zero coefficients will be at multiples of $n$ and so on.
The main problem would then be to convert this into a single number. I know that there are some ways to measure sparsity often used in numerics, but I know little about the details and how well they react to patterns (such as "precisely every $n$-th coefficient", i.e. symmetry, in contrast to "on average $1$ in $n$ coefficients").


Using webdriver plugin in jmeter

Question: Iam trying to use webdriver plugin to automate an application in jmeter. Can anyone help me to do scripting in selenium by using webdriver plugin. Thanks in advance.
Answer: It depends on the browser and programming language you're using.
Currently Chrome is the most popular browser and JavaScript is the default language for the WebDriver Sampler so:

Download chromedriver (make sure that the version you download matches your Chrome browser version)

Add Chrome Driver Config and specify the path to the chromedriver in the "Chrome" tab:

Add WebDriver Sampler and implement the code for your test scenario. WDS.browser will stand for the ChromeDriver instance so you will be able to: use

WDS.browser.get('some url') - open a page
var element = WDS.browser.findElement(org.openqa.selenium.By.id('some id')) - locate a WebElement
element.click() - click the element found in the previous step
etc.

More information: The WebDriver Sampler: Your Top 10 Questions Answered

Using webdriver plugin in jmeter


Can I transfer an app to somebody else without having them to pay for developer enrollment?

Question: I am an app developer, and I have to transfer an app to a company, so the app won't have my name under the title in the AppStore. So it looks like they made it. My question is, do they have to pay for an enrollment, or can they just recieve the app for free, and if they can, how can they recieve it from me? I know you can transfer it from itunes connect, but it seems to me that you have to pay for an enrollment to get access to itunes connect.
Comment: app developer? iOS or Mac apps? Visit [this](https://developer.apple.com/). Look for Enterprise Distribution.
Answer: Yes, the receiving company will need an Apple Developer ID and membership of the Mac and/or iOS Developer Programs.
Part of the process of enrolling involves agreeing to Apple's terms of service and other legally binding agreements. Without agreeing to Apple's agreements, the receiving company will not be able to participate in Apple's App Store or Mac App Store.
It is possible to have applications associated with your Apple Developer ID but not participate in the App Stores. However, those applications will not be eligible for sale through Apple.


How do you work out the coefficients of the metric tensor?

Question: The definitions of covariant and contravariant tensor quantities are that they transform as $A' ^i=\frac{\partial x_j}{\partial x'_i} A^j$ and $A'_i=\frac{\partial x'_i}{\partial x_j}A_j$ respectively, and they are related by the metric tensor $g^{\mu\nu}$ so that $A^\mu=g^{\mu\nu}A_\nu$. Is there any way to calculate the coefficients of $g^{\mu\nu}$ from the coordinate transforms listed above (e.g. $g^{\mu\nu}=\left(\frac{\partial x_j}{\partial x'_i}\right)^2$) or is the metric the most intrinsic (read: incalculable) part of the spacetime?
Comment: You can certainly not compute the metric tensor from the coordinate transforms for tensor quantities, which don't have any dependence on the metric tensor and will always look the same.

On the other hand, if you are given $A^\mu$ and $A_\mu$ in coordinates, that does gives you 4 linear equations the coefficients of $g_{\mu\nu}$ have to satisfy, so if you have a enough of such pairs, you can reconstruct $g_{\mu\nu}$.

Finally, the metrix *is* the most intrinsic part of the spacetime, but that doesn't necessarily mean that it cannot be reconstructed from derived quanitities.
Comment: Your transformation is wrong. Coordinates should have upper indices and the primes should be on the top of the partial derivative.
Answer: What we tipically call the metric is $g_{\mu \nu}$, whereas the inverse metric is called $g^{\mu \nu}$ and it is defined through $ g^{\mu \alpha} g_{\alpha \nu} = \delta^{\mu}_{\nu}$. The metric transforms as a 2-rank covariant tensor, i.e. $   g'_{\mu \nu} = \frac{\partial x'_{\mu}}{\partial x_{\alpha}} \frac{\partial x'_{\nu}}{\partial x_{\beta}}g_{\alpha \beta}$. All the information about the geometry of spacetime is encapsulated in $g_{\mu \nu}$. In the context of general relativity, it is obtained from the energy and matter content through Einstein equations: $G_{\mu \nu} = 8 \pi G T_{\mu \nu}$, where $G_{\mu \nu}$ is a very complicated tensor that depends on the first and second derivatives of the metric, and $T_{\mu \nu}$ is the stress-energy tensor. Hope this helps.


What is the difference between Xbox 360 and a Xbox 360S?

Question: I am planning on buying an Xbox but am confused by all the different versions. What are the differences between them?
Comment: Edited so the question is no longer an opinion as to which is best or a shopping recommendation.
Answer: The Xbox 360 hardware has gone through 3 major revisions:
Here's a photo from a CNet review of the E:

The one on the left is the "old style" Xbox 360.  There were a couple of versions of this, and some early models did not come with HDMI ports.  You're only going to buy one of these if you buy used, and I'd be careful.  A lot of the early run had issues with overheating and were prone to the "red ring of death."
The middle unit is the Xbox 360 S.  It improves on the classic 360 by adding a dedicated Kinect port, additional USB ports, integrated wireless and a built-in optical audio out.  All of these models come with HDMI ports.
The one on the right is the Xbox 360 E.  It is a redesign of the 360 S to make it look a bit more like the Xbox One.  However, they dropped the optical out, the AV port (for Component video) and a USB port.  There's no real advantage to this model as far as I can see. (The linked CNet review says the pros are it's "slightly quieter")
In addition to these models, there are also variants of each of these.  Usually it amounts to a difference in the amount of storage.  There are also various bundles that pack in games or peripherals like Kinect.
It seems clear that for almost any way you could judge the three, the Xbox 360 S is the winner.  The only advantages the E has is similarity in design to the Xbox One, and perhaps a slight noise difference.  The "classic" 360 is probably cheaper, but buying one used is slightly risky.
There's some more detail and a somewhat-useful table in this Wikipedia article.
Comment: @Thebluefish, yeah, there are other risks as well.  I just figured if I said "it's really risky" I'd get a comment saying "well, I bought one used and it was fine, so your statement is overblown" :P  Instead I erred on the side of not risky enough, and got a comment saying my statement was under-blown...  I'd also be afraid of getting a modded console or one where it was banned from XBL.  At any rate, if you go used, get it from a place with a good return policy.
Comment: "Slightly" risky is an understatement. Many older consoles like that are prone to "quick" fixes that allow someone to sell it off real easy. A word of advice though, if you do plan to purchase the older style, look for the ones that have 256mb or 512mb of built-in memory, and I'm not taking about a separate memory card or HDD. If it has that built-in memory it means it's a Jasper model which have failure rates akin to the 360 S models.
Comment: I do not see why you think there is only a market for the original console in used form. You can find used Xbox Ones and PS4s for sale at retail outlets like GameStop, so the age of the console really does not correlate strongly. People will turn around and re-sell a console they only owned for 5-6 months as weird as that may seem. I would definitely avoid the *very first* run of consoles though, not for a lack of reliability but because many televisions do not support 1080p over component or require breakout cables just to use component at all these days. The HDMI models are more versatile.
Comment: @AndonM.Coleman, I don't believe there is only a used market for the original console.  I'm sure there are used E's and S'es on the market.  However, the original model is discontinued and has been for some time.  Thus, you're unlikely to find a new one.  Since it's hard to determine what vintage an original style 360 is, you might end up with one that is at higher risk.  I agree with you that I find used Xbox 360's (of any model) to be higher risk than other consoles, though.
Comment: @agent86 Oh.... I get you now. You meant to say that the only way you will be able to buy an original 360 model is if you look in the used market. I thought you were saying that the used market only consisted of original 360s ;)
Answer: Aside from appearances,
the most significant differences between an original 360 (2005-2008) and the "S" / slim version (2010) are related to the processor type, power consumption, and heat dissipation, all of which significantly affect the long-term functionality of the system.
From the table below (source), we can see that the Valhalla processor used in the Xbox 360S
is the most power-efficient and therefore has the least amount of system heat. The best possible processor for the regular 360 is the Jasper; any processor revision prior to that will have higher chances of failure due to overheating.

Also, the 360S has a more sensible ventilation design where the grill is on the top of the system (vs. the back of the system like in the original 2005 design), allowing heat to dissipate naturally upwards.
Answer: I got an original No-HDMI out Xenon model from a friend for precisely the price it's worth.
Free.
That's the only way anyone should bring one home.
One way to spot the Jaspers is by looking at the label on the back.  If your looking at one of the original chassis, you do NOT want one that says 12V 16.5A or 198 W.  Again, unless it's free.  I'd honestly recommend never buying one from EBay or Craigslist at any price, since you don't even know if the insides match the label.
From Gamestop, might be worth it on a refurb since if it's a Jasper (12V 12.1A or 145.2 W) according to the label, it almost certainly really is one.  However, I'm seeing refurb bundles on the classic chassis for only 10-20 bucks cheaper than a refurb bundle on a similar S or E.
Take the S, or the E in a pinch.
I only even took the original non-HDMI Xenon for free as a Windows Media Extender point for a 20" tv I don't care about HDMI on.  When it dies, it dies.


Beginner book on logistic regression

Question: I have some understanding of linear regression and want to understand logit and probit models. What is a good beginners book that helps you develop intuitive understanding of this topic of logistic regression?
Answer: I personally like

Introduction to Categorical Data Analysis by Alan Agresti

and

Introduction to Generalized Linear Models by Dobson and Barnett

Both are very readable and specifically designed for beginners. Math is kept at a minimum while emphasis is given on intuition and the methods are illustrated with many examples.
If you are only interested in Logistic regression, then Agresti would be the best choice but if you would also like to see what else is there, besides the classical linear model, Dobson is a good reference.
Hope this helps.
Answer: Introduction to statistical learning by James, Witten, Hastie and Tibshirani has a chapter (4) with an introduction of logistic regression. Also provides plenty of alternatives and context as well as code examples in R. You can download it as pdf for free from James' website.
http://www-bcf.usc.edu/~gareth/ISL/

Beginner book on logistic regression


Firefox - weird rendering for select element on changing background colour?

Question: I have a HTML page which has a <select> dropdown box.  I want to change the background of the box to be red upon certain conditions.
This works, but with Firefox 3.6.16 the dropdown box loses its 'downarrow' widget symbol (on which one clicks to list the options of the dropdown box) when the background colour is changed.  The widget is still there - one can click on it, the dropdown box expands as normal - but Firefox, in rendering the background colour red, 'loses' the actual 'downarrow' symbol.
After making another selection in the dropdown box Firefox re-renders the element and the symbol pops back again, everything looking normal.
I've got this little sample page to illustrate the problem:

          <title>Showing weird bug with select background colour change</title>

          <select id="test_select" onchange="select_changed(this);">
               <option value="one" selected="selected">one</option>
               <option value="two">two</option>
               <option value="three">three</option>
               <option value="four">four</option>
          </select>

The option 'one' is supposed to be the default background colour; any other option is red.
Making a transition from one to the other causes the javascript to set the background colour of the <select> element to either #FF8888 or ''.
If the dropdown box is looking normal with 'one' selected and you select 'two' you'll see - in Firefox - that the element will turn red but the symbol thingie will disappear.  The next time you select an option it will come back again.  Going the other way - from red to normal background colour on select the 'one' option - doesn't have the same problem.
Is this something I just have to live with, or is there a better way to change the background colour?  Thanks!
Answer: I don't sure I understand correctly what would you do, but can you try:
<select id="test_select" onchange="select_changed(this);">
   <option value="one" selected="selected" style="background-color:#FFFFFF;">one</option>
   <option value="two" style="background-color:#FF8888;">two</option>
   <option value="three" style="background-color:#FF8888;">three</option>
   <option value="four" style="background-color:#FF8888;">four</option>
</select>

I just want to recommend you that, you should override child elements of parent, if not it will be inherited from its parent.
If you don't clear please let me know.
Comment: Setting the background colour for the individual options makes them have that colour when one clicks on the dropdown box and the elements are all 'expanded', but doesn't leave the dropdown box itself coloured red in its normal 'unexpanded' mode.


Thicker string gauge

Question: I'm a beginner and im thinking of getting thicker gauge strings and a guitar store employee told me that if the thicker string doesn't fit around the nut I'll have to go and get my guitar a setup.
If I get strings with thicker gauge that does fit without getting a setup is there any potential damage to my guitar?
Comment: Electric, acoustic? Truss rod or not? The answer is going to be 'it depends'. tbh, someone would have to see it before & after to really be able to give a 'correct' answer. Actual *damage* is unlikely, but playability may suffer.
Comment: Without actually holding the guitar and whchever strings you want, and trying them, there's no knowing.The guitar guy may have erred on the safe side, may be ignorant, or may be drumming up business. We can't help.
Comment: I think the employee mean that the slots in the nut may be too narrow for thicker strings and the guitar will be hard to tune until the slots have been filed wider.
Answer: Swapping to the next-thicker (or next-thinner) gauge of strings of the same type (from electric to electric, or from acoustic to acoustic) is normal and almost never requires any sort of setup adjustments.
For example, my guitar had 9s on it when I bought it. I have used 13s on this guitar without needing to modify the nut.
However, if you want to put acoustic guitar strings on an electric guitar, the gauges can be much larger and warrant the use of a luthier's narrow files to enlarge the grooves in the nut to accommodate the thicker strings.
You can do a quick check before you buy the strings:

remove the new string from its packaging (ask the manager first)
gently lift the old string from the nut and hold it aside
see if the new string touches the bottom of the groove in the nut

If it touches the bottom, no modification is necessary.
As you get more experienced, you may still benefit from a professional setup to match the string action and intonation with your playing style, string gauge of choice, tuning, etc. But for now you probably don't need a luthier's help.
Answer: Thicker strings engage more force. I've heard of a case where the guitarist changed strings to thicker ones, and the neck couldn't support the force needed and hence doubled.
I suggest you take your guitar to a luthier who can inspect it and advise you on your specific case (guitar/string gauge).
Hope this helps!
Answer: You will definitely need a set up with a gauge change, there is no way around that.
But if the thicker string doesn't even fit in the nut groove it will not be properly mounted and likely slip off the nut and that will cause damage to the nut and the finish on the side of the fingerboard, not to mention the guitar will likely not be playable.  In the rare chance that the string stays in place it will be too high to fret the first position, etc.
By the way, fixing the nut because the string does not fit properly is more work than a standard set up (I think).  Changes in weather, any playing, will also cause the set up to slip out of adjustment slowly over time.  It has been recommended to me that I get them set up at least once a year, maybe twice.  So being a guitar owner comes with that maintenance responsibility.

Thicker string gauge


Why do the 2-SAT techniques do not work for 3-SAT?

Question: i just want to know why the techniques we use to solve the 2-SAT problems cannot be used to solve the 3-SAT problem. So the techniques that i know that help solve 2-sat are the naive on where you just take one variable in the formula and give it a truth value and go on from there by deleting clauses where the variable is in it and so on. Why won't that work for 3-SAT so when i chose a variable and set it to true i do the same things that i would do for 2-SAT, shouldnt that also work?
The second technique that i know that helps solve 2-SAT is where you create an implication graph from the formula with 2 implications per clause with the form being a$\Rightarrow$b, where a and b are literals. if i convert that to 3-SAT i could generate 3 implications per clause a$\Rightarrow$b$\lor$c where a,b and c are literals. why won't that algorithm work for 3-SAT the same way it works for 2-SAT?
Comment: Can you explain why your algorithms work for 2SAT? Do the same proofs carry on to 3SAT?
Comment: Have you tried your proposed algorithms on 3SAT?
Answer: The issue might be that 2SAT can be solved in polynomial time while 3SAT is NP-complete and can't. When the computational complexity becomes much higher, the proofs of your 2SAT method might not translate well to 3SAT or it works but you just don't have time to wait around for the solution.
It's hard to say unless you tell us what you mean by "work".
As I understand, going from 2 to 3 SAT you're changing the information structure of the problem a lot. So a lot can go wrong.


Importing Ethereum Account Private Key and Using on Different Servers

Question: Let's suppose i have a wallet on one remote server, R-1. Is it possible to get the private key of that wallet/account and use it on different remote server R-2? I know, i can copy/paste files under keystore of a specific wallet from R-1 server to R-2 server but I am more interested in getting a private key, which when exporting from CLI on R-2 will import my account without copying/pasting. As per my understanding, Ethereum must have my account's information, if yes, then how can use that available information to use my wallet on different server.
Answer: You can use one pair public and private key to do blockchain movements from many different servers or wallet apps at the same time. Your address history is not stored in some back-end server or in the server of the wallet application you use on your mobile phone, but on the blockchain, because it's peer to peer network. Every running node have copy of what you did in the past and that's the point of the decentralization. People get easily confused that if they remove the wallet app they use on their phones they will lose their crypto funds - no, these apps are just reading the funds from the blockchain network.


Brown Coconut water safety issue?

Question: If you like sour and the water is not rancid is there any danger in drinking the water?
If so what is the danger? I grew up drinking it and it was delightful. There were never any warnings against drinking it. If the water is unsafe to drink then in my opinion the coconut meat is also unsafe. Please be specific.
Comment: There are (sometimes) warnings against drinking the water of mature cocomuts: http://cooking.stackexchange.com/q/51575/23376
Answer: The only thing I know is that sour water from the brown mature coconuts is horrible tasting but I didn't get sick from it. I would not keep the meat if the coconut water is in question. The meat is not safe from bacteria if bacteria has touched it.
Check out what organyx has to say here about the pink and purple color: http://www.chow.com/food-news/84067/is-coconut-water-bad-for-you/
I've found this site very informative in the past: http://foodbabe.com/2014/07/15/how-to-buy-the-healthiest-coconut-water-and-avoid-the-worst/

Brown Coconut water safety issue?


compare block disappears from sidebar

Question: I see when there is no item selected to compare, the block-compare is not displaying in sidebar and after adding product to compare it is there. How to display this block always and show message"You have not selected any items to compare" for example?
Answer: Natively the block should show regardless, and if there's no item, it would say "You have no items to compare.". So it looks like your theme is customised, or a module overwrites it. Make sure your catalog/product/compare/sidebar.phtml in your theme folder is similar to the default one.
Comment: Even I copied the sidebar.phtml from default to my theme folder, still I can not see the block :(
Answer: The core code will not display the block if there's no recently compared products:
protected function _toHtml()
{
    if (!$this->getCount()) {
        return '';
    }

    $this->setRecentlyComparedProducts($this->getItemsCollection());

    return parent::_toHtml();
}

To change this behavior you can do a rewrite of the Mage_Reports_Block_Product_Compared block to fulfill your needs.
Comment: This is for the "Recently Compared Products" block, that doesn't apply here
Answer: I figured it out. In default sidebar.phtml there is a line $_items = $_helper->getItemCount() > 0 ? $_helper->getItemCollection() : null; I removed the condition of checking $_items and it works now.
Comment: Can you paste the content of your `sidebar.phtml`, your solution isn't really one.

compare block disappears from sidebar


Why does Maxwell's equations $\partial_{\mu} F^{\mu \nu} = 0$ have 3 independent components (DOF) in $D = 4$?

Question: And how can we generalize this to the statement that it has $D-1$ independent components in dimension $D$?
I know that $F_{\mu \nu}$ has six independent components (because of antisymmetry), how do I go from there?
Comment: Related: https://physics.stackexchange.com/q/20071/2451 , https://physics.stackexchange.com/q/457269/2451 and links therein.
Answer: First, I'll address a language issue: "independent components" can mean two different things:

The statement that $F_{\mu\nu}$ has six independent components is a statement about the number of parameters required to specify an antisymmetric matrix.
The statement that $\partial^\mu F_{\mu\nu}=0$ has three independent components is a statement about the number of functions required to specify a solution of this differential equation.

The question about the number of independent functions required to specify a solution is answered in https://physics.stackexchange.com/a/20072/206691, so this could potentially be marked as a duplicate question. That other answer uses a different notation, though, so I'll offer a version that uses the current OP's notation.
I'll write $j,k$ for spatial indices and $0$ for the time index.
Suppose that we have some solution $F_{\mu\nu}$ to
$$
\partial^\mu F_{\mu k}=0
\tag{1}
$$
 but that we have not made any attempt yet to enforce
$$
\partial^\mu F_{\mu 0}=0.
\tag{2}
$$
Equation (2) is equivalent to
$$
\partial^k F_{0k}=0.
\tag{3}
$$
Equation (3) is a constraint: it is a condition that the initial data must satisfy, rather than a condition on how that data evolves in time.
Applying $\partial^k$ to equation (1) gives
$$
\partial^\mu \partial^k F_{\mu k}=0.
\tag{4}
$$
In equation (4), the terms with $\mu\neq 0$ are identically zero because $F$ is antisymmetric, so equation (4) is equivalent to
$$
\partial^0 \partial^k F_{0 k}=0.
\tag{5}
$$
In words, this says that if the functions $F_{\mu\nu}$ that we chose to satisfy equations (1) for all times are also chosen to satisfy the constraint (2) at some initial time, then they automatically satisfy the constraint (2) for all times.  In this sense, a solution to $\partial^\mu F_{\mu\nu}=0$ in $D$-dimensional spacetime has only $D-1$ independent components.
Summary: Equation (1), which involves time-derivatives, has $D-1$ components (the number of values of the spatial index $k$). Equation (3) does not involve time-derivatives; it imposes a constraint on the initial data rather than an additional condition on how that data evolves in time.
Answer: There are 3 dofs for the three spatial directions. Then there are two directions - time reversal symmetry. Taking polarisation into account one has 12 independent solutions, per frequency. That is a lot of dofs.


Can $\sum_{n=2}^{k} \sqrt{n}$ be rational?

Question: It is easy to see that $\sqrt{2}$ and $\sqrt{2}+\sqrt{3}$ are irrational. So $\sqrt{2}+\sqrt{3} + \sqrt{4}$ is irrational. But what about $\sqrt{2}+\sqrt{3} + \sqrt{4} + \sqrt{5}$? I suspect that
$$\sum_{n=2}^{k} \sqrt{n}$$ is always irrational, is it true and is there a simple way to proof that?
Comment: @PeterForeman I don't disagree, but most likely the OP doesn't quite understand the terminology used in the question and answers there. They likely don't know what a $\mathbb Q$-vector space means, for example.
Comment: Not quite a duplicate, as a sequence of roots of adjacent integers are added, not (typiccally) primes,.
Comment: See [this](https://math.stackexchange.com/questions/442259/is-sqrt1-sqrt2-dots-sqrt-n-ever-an-integer) post which provides an answer to your question.
Answer: $\sqrt{2}+\sqrt{3}+\ldots+\sqrt{n}$ cannot be rational. Let $p$ be the greatest prime in $[2,n]$. By quadratic reciprocity and Dirichlet's theorem there is some huge prime $P\equiv 1\pmod{4}$ such that $p$ is a quadratic non-residue $\!\!\pmod{P}$, while the primes less than $p$ are quadratic residues $\!\!\pmod{P}$. It follows that $\sqrt{2}+\sqrt{3}+\ldots+\sqrt{n}$ does not belong to $\mathbb{F}_{P}$ but it belongs to a quadratic extension of $\mathbb{F}_P$. In particular it cannot be rational.
Comment: A delightful point of view!
Comment: I don't have the skills to understand the answer so I'm not sure that Exchange stack etiquette requires me to vote it. On the other hand, it seems a pity to me because there is a risk that good answers will go into oblivion.


Why is my car missing, surging, shaking and rpm's idling radically

Question: 1999 Toyota Camry 2.2 automatic
     Car is trying to die. RPM's are jumping from 1-5. Only way to accelerate is very slowly when it starts to buck. Have replaced fuel filter, air intake molded piece, IAC sensor, TPS sensor, oxygen sensor, plugs,... Was told air filter is fine, and wires need to be changed because it shocks if you touch a wire, catalyst converter, etc.  Ready to drive it off a cliff. Please help.
Comment: If the ignition wires will shock you if touched replace them as the next step. Poor wires that are shorting out can cause exactly the symptom described.
Answer: your problem is too hard to figure on the info provided, however no worries. My advice for the cheapest solution is to remove the intake pipe to the throttle body and drown the throttle body butterfly and intake with carburettor cleaner until it looks showroom shine (please wear safety glasses, this stuff stings eyes), then fit the old plugs, wait about ten minutes then crank the engine with full throttle until you get some sign of life (same as if you flooded an old carb engine). Then remove and replace the old plugs with your new ones and carry out a thorough check on all ignition leads, max resistance on leads is 4K OHMS per foot, (requires multimeter, cost approx. 20 dollars) if resistance reading is higher than this for any lead,  replace all leads and restart engine. If this fails to improve performance, you may have a compression / head gasket type problem. Unfortunately that requires more work, but your efforts with the carb cleaner, plugs and leads will still be good when the compression issue is resolved, and you wont spend any extra. Basically if an engine has spark (timed and correct firing order), compression (@180psi per cylinder on average) and fuel (use 98 octane wherever possible) it will run well, as advertised. My feeling on your post is that your ignition leads are at fault, so once again, check resistance and further more, carry out a physical inspection /integrity check of their outside insulation.
Cheers Hutch
Comment: First off, carb cleaner is super toxic stuff. Use sparingly, and with gloves, if at all--and it likely will not help with op's problem, in this case.

98 octane gas is bad advice. OP should use the lowest octane available at your local gas station for the car described. Higher octane gas does not confer any benefits to normal cars, and can, in fact, reduce their performance.

This answer sounds like a 'clean everything and hope something fixes it' answer--a reasonable catch-all troubleshooting procedure when lost, but hardly a useful specific response to this question.
Answer: Erratic idle: lack of steady air and/or fuel supply, or erratic/weak sparking. IAC sensors are the first I would blame, since it controls idle: check it is working by taking it off its supporting mount, leave it connected and get someone to help you turn on the key (not to crank). The sensor tip should move to reposition itself. Put it back if it did, change if it didn't. Check intake pipe where the IAC gets mounted, be sure there isn't any gunk where the IAC actuator sits. Check sparks, they should be dark "white-ish" brown, if they are carbonized means too much gas. In any case, put new or clean sparks. Try to crank without the air filter connected, that excludes the breathing part :)
Nada? Then go further: injectors may be dirt. In some engines you can disconnect them without too much trouble, see if with the engine running you can selectively disconnect them one by one, then connect them back, like checking for a bad spark plug. Injectors works with low current, won't zap you doing this. :)


Jagged lines on Barcode

Question: Trying to insert a barcode into some artwork on Illustrator CC when i try to free rotate the barcode and downsize the lines are becoming jagged? I have anti aliasing checked and the barcode is in an eps format?
Comment: Is it a raster or vector?
Answer: Sounds like your barcode is a bitmap / raster (even if .eps, the format is capable of embedding bitmaps).
If that is the case, you can solve it by tracing the barcode first.

LiveTrace while the barcode is still vertical
find the preset that gives best results, check Ignore White
Expand
rotate and say goodbye to jaggies, win.
Comment: Very likely; but I'd rather try and regenerate the barcode as an actual vector image first.

Jagged lines on Barcode


Which directories to exclude before full Ubuntu synchronization?

Question: I have two Ubuntu server which I want to synchronize so that I can run the second server when the first server is down. I want to do the synchronization using rsync. But I read, not all the directories (for example /boot and hardware specific files) are not supposed to be synchronized. Can someone tell me exactly which files and directories I need to exclude before doing full synchronization?
Answer: Just don't backup things that aren't really files
<code>/proc, /sys, /dev
</code>
You also may want to stay away from extra disks (like USB disks) or network mounts that you don't want being copied.
This question has more details
Linux - What directories should I exclude when backing up a server?


How do Intrusion Detection Systems (IDS) work?

Question: How do Intrusion Detection Systems (IDS) work? As I understand it, they monitor network traffic but what exactly do they look for? How can they tell apart regular activity from intrusions?
Answer: You typically have two kinds of IDS, network-based and host-based, and these can be signature or statistical response types.
Signature IDS's are simple, fast, and can be updated easily. Usually vendors supply signature files - similar to the way anti-virus vendors supply virus signatures. For this reason most IDS's use signature recognition. The downside is that they will not identify a new attack unless it has a signature which matches an existing attack.
Statistical or heuristic IDS's learn what 'good' or normal traffic looks like and alert on anything which isn't normal. This does mean they are much better at spotting new attacks, but does require learning periods when initially installed and regularly on the implementation of new servers, services and when any new traffic types or volumes are expected.
Network based IDS's are usually implemented at the perimeter of an organisation, and they have visibility of all traffic entering (and sometimes exiting) the organisation. Where traffic has indications that it may be malicious it is logged or flagged to a response system or person.
For a large organisation, the amount of different valid traffic types can be very high, and the traffic types can vary over time so the ongoing configuration and tuning of a perimeter network based IDS can take a lot of resource. For this reason, the majority of large companies outsource this to vendors who provide the service to many organisations. These vendors have better visibility of attacks happening, a scale advantage on tuning and response, and the ability to update signatures for all their clients at once.
Host based IDS's are more usually implemented in-house for specific high-value servers. The traffic types and load are usually much lower and more predictable so the resource requirement is usually lower.
Also have a look at this question - some discussion on anomaly based (statistical) IDS.
Comment: There is (sort of) a third kind of IDS, application-based. I dont personally have much experience with this, but see e.g. OWASP's AppSensor project.
Comment: Yeah - was wondering whether to have that in the list but erred on the side of counting it as the normal app logging and alerting. You are right though - it is still intrusion detection
Comment: Weeellll... *sort of*...
Answer: Wikipedia has a good start on an answer to this question.
An excerpt:
All Intrusion Detection Systems use one of two detection techniques:

Statistical anomaly-based IDS:
A statistical anomaly-based IDS
  establishes a performance baseline
  based on normal network traffic
  evaluations. It will then sample
  current network traffic activity to
  this baseline in order to detect
  whether or not it is within baseline
  parameters. If the sampled traffic is
  outside baseline parameters, an alarm
  will be triggered.
Signature-based IDS:
Network traffic is examined for
  preconfigured and predetermined attack
  patterns known as signatures. Many
  attacks today have distinct
  signatures. In good security practice,
  a collection of these signatures must
  be constantly updated to mitigate
  emerging threats.


Around how much difference between two classes can be accounted as an imbalanced dataset?

Question: I am building a binary classification model which has 17K values as class A and 10K values as class B. I want to know when a dataset can face the issues of "IMBALANCED Dataset" ?
Answer: In general there will be not a hard rule about this, but this dataset seems to be like balanced.
The point about disbalanced is that you have to keep in mind that the accuracy of your model will have a different starting point. For this python has the function to run a baseline with dummy classification. https://scikit-learn.org/stable/modules/generated/sklearn.dummy.DummyClassifier.html. This gives you a good indication what is your baseline (if you choose the the strategy to most_frequent). In your case, any prediction model which always forecast class A, would have a accuracy of: 17/(10+17) = 0,6296... , so any binary classification with a score under this value would be really bad
Comment: Okay, makes more sense now!
Answer: There is no strict definition about when to call a dataset "imbalanced", but generally speaking it's when the imbalance is likely to cause a problem with the model. Typically the problem is that the model will use the majority class as a default, because assigning the majority class is far less likely to be an error: for example if the data consists of 99% class A and 1% class B, a model which always predicts A will achieve 99% accuracy.
Your case would not be called imbalanced: the minority class makes up a large proportion of the data so it's very unlikely that the model would ignore it.
Comment: Thank you for the explanation.


What did the Howler to Aunt Petunia mean in Order of the Phoenix?

Question: What does "Remember my last, Petunia" mean in Harry Potter and the Order of the Phoenix?
From what I gather, most people seem to think it refers to the letter Dumbledore left for Aunt Petunia when he left Harry at the Dursleys' doorstep -- but is it?
Aunt Petunia seems to act as if she was compelled to keep Harry at her home after listening to the Howler, but what would compel her to keep him there?
Uncle Vernon had, I suppose, made a logical decision to throw Harry out after he found out that a dangerous criminal was looking for Harry, and Harry certainly couldn't call it his home if the owner did not let him live there, which was the sole cause of the existence of the protection. Could it be that the charms that protected Harry from Voldemort at the home of his relatives was also protecting his relatives as well?
But then again, did Voldemort know about the Dursleys at all? He probably would have tried to find out where Harry went when he was away from Hogwarts, and the Ministry certainly knew where Harry was and with whom. Even if we believe Voldemort and any of his crew didn't know about the Dursleys prior to the Order of the Phoenix, Lucius Malfoy did have good connections and a certain level of control and influence over at the Ministry. Wouldn't it have been possible for him to find out about Harry's relatives after the Ministry decided to go against Dumbledore at the end of Goblet of Fire?
In Deathly Hallows, Harry states that the Dursleys should go into hiding because Voldemort might try to take them hostage to make Harry come and save them, so the Order itself takes it upon themselves to ensure the Dursleys' protection.
Even if we assume that all of the residents couldn't be harmed inside No. 4 Privet Drive (and not just Harry) by Voldemort, wouldn't it be easy for him to, say, attack uncle Vernon at the office or Dudley at school? They certainly knew about the general location of the house, wouldn't they know exactly who the residents were and where they worked?
Comment: There's some good questions in here, but it might help to break some of them out into separate questions (i.e. "What did the message to Petunia mean?", "Why didn't Voldemort try to get at the Dursley's outside of their house?", "When did the Death Eaters learn about the Dursleys/No. 4 Privet Drive?", etc.).  It'll help focus the answers, and perhaps give you more opportunities for reputation :)
Comment: All of your questions are great ones, but there's waaaaaaaaay too many of them for one post. I count six different questions in your post on different topics. I love *Harry Potter* questions, so I'm glad you're participating -- for the future, you may want to consider just one question/topic per post. Just some food for thought :)
Answer: Part of this question is answered in an FAQ from J.K. Rowling's website:

What did Dumbledore's Howler to Aunt Petunia mean? ("Remember my last"?)
So: Dumbledore is referring to his last letter, which means, of course, the letter he left upon the Dursleys' doorstep when Harry was one year old. But why then (you may well ask) did he not just say "remember my letter?" Why did he say my last letter? Why, obviously because there were letters before that…

I doubt the protection extended to his relatives, but the charms Dumbledore placed on the house to extend Harry's protection in Philosopher's Stone probably did.
I don't think we know what was in Dumbledore's previous letter to Petunia, but it must have convinced her to take Harry. I can't find a reference right now, but I think Petunia makes reference to Voldemort at some point; clearly she had some idea of how dangerous he was, and the risk Harry would be in (and perhaps even Harry's importance to the anti-Voldemort fight).
Petunia probably realised that casting out Harry would put him in huge danger, even if she could't explain why to Vernon. I don't think Vernon would appreciate the magnitude of the threat posed by Voldemort.
Comment: *"I thought," said Dumbledore, inclining his head slightly, "that she might need reminding of the pact she had sealed by taking you. I suspected the Dementor attack might have awoken her to the dangers of having you as a surrogate son."* OotP, The lost prophecy
Comment: Thanks! I hadn't seen that FAQ answer before. I am in the middle of rereading the books, and was wondering this again. Also, Petunia explains to Vernon what Dementors are, nothing about Voldemort in the beginning of Order of the Phoenix.
Comment: @krillgar There is. It's clear from the way she says "Back?" and is visibly terrified afterwards that, to quote, "he was not the only person in the room who had an inkling of what Lord Voldemort being back might mean".
Comment: I would give a fuller answer, but I need to go out, and there are also really too many questions to be answered at once.
Comment: Speculating, but Petunia was envious of her sister / *had a link with her*. That's a reason why she'd be so worried by her murderer returning and going after Harry.
Answer: It seems that Arthur Weasley and Kingsley Shacklebolt (and probably others in the Order of the Phoenix) believed that Voldemort would be able to get at Harry when he turns seventeen and the protective charm that Dumbledore put on the house would break.
In the Deathly Hallows (page 33 of Scholastic paperback), Harry is telling the Dursleys:

Once I'm seventeen, the protective charm that keeps me safe will break, and that exposes you as well as me.  The Order is sure Voldemort will target you…

Also, earlier, Snape is discussing with Voldemort and the Death Eaters about the plans to move Harry.  While Snape doesn't mention the Dursleys or Privet Drive by name, he does imply that he (or Voldemort already) knows the location.  Snape tells Voldemort (Deathly Hallows, page 3 of the Scholastic version):

My Lord, the Order of the Phoenix intends to move Harry Potter from his current place of safety  on Saturday next, at nightfall (emphasis mine)

It is unclear how much Voldemort and the Death Eaters knew about the Dursleys themselves.  But obviously they knew Harry was at Privet drive.  When Harry and the others left, the Death Eaters were waiting.  As Harry told Mrs. Weasley (Deathly Hallows, page 68 of the Scholastic version):

"The Death Eaters were waiting for us," Harry told her.  "We were surrounded the moment we took off…"
Comment: that still doesn't explain why Voldemort wouldn't attack them outside the house during the period of the Order of the Phoenix or the Half Blood Prince
Comment: No, it only addresses if Voldemort and the Death Eaters knew the Dursley's location
Comment: Petunia had written to Dumbledore when Lily was admitted to Hogwarts. So Dumbledore was aware of Petunia.
Comment: they certainly didn't know the location, but only the general area
Comment: I agree.  Not nowing the exact house is a result of the charms, I believe.
Comment: For his advantage. Look Voldemort can attack on the ground but in the sky its too dark so the o of p can't see them. Plus Voldemort was the one who would kill Harry, so on the ground there would obviously be someone protecting Harry. Anyways I also ask the question What does "Remember my last, Petunia" mean in Harry Potter and the Order of the Phoenix?
From what I gather, most people seem to think it refers to the letter Dumbledore left for Aunt Petunia when he left Harry at the Dursleys' doorstep.
Answer: Well for your first question about why didn't petunia throw Harry out I believe it has two parts for answers:

One is the letter Dumbledore left with Harry when he left Harry on their doorstep. The letter explained how Harry's parents had died and that if Harry wasn't given a home he would be in extreme danger his entire life. It further explained that their home was the only place that could be made safe. In the letter Dumbledore tells Petunia to remember that if she kicks Harry out that he will be most likely murdered; she has enough love in her heart still for her sister and for Harry that she couldn't let that happen (mostly because I personally think that she felt guilty for distancing herself from Lily - they were super close until Lily got her letter and Petunia didn't get one even though she wrote to the school asking to be let in.)
Second is the charm your talking about, which Dumbledore bound to the house where Harry's mothers blood flowed and called home. It would be the place where Petunia called home and lived. While they each called the place home they were protected. Which is why in the book, when they decided to break the protection spell early, both the Dursleys and Harry had to leave with the intent of never returning to that home - so the spell would break and the Dursleys would be moderately "safer."

For the question about wether or not Voldemort knew where Harry's aunt lived and why he just didn't attack them when they went to work: Voldemort had a general idea of where the house was but because of the spell cast by Dumbledore, using Harry's mothers sacrifice, it sort of produced a fidelius charm,

"An immensely complex spell involving the magical concealment of a
  secret inside a single, living soul. The information is hidden inside
  the chosen person, or Secret Keeper, and is henceforth impossible to
  find — unless, of course, the Secret Keeper chooses to divulge it. As
  long as the Secret Keeper refused to speak"

Which is the same spell Harry's parents used to hide from Voldemort. However, the trusted Peter Petigrew was Voldemort's supporter, so it failed. I believe Dumbledore did a similar spell that duplicated the effect of the fidelius charm but used Harry's mothers sacrifice-protection and extended it to cover anyone who shared her blood.
So Voldemort knew the general idea, but could not find the specific house - or harm them when they were within its protective barriers. The range of the protection was, I believe, a 5+ mile radius around the house (the Dursleys had to drive 15 mins before the broke free of it).
Also I think that although the order thought Voldemort would take the Dursleys - I don't really think he would of bothered. If he finds anyone who isn't a pureblood wizard to be unworthy of his notice, then a muggle probably wouldn't be worth his time.
Since he doesn't understand love, which is why he couldn't possess Harry, he wouldn't really comprehend using someone's love for another to compel them to do something. Dumbledore did say that the closest Voldemort has come to feeling love his in his closeness to the snake Nagini. But that could just be because Nagini is a horcrux, so caring about what happens to yourself isn't really close to loving others at all.
Comment: "..he wouldn't really comprehend using someone's love for another to compel them to do something." -  he lured Harry into the Ministry using images of Sirius being tortured as bait; he knew Harry would reach the Ministry somehow
Comment: couple of things: "Also I think that although the order thought Voldemort would take the Dursleys - I don't really think he would of bothered" - they attacked muggles even just for fun, and attacking the Dursleys actually had a purpose, of luring Harry towards them
Answer: I kind of wonder if "Remember my last" wasn't a reminder of the last time that Dumbledore personally wrote to Petunia. The letter left with baby Harry would surely have been addressed to the Dursleys and not just Petunia. However, there was a letter Dumbledore wrote to her, when she was a child and had wanted so badly to join her sister as a student at Hogwarts. Perhaps he was referring to the fact that he once was compassionate and kind to her, even in rejecting her admission to Hogwarts, or even that she appreciated magic and the wizarding world at one time.
I think maybe he was asking her to recall those memories and to extend those same decencies to Harry when Vernon wanted to wash his hands of him.
I also think there is some credence to her appreciating how dangerous and deadly Lord Voldemort was, and that she knew the entire family, including but not limited to Harry, would be endangered in turning Harry out of her home.
That being said, if we go with my theory that it could be Dumbledore referring to the rejection of admission letter, it could be a veiled threat of exposing that Petunia had once wanted to go to Hogwarts as well, which would certainly be enough motivation for her to refuse to kick Harry out. I mean, we know how the Dursleys feel about magic and "those kind" of people. How would Vernon react to his own wife having once coveted her sister's abilities? It would definitely explain why she didn't want to explain what the letter meant it discuss it further at all when asked to... Thoughts?
Comment: Interesting points, however, if you have a new question, feel free to ask a new one using the [Ask Question](http://scifi.stackexchange.com/questions/ask) button at the top right.
Comment: great point. Petunia would definitely not want dursley to know of her connection. Though personally the first time i read it, i thought it was more like a warning. So i thought maybe this wasn't the first time the discussion of keeping harry came up in the house. Maybe petunia was always calming down Dursley without drawing attention to herself. But of course we know later on that there Were other letters before this one.....
Answer: I thought somehow Lily Potter sent the Howler to her sister. Harry wouldn't know his mother's voice. Maybe "remember my last" meant remember my last dying wish?  Which would be to protect Harry.
Comment: Well, this isn't really the answer, though. Dumbledore sent the Howler. It's mentioned later in the book, as well as in some of the other answers.
Comment: Welcome to the site :) Lily's dead, which tends to be a bit of an impediment to sending and receiving post, so you should try and back this up with some evidence from the books. It's not your fault, you're a little new here, try taking [the tour](http://scifi.stackexchange.com/tour), but unlike some forums and stuff, we take our scifi pretty seriously here. We're a Q&A site and the A's are usually expected to have some supporting evidence. And unfortunately, the evidence from the book is quite plain that it was Dumbledore
Answer: I think it must be Snape in the howler. I mean, he really cares about Harry's safety. It must be a dreadful man's voice like it says in the book. Mad Eye Moody? Not Dumbledore! It can't be Sirius either. Snape knew Aunt Petunia from his childhood (Lily Potter) so it's more likely its him.
Comment: What makes you think it isn't Dumbledore?
Answer: It can't be dumbledore who sent the howler because Harry asks himself who's terrible voice it could be.. Since Harry has known dumbledore for 4 years now he had to recognize his voice, no matter how much more 'terrible' than normal.
Answer: I think Dumbedore did something, like making sure Petunia can't have more children for example, in case she gave Harry to an orphanage or did something Dumbeldore did not want.
So he means his last action. "Remember my last" Its a clear threat formulated in a way Dumbeldore would say it, describing / hinting at : Remember what I did last time. YOU WILL OBEY!
Comment: Do you have evidence that Dumbledore may have done such a thing?
Answer: I have no theories about this, but Snape doesn't care about Harry (J.K. Rowling has said it herself). So, it clearly wasn't him.
Comment: "JK said it herself" <-- Where did she say this?
Comment: conversation bet snape & dumbledore in 7th part:{S>so you let him live all this time only to be slaughtered like a pig at the right time? D>dont tell me you finally grew fond of the boy S>The boy?...[patronus charm same as Lily] D>all this time? S>Always.}Basically he doesn't have any love for harry but only as a human. Snape is not a monster like Voldy! he does have humanity. Also, however much he looks like James, he knew he can never harm someone with Lily's blood. All for Lily. I hope people wont down vote me just for saying Snape doesn't really care for harry more than any other student.


pages accessible only by specific groups

Question: I have made those pages on my Drupal 6: Alpha, Bravo, charlie, Delta. At time of writing, those pages are visible by anybody.
I need that those pages are visible only for limited groups, for example:
Alpha is visible only for group One and Two
Bravo is visible only for group Two and Three
Charlie is visible only for group Two and Three
Delta is visible only for group Three.
I have other nodes and other groups on the Drupal installation that needs to stay untouched, I need to change permissions only for those four pages, and of course their menu links need to reflect that (so each group will see only the menu links that they have granted access)
How can I do that?
Answer: Try the Nodeaccess module, here is a quote from its project page:

... is a Drupal access control module which provides view, edit and delete access to nodes. Users with the 'grant node permissions' permission will have a grant tab on node pages which allows them to grant access to that node by user or role. Administrators can set default access controls per content type, and also define which roles are available to grant permissions to on the node grants tab.


Can I use LINQ to lists in sharepoint online?

Question: Can I use LINQ with lists in sharepoint online? If so, can someone provide me with an example?
Answer: Yes, you can use it in SharePoint online

SharePoint 2010 includes the LINQ to SharePoint provider, which
  translates LINQ statements to CAML queries. LINQ is a standard query
  language and, when applied to SharePoint, it enables developers to
  query SharePoint lists without requiring them to learn the CAML syntax
  that is used internally by SharePoint.

Source:
http://www.microsoft.com/download/en/details.aspx?id=17069
Comment: Yes, I've read the developer guide. I also tried spmetal, but as expected it did not work. I can't figure out how to write the query. Can you provide me with an example?
Comment: Here is an example from the ms forums: http://social.technet.microsoft.com/Forums/en-US/sharepoint2010programming/thread/35c5316f-379f-424c-b03a-0e09b94a4ad5/
Comment: Thanks a lot, to the both of you. So far, I have tried the following: Create a site locally in my sharepoint 2010 developement environment. Use SPMetal to generate the entities from the lists. Export the lists and the webpart to my sharepoint online site.
I  was able to get it up and running, but I had to change something in my javascript. After deploying the webpart for the second time, I got the errormessage about running in a sandboxed solution
Comment: Could you post the error log/message in your above post?
Comment: FreeStyler, there's not alot of details in the errormessage, I forgot it and have been unable to reproduce. Anyway, I managed to run my webpart now. As mentioned, I used SPMetal to generate entities from the list in my local sharepoint developement environment. I then created the same lists in sharepoint online in a new site, and added the visual webpart solution to sharepoint online. I was hoping I could just export my local site as a solution (with webpart included), and import it into my sharepoint online site. But all attempts have failed, giving me a varierity of errormessages.
Answer: Yes, We can use LINQ in SharePoint Online. I used it in a Visual web part which stores the information from the form to the list. Here are the steps to follow:

Create a ListContext File with the help of SPMetal
Now use the ListCotext file as the entity layer for the data saving and modification functionality


DDD why create interface on domain service?

Question: I am using DDD for 3 of my applications that use the same domain layer (domain services) via 3 different application (workflow) services. Infrastructure & UI layer don't have access of domain services. I'm also passing all external dependencies like repository via domain service constructor. So, do I need to create interface on domain services? Note that I have defined interface for application services.
Comment: You are right. Actually, I was thinking this because my own object factory class had a limitation of auto detecting the object constructor's parameter types and couldn't resolve it from registered dependencies. I replaced factory class with StructureMap and now everything is fine. :-) This has changed the way I was thinking about DI.
Comment: An interface might help you with more granular testing, but if you don't need it, don't create it.
Comment: @JefClaes, well that's the thing. I don't think (may be not realizing) domain service interface would help for even granular testing as the implementation of domain service would be concrete. In other words, the domain service business logic should not be swapped. Not only that but the only reason I see to create interface on application services is to mock it while creating unit test on controller (if we do!!). I would be much appreciate if anyone give me a reason to create interface for service.
Comment: I think you've covered the 2 main reasons, if you code to an interface and use DI, you can swap the class out for another and reduce the dependencies and it makes it possible/easier to unit test.
Comment: @ParthPatel Why are you looking for a reason? You can always extract an interface once you see a need for it.
Comment: @JefClaes, is there anyway I could mark your comment as an answer?
Comment: @ParthPatel I answered your question below as well.
Answer: Why are you looking for a reason? You can always extract an interface once you see a need for it.


What is a proper way to send information from an HC-05 (Bluetooth module) to a phone?

Question: I have 12 simple touch sensors and each of them produces an analog output. I want to send that information to a phone (or some other device). To do that I would need to use the HC-05 Bluetooth module. I don't want to use an Arduino. I looked up some  alternatives and I can program an ATtiny85 chip (using the Arduino) to somehow function with the HC-05 and from there send the information to the phone. I don't know if that's the right way to do it. Also, I have 12 analog states that I want to send and only 6 ATtiny85 pins. The project is small so I don't want to use any big components or chips.
Comment: if you have an Arduino, then use it to develop the code
Comment: Sure but my concern is how to fit 12 analog states into the ATtiny85 and is my whole setup even right?
Comment: There are other Bluetooth modules available that have native analog inputs, are fully programmable, and the same size or smaller than the HC-05.  *Depending on your budget* perhaps you should reconsider your choice of Bluetooth module.  You would then no longer need the ATtiny at all.
Answer: The usual way of doing this is to use a microcontroller to interpret the inputs and then send your information to the HC-05 module via UART. You need to make the communication protocol yourself, which is probably a good thing. Since you want 12 analog inputs you might be facing some difficulty finding a small microcontroller with that amount.
My suggestion for this task would be to use an analog to digital converter chip (ADC) for the inputs and send this to the microcontroller, which interprets the information and codes it according to your communication protocol and then sends it to the HC-05. An 8-channel ADC can be seen below. Such an ADC connected to the ATtiny85 communicates with SPI or I2C, which are bus protocols needing few wires.

Alternatively, you can use something like a Teensy 3.6 as it already has enough analog inputs. Of course it's bigger and way more powerful than what you need for your purpose. This will be up to you and how much convenience you want in your project. Teensy 3.6 pictured below:

If you want real good convenience Adafruit designs a series called Feathers. These are Arduino compatible microcontrollers with built in Bluetooth functionality that are easy to use. The Feather M0 features 10 ADC channels, which is almost what you want. By searching their range of products, you might be lucky to find a 12-channel one or you could simply use the Feather M0 and a small 2-channel ADC so you have 12 channels.

https://www.adafruit.com/product/2995
Comment: Thank you. I will try the first method but I'm looking at the MCP3008-I/P ADC and it has only 8 analog inputs, and I need 12.
Comment: Great. You can use two of the 8-bit ADCs of course, or find another 4-channel ADC.


Configure an IP address instead of localhost:8080 tomcat eg:192.260.0.2:8080

Question: Can anyone suggest me how to configure my localhost:8080 in tomcat to connecting to an IP address instead of localhost such that i access my localhost in any system in my workplace... i am using tomcat6 and netbeans 6.9.1
Comment: For the record, 192.260.0.2 is an invalid IP.
Comment: Alex i meant similar to it.... i know its a invalid IP
Comment: Which operating system ?
Answer: Your question is ambiguous dude.
If you are asking how you can make the hostname "localhost" resolve to something other than 127.0.0.1 then the answer is:

change the localhost entry in your "/etc/hosts" file, and mess with the network configs, and
DON'T DO IT because you are liable to break things.

If you are asking how to get Tomcat to accept requests on some other IP address, then the answer is to edit the $CATALINA_HOME/conf/server.xml to add or change the relevant <Connector> element.  Of course, you can only listen on the configured IP address or addresses of your computer's network interface / interfaces.
Answer: Tomcat by default binds to all IP addresses (0.0.0.0)
Check your inbound firewall rules next.  8080 or whatever port you're using may be blocked.
Comment: for Tomcat 7, there is an entry `` that can be extended with another `` entry pointing to your desired IP/external hostname. Then both localhost and the external name will work.
Answer: This is the general method for nearly any (non-microsoft) service:

Go through your configuration files for the service in question, and locate any references to either "localhost" or "127.0.0.1".
Ponder for a moment about the wisdom of giving anybody unrestricted access to the service.
Change the reference to reflect your external ip address or dns name.

You might need to change network routes on any gateways or routers that may exist between you and "everyone else".
Comment: @Arafangion - in this case, he probably needs to do it differently than this.  While he says "instead of", he probably doesn't *really* want to stop his Tomcat from listening on 127.0.0.1.  (Why would you?)
Answer: You just need to use the IP of the machine, e.g. your dev machine is 10.0.0.100, then in the other computer type 10.0.0.100:8080, remember to check firewall rules
Comment: That will only work if the service is listening on that IP address.
Answer: So if I understand you, you want to remotely access your Tomcat which runs locally on port 8080?
You need to know the IP address of your machine first;
windows:
- open a dos box
- type:
ipconfig /all
linux:
- go to shell
- type
if config
Your local IP address is listed.
Now on your remote (at work) machine's browser enter the following URL:
http://the-IP-address-you-found:8080
If there are no firewalls blocking or other network problems you will see your tomcat page.
Answer: Since It is too late to respond, but today I faced same problem and resolved by allowing 8080 port in firewall rule.
sudo iptables -A INPUT -p tcp --dport 8080 -j ACCEPT
Note- In my case was able to access my server by url localhost:8080 but unable to access by url like 172.16.16.10:8080 from local as well as from remote machine.


What will be the effect on a rigid body if two unlike unequal parallel forces act on the body?

Question:
What will be the effect on a rigid body if two unlike unequal parallel forces act on the body? Will the body translate or rotate or both? If they rotate, with respect too which point will they rotate?
Comment: Related question - https://physics.stackexchange.com/q/95234/37364
Comment: What are your ideas? You can change this into a forces at the centre of mass (which cause translational acceleration of the centre of mass)  and a couples (which cause angular acceleration)  problem by adding forces acting at the centre of mass as shown here  https://physics.stackexchange.com/a/285167/104696
Comment: To provide complete information, the location of the forces relative to the center of mass in quite important.
Answer: Answer: both.
Now, to determine the point about which they rotate, calculate the linear acceleration (of center of mass) and the angular acceleration (using the torque equation about the center of mass). Then, the acceleration of any point on the body would be the vector sum of linear acceleration and distance times the angular acceleration. The required point is the one for which this vector sum vanishes.
Comment: //Then, the acceleration of any point on the body would be the vector sum of linear acceleration and distance times the angular acceleration. The required point is the one for which this vector sum vanishes.//  From which point should I calculate the distance?
Comment: The center of mass.
Comment: Maybe try H C Verma
Comment: Thanks, Abhijeet. Can you please, help me here?: https://physics.stackexchange.com/questions/331674/whats-the-significance-of-the-point-at-which-the-resultant-of-two-unlike-unequa
Answer: The same laws of motion apply here too. Get the net force to see how the center of mass moves and the net torque about the center of mass to see how the body rotates.
So unless the net torque is zero about the center of mass, the body is going to translate and rotate at the same time.
Consider the general case of two unequal opposite forces $F_1$ and $F_2$ each at a distance $d_1$ and $d_2$ from the center of mass at opposite sides. The force act along the y axis and we track the vertical acceleration of the center of mass $\ddot{y}$, as well as the rotational acceleration $\ddot{\theta}$. Finally we find the center of rotation along the x-axis relative to the center of mass as $ r= - \frac{\ddot{y}}{\ddot{\theta}} $.

Linear Motion

$$ (F_1)  + (-F_2) = m \ddot{y} \;\Rightarrow\;  \ddot{y} = \frac{F_1-F_2}{m} $$

Rotational Motion

$$ (d_1) (F_1) + (-d_2) (-F_2) = I_C \ddot{\theta} \; \Rightarrow \; \ddot{\theta} = \frac{d_1 F_1 + d_2 F_2}{I_C} $$
Where $m$ is the mass and $I_C = m \kappa^2$ is the mass moment of inertia about the center of mass, and $\kappa$ the radius of gyration.

Center of Rotation

$$ r = - \frac{\ddot{y}}{\ddot{\theta}} = -\frac{ (F_1-F_2) I_C }{ (d_1 F_1 + d_2 F_2) m} = -\frac{(F_1-F_2) \kappa^2}{d_1 F_1 + d_2 F_2} $$
Now lets consider the fringe cases:

Rotation about the center of mass, $r=0$. This happens only when the two forces are equal in magnitude $F_1 = F_2$.
Pure translation, $r = \infty$. This happens only when the net torque about the center of mass is zero $d_1 F_1 + d_2 F_2 =0$.

In any other case you have simultaneous translation and rotation about the center of rotation.

There is a shortcut to finding the center of rotation. Consider $d$ the distance where the combined forces act through. Essentially $\sum M = d \sum F$, or $$ d = \frac{d_1 F_1+d_2 F_2}{F_1-F_2} $$
Now consider the mass moment of inertia $I_C$, and the radius of gyration $\kappa$. They are related by $I_C = m \kappa^2$.
The center of rotation is found by
$$ r = -\frac{I_C}{m d} = -\frac{\kappa^2}{d} $$
The negative sign here means that $r$ is located on the opposite side of $d$. This means that the center of rotation is purely a geometric quantity as only relates to distances and the radius of gyration. All of these quantities are a function of the shape of the body only.
Stated backwards, if the center of mass is a distance $r$ from the pivot point (center of rotation) then the axis of percussion (axis where momentum needs to be applied to stop the motion) is at a distance $ d = \frac{\kappa^2}{r} $ from the center of mass in the opposite side.
That is why a baseball bat (approx. cylinder of length $\ell$ with center of mass at $r=\frac{\ell}{2}$) has its axis of percussion (sweet spot) at a distance $d$ from the handle of
$$ d = r + \frac{I_C}{m\,r} = \frac{\ell}{2} + \frac{\ell}{6} = \frac{2 \ell}{3} $$
Answer: This answer applies equally to your related question in which the two forces are parallel and unequal but act in the same direction.
The pair of forces can be replaced by a resultant force $F$ and a torque $T$. The resultant motion is a combination of :

acceleration of the centre of mass in a straight line, according to $F=ma$, where $m$ is mass and $a$ is linear acceleration; and
rotation about the centre of mass according to $T=I\alpha$ where $I$ is moment of inertia about the CM and $\alpha$ is angular acceleration.

In the anti-parallel case shown here, in which the forces point in opposite directions, the resultant force and anti-clockwise torque are given by $$F=F_1-F_2$$ $$T=F_1 x_1 + F_2 x_2$$ assuming $F_1 \ge F_2$. For the parallel case replace $F_2$ by $-F_2$.


What is a body strength?

Question: I want to know what is the body strength? Is that muscle building help for make a body strength?
Answer: Strength is dictated by the ability of your central nervous system to use as much muscle fibers as possible.
Having big muscles doesn't make a person stronger, having big muscles makes the person slightly more resistant to strength training, thus making it easier to build strenght.
Big muscles require more effort to reach failure, having big muscles allows you to use heavy weights for more time so your nervous system receives a bigger stimulation.
Comment: Can you explain it in more deeply?

What is a body strength?


Converting AE animations to SVG with bodymovin

Question: I have been trying to export an After Effects animation of an aurora borealis (made with Flux) as an SVG for a website, but every time I export it looks a bit low res and takes up to much space to be applied to a web page. I tried to render it first as a PNG sequence and the render it with bodymovin but the problem still persists. Can anyone help pls?
Answer: If you have no alpha-channel and would like to run the animation on your website (as a background for example), I would highly recommend you encode it in h264 .mp4 format, with about 3-6 MBit/s and 2 passes VBR to lower the filesize further. Also take a look at your framerate, as you could get away with 20 frames for a slow animation. I wouldn't go above 30fps. I hope I could help you, if not, please comment on my post.
Answer: Bodymovin works best for things like shape and text layers, that can be described by the SVG language. SVG is a bit like html, it contains instructions on how to render the content, rather than the actual images. For example if you wanted to draw a circle 256px wide, rather than create a raster image of a circle, and provide the colour info for each pixel in the 256x256 pixel grid (over 65 thousand pixels, each with red, green and blue value) you can make an svg that does the same thing thus:
 <svg height="100" width="100">
     <circle cx="256" cy="256" r="40" stroke="black" stroke-width="3" fill="red" />
 </svg>

It's also scaleable. If you resize it up to 500px there will be no quality loss, because it's rendered on the client machine.
However, if you have content that can't be described in the SVG language, such as raster (pixel grid) images such as jpegs or pngs, or video, it can't just describe it, it has to embed the raster into the output, meaning that SVG will not be any more efficient than just using an  or <video> element.
PNG sequences are great as a lossless intermediate for transferring video on a local machine with lots of storage, they completely fail as a web codec. Web codecs are highly efficient at reducing file size with minimal perceptible quality loss, so they're what you need for the flux output.
TL;DR as Florian says: use h.264 encoded video.


Old story about slow light

Question: I remember reading this story before I was in high school, so it must have been written before 1972. It was about a house having windows made of a special kind of glass with such a high index of refraction that it took on the order of a century for light to pass through it. Visitors to the house "witness" a murder that took place many years ago by looking in or out the windows. That's all I can remember, but I was struck by the idea and would like to track down the exact reference. The title or text may have contained a reference to "slow light".
Comment: What a fantastic story idea.  I'm going to buy the book.
Comment: Terry Pratchett's discworld also famously features slow light. However, it's faster than in the story you are looking for.
Answer: It could be Light of Other Days by Bob Shaw (1966)
Plot

The narrator is touring northwest Scotland with his wife Selina. Their
relationship is in trouble, and news of her pregnancy has made it
worse. Their holiday, intended to improve the situation, has not been
a success. Travelling through a remote area, they find a place that
sells panes of slow glass. This is glass that light takes a long time
to pass through, even years, so that a pane of this glass shows a
scene from the past. People buy slow glass that has been placed in
picturesque scenery so that later they can enjoy the view in their
homes or workplaces. The best quality slow glass is priced by its
"thickness", corresponding to the number of years of scenic view
contained within it. The narrator thinks that an extravagant idea like
buying slow glass might set right his relationship with Selina.
Leaving the car, they walk along a path, where they see panes of slow
glass facing a view of a loch. They meet Mr Hagan, who is sitting on a
low wall in front of his stone farmhouse and looking towards the
house. Inside, through the window, they see a young woman, presumably
Mrs Hagan, and a small boy. Hagan fetches a rug from the house so that
Selina can sit on a wall. The narrator, sensing that Mrs Hagan,
looking towards them from inside, is not aware of them, wonders if she
is blind; Selina remarks that her dress is out of fashion.
The narrator discusses with Hagan the price of his slow glass. Hagan
says how fine the view is from there; as he talks, the narrator, still
looking at the house, wonders if the small boy is blind as well.
Eventually, although Selina is not impressed, the narrator decides to
buy. While Hagan is fetching the glass, rain begins, so Selina goes to
the house to return the rug. When the door is opened, the interior is
seen to be disordered and squalid; Hagan is living alone. The family
scene of wife and child was of years ago, seen through slow glass. As
the couple leaves with their purchase, Hagan sorrowfully tells them
his wife and child were killed in a road accident six years
previously.
The narrator, walking with his wife back along the path, feels their
relationship is somehow strengthened. He looks back at Hagan, who is
sitting at his usual place, looking towards the house.

Full story
Comment: Other Days, Other Eyes is actually a fix-up consisting of all the slow glass stories and other material. Chapter 2 is "Light of Other Days".
Comment: It's been asked about a few times here. Here's one of them https://scifi.stackexchange.com/questions/258445/novel-with-replaceable-eyes-that-store-memory
Comment: @OrganicMarble seems that TS asked for a story, and "Other days, other eyes" is a novel, based on this story.
Answer: It's definitely one of Bob Shaw"s "slow glass" stories and I suspect the specific story you're after is "Burden of Proof" (first published in Analog in May 1967), but I don't have any sources to hand with which to verify that.
Comment: "Burden of Proof" is about a judge waiting for confirmation of a verdict by something that finally shows up in a slow glass window. It's in *Other Days, Other Eyes* chapter 4.
Comment: It could well be:
"Ever since slow glass appeared in several fashionable stores, people have wondered: what would happen if a crime was committed in front of it?"
See full text: https://koapp-narod-ru.translate.goog/hudlit/sfiction/book566.htm?_x_tr_sch=http&_x_tr_sl=ru&_x_tr_tl=en&_x_tr_hl=uk&_x_tr_pto=wapp

Old story about slow light


How does one tell if your paying a ETC or ETH address?

Question: How can you tell if your paying an ETC or ETH address? Which wallets can check for you? It's scary if I have both eth and etc wallets and when I make a payment it deducts from both.
Answer: Your ETH wallet, is also an ETC wallet. A transaction for the ETH chain can be replayed on the ETC chain, and vice-versa.
See here to protect yourself https://blog.ethereum.org/2016/07/26/onward_from_the_hard_fork/


Ubuntu 9.10 will not boot

Question: After an update yesterday my Dell X300 will only boot to a screen with an option to perform 2 memory tests. If I perform these tests, it will only go back to this same screen, offering a choice of memory test again, 86+ or 86+ serial console 115200. This screen is headed - GNU Grub version 1.97 beta 4.
This is a full install on my hard drive.
How can I get back to booting normally?
Answer: Looks like your GRUB is corrupted, check here for a possible solution:
http://ubuntuforums.org/showthread.php?t=24113
Answer: There are two possibilities here:

Grub hasn't recognized your kernel after an update
There's no kernel installed right now

So my suggestion is that you chroot from a Live-CD into your system, reinstall the kernel and update grub.
Comment: there's actually a few other possibilities, mostly of the "OP hasn't given all the details of what he actually did" variety.  for example, this result could happen if he's been monkeying with the grub scripts in */etc/grub.d*.
Comment: @quack quixote: True, my assumption was that he did nothing but an update...

Ubuntu 9.10 will not boot