Found a serious bug in my question after posting it

Question: What is the correct way to go after I found a bug in a question that I posted?
Shall I delete the question, vote to close it or flag it as off-topic?
Comment: Possible duplicate of [Should mostly working but a little bit broken code stay or be deleted?](https://codereview.meta.stackexchange.com/questions/801/should-mostly-working-but-a-little-bit-broken-code-stay-or-be-deleted)
Answer: If the question has been answered, you must not edit it in a way that would invalidate the answers; that is not permitted on Code Review.
If the question is still unanswered, and the bug is easily fixed, then edit (consider temporarily deleting the question if the edit will take more than a few seconds, e.g. due to testing).
If the bug is non-trivial, then I recommend that you write an answer pointing out the bug, just as if you'd spotted it in someone else's code.  If you can, show the inputs that trigger the bug.  In short, you've spotted the bug because you posted it for review (regardless of the fact you're the author), so address it like any other latent bug you find when reviewing.
You shouldn't normally consider it off-topic, provided you weren't aware of the bug when posting the code for review.  Spotting errors is an important part of code review; questions are off-topic only when the author already knows the code is broken, and is asking for help to fix it.  If you didn't know the bug is there, you're not breaking the rules.
Comment: I upvoted because I agree in general, but on reflection I think there is a subtlety about self-answering: if the self-answer gets an upvote it will remove the question from the "Unanswered" list, which from the point of view of the asker is probably undesirable.
Comment: Perhaps it's also a good idea to post a new question incorporating the fix?  The self-answer is often useful to others (perhaps it's the kind of bug to watch out for in other reviews), but a new question incorporating advice is something we always recommend when improvements/fixes have been made.
Comment: To clarify, we still expect some level of due diligence before posting, correct? So if there are obvious syntax errors preventing it from compiling, but they weren't aware (b/c they never tried to compile it), it would still be off topic, yes?
Comment: Fair enough. I suppose it's really a question of where you draw the line for "*non-trivial*".
Comment: @Dannnno Absolutely, although I'm having a little trouble finding the related meta at the moment.
Answer: As others have already said, the correct answer is to delete it or edit it if there are no answers.  (Or delete it, write up the edit, undelete it, and then edit it if the edit would take a long time.)  But I think that it would also help to explain what happens in your other options.
Voting to close is just asking for four more people to vote to close it.  While closing the question under that circumstance isn't horrible, it also means that you'd have to get votes to reopen it.  Meanwhile, you can delete and undelete your question yourself.  Voting to close your own question is not really something that you want to do except if it's a duplicate, which doesn't really apply on this stack.
Flagging just puts it in a queue to check if it should be closed or deleted.  Or sends a message to  a moderator.  You don't need to tell a moderator that the question has the bug.  All they could do is close or delete it.  Since you can delete unanswered questions yourself, you don't need a moderator (or a queue) for that.
Deleting it is the least work for both you and everyone else.  And it puts getting the question back in your control.  Closing or flagging it means that you may have to ask someone else to fix it.  Deleting your own question is something that you can reverse yourself.
If you delete your unanswered question and later fix the bug, you can undelete and edit the question with the new code.  And that's what you should do.  Replace the flawed question with a better one.  Because there is a statistic that tracks the number of deleted, closed, and down-voted questions (and one question can count up to three times) that you have posted.  If you have too many, your account won't be able to post questions.  So the best thing is to delete the question before it gets closed or answered, fix the bug, and then undelete and edit the question.
If it gets closed or deleted, or you don't want to go through the work of fixing it, you don't need to panic.  It's okay to occasionally show up on that statistic.  My main point here is that you shouldn't delete it and then post a fresh question.  Fix the original and get that undeleted or reopened.  Then you can keep that statistic down.
Answer: If you still want your question answered, leave it open.
If you no longer want your question answered (because the bug makes the whole thing moot), then simply delete the question. There's no reason to leave a question hanging around if the author himself doesn't even want an answer.
Any solution involving "voting" or "flagging" is just making work for someone else. (I mean, so is posting on Meta, for that matter.) Leaving the question open is also making work for someone — specifically, for the people who read it and the smaller number of people who write answers. If you don't want that work done, then the way to cancel your request is simple: just delete the question.


vTiger doesn't recognize any user

Question: I having a problem using vTiger.
Actually i had no problems but we had to format one of our servers and as the SQL has been backup'ed i had no worries about vTiger.
Just got vTiger folder backup'ed too, but when i had my server back and put the folder of vTiger into www and restored the database i had a few errors, mostly resolved by re-configuring config.inc.php because paths had changes. But after all the configuration is done again i can't login to my vTiger. He reachs the database but does not recognize any of the users. I alway get username or password is wrong. Have anyone experienced this? Is there any possible solution?
Answer: Had a similar problem a few days ago, it turns out my cookies were corrupt (Firefox). I cleared all related cookies, problem solved. Try first using some other browser to confirm. hth.
Answer: First I would check if the db is reached and populated with your data. Try connecting from the command line.
If you are working on Windows and have changed Vtiger's path, and you are using the bundled version, MySql path changes as well, but the Windows service can still be alive on the old path: in this case you must manually cancel the service and afterwards use the "Start Vtiger" button in order to create the new service.
Second, I would turn on the php debugging as explained here. In most cases, these steps should help you.
Answer: Have you checked user_privileges folder? in that you must have 2 files for particular user. eg. your user admin having id 1 in database then you must have user_privileges/sharing_privileges_1.php and user_privileges/user_privileges_1.php file
and open user_privileges_1.php file and check username/password in that file, if it's not same with your DB then change it manually.
Try this solution and let me know if it works.
Comment: try deleting the user_privileges_1.php file (make a copy first). I suspect the file will be re-created automatically, with the correct contents.

vTiger doesn't recognize any user


Why do insect legs look so different from mammal legs?

Question: Why do insects (and spiders and centipedes) have legs that are angled acutely upward then downward to meet the ground whereas mammals large and small all have straight legs that curve only slightly when walking or running? Why are they so different in morphology when they serve exactly the same function, i.e. load bearing and level attitude keeping of the central torso when in rest and motion? If the larger mammal's leg is evolved to favor large ground clearance for eyes to better spot threat/prey, then how come very small rodents also have straight legs when they should evolve to favor rapid gait because they are so small that they can not stand above grass or most obstacles in nature?
Answer:
Evolution is based on chance.  AFAIK, the ancestors of arthropods and vertebrates diverged before legs really evolved.  From there, whatever bunch of random changes that improve fitness get adopted.  (And not all arthropod legs fit that insect architecture: for instance millipedes.)

You're up against the square/cube law. That is, body mass increases as the cube of size. That means that the larger an animal is, the stronger splayed-out insect-style legs would have to be, and the more muscle power is needed to support & move them.  For example, try doing a pushup with your arms extended out to the sides: it's a LOT harder than the normal arms underneath you one, isn't it?
Comment: Evolution *includes* chance, but it's not *based* on chance.
Comment: Plus one for point 2.


In Kimi no Na Wa., how long does the comet fragment take to reach Itomori Town?

Question: In Kimi no Na Wa, Mitsuha is able to save everyone in the town even as the comet fragments were falling. So I'd like to ask, just how long would the comet fragment take to fall, in a timeframe that still allows everyone to reach the High School before the comet strikes?
Comment: Since it's a happy ending *anime*, the comet will wait for everyone to safely reach the school. Then it'll fall. As for the actual answer to your question, I don't know and would like to know as well..
Comment: Haha! I suppose. Let's see if anyone has an answer other than yours, 'cause it'll be interesting.
Comment: This is bugging me too - in the original timeline, the fragment seems to have impacted Itomori almost immediately (within a few minutes, say) after it split off. But in the revised timeline, it must have been at least an hour between the split (Mitsuha is still running towards city hall at this point) until impact (by which point evacuation would've been complete).
Comment: I agree with @Nightshade, it's an anime, don't think too much! :P
Answer: Personally I think the comet's fragment has gone through a full orbit around the Earth, because it is unlikely that it has diverted so much from the original comet trajectory (which is tangential to Earth) to fall almost perpendicularly, since the break was above Japan and the fragment also fell in Japan

So it is likely that more than an hour has passed between the break and the impact.
By the way, I'm not an aerospace engineer, maybe my reasoning does not make sense ^^
Answer: It reached its perigee at 7:50. At 8:52 it impacted. In the light novel, it was written that everyone stood in awe until it was to late.
Comment: Do you have a screenshot or page number for the source?


Job Offer Switch

Question: I have completed 2 interviews for a position selling furniture. The second interview was with the person doing the hiring and she seemed really positive about me getting the job. Today I was called and asked if I would be OK working in the warehouse/delivery for about 3 days a week and selling for just 2 days. Looked on the net and sure enough they have had an open position for a Delivery driver longer than the sales position. I really do not want to work in the warehouse/driver/delivery job, but I agreed to do an interview with the supervisor of that position tomorrow. How do I handle this.
Comment: do you have a truck licence? If you don't then your job will be muscle loading and unloading
Comment: what do you want to happen? i'm assuming you only want the sales job? do you need the job come what may, or can you keep looking for a different role? do you want to instead do 3 days sales, 2 days delivery? you need to know what your bare minimum is, and then stick to that.
Comment: Are you asking (a) what choice we would recommend you make (which would be off topic), (b) how to decline working in the warehouse (even if this costs you the job) or (c) how to politely indicate that you'd *prefer* not to work in the warehouse (but you'd still take the job if that's required) or something else?
Comment: @paparazzo yeah, still have to do the loading, but at least you get to drive a truck which is always fun. Forklift is pretty cool as well
Comment: In my experience, this means you'd be a full-time warehouse and delivery guy, with vague promises of being in sales as requested.
Answer: You know what this smells like? This smells like the person who needs to hire a salesman has no political power in the company compared to the person who wants to hire a delivery driver.
I would not even be surprised to learn that the hiring person has been yelled at for trying to fill the sales position before the warehouse/delivery position is filled.
You probably don't want to be either a driver or a salesman in that company. You certainly do not want to devote three days a week of your valuable sales department time to driving a truck.
Tell the company that you are a salesman and not a truck driver. There is no skill set overlap between those two jobs.
Comment: @paparazzo: "open position for a Delivery driver" appears in the original question.
Comment: Working in the warehouse/delivery is not truck driver.
Comment: Yeah. Unless equipment is involved, working in the warehoue is BELOW working as truck driver. How comes they od not need sales people ;)

Job Offer Switch


Is there a command or tool to delete unneccessary settings in the home-folder automatically?

Question: I want to get rid of all the settings eg. of wine which aren't deleted when purging it, but don't want to delete them all by hand. Is there any tool, that lets me do this? If not that might be a coding-idea for some one?
Comment: Maybe there should eventually such an info added to the debs from the repository, or the file standard should be extended, as I think many people would like to keep their system clean
Comment: Good question, but I'm afraid that such program does not exist as programs are free to have their own locations for storing user data. If programs have a path to the configurations in their manual page, a script might retrieve these paths. Unfortunately, most GUI applications do not have such a manual page.
Comment: @Lekensteyn Make it an answer and get an upvote! ;)
Answer: There is no such tool I am aware of, but some programs do store the path to the users configuration files in their manual page under the Files section. The next text is retrieved from man chromium-browser:

FILES
   ~/.config/chromium
          Default directory for configuration data.
   ~/.cache/chromium
          Default directory for cache data.  (Why?  See
          <http://standards.freedesktop.org/basedir-spec/latest/> .

If you've just installed an application and want to purge it including any settings, you might want to look for these files based on their modification time. That can be done using find with the -mtime (modification time in days) or -mmin (in minutes) options. For example, if you've started the program within an hour (=60 minutes), you should run:
find ~ -mmin -60 -ls | less

The output will contain the modification date, owner/group (usually your user/group), size and name and will be piped into less so you can use your arrow keys, page up and down keys for navigating.
Finally, you can look at the source (you're using FOSS software, right :) ) if you understand the language. If the program is small, this is doable. But if the program is large, it gets more difficult.
Comment: So anyone who wants to code, but doesn't know what to code: That would be a really cool project, as at the moment quite some space is wasted by old applications and at least I would like to have a clean home folder
Comment: @Toxicbits: the problem is that most programs do not provide such a nice documentation, but hey, it's possible :) Another approach would be indexing all files in the home dir, so you can tell which files has appeared on what time.
Answer: If you want to reset all of your personal settings, you should delete all hidden files in your home directory, with the excpetion of .bash_logout, .bashrc, .profile (these three files are copied from /etc/skel on user creation).
It is not always simple to delete settings for a particular application, because the hidden file or directory it create, usually on first run, can have sometimes a name very cryptic (for example the hidden directory for pidgin used to be .purple).
Apart from pathological cases, a search like
find ~ -iname '*appname*'

should give a good hint of what to delete to reset the application.
EDIT:
As @htorque pointed out in a comment below, in hidden files and dirs there could be also data, as emails under .evolution, browser bookmarks under .mozilla/firefox, and the like.
So, as always, have a backup of your sensible data somewhere.
Comment: @htorque, I edit the answer to include your observation.
Comment: No, I want to keep the settings of all installed applications
Comment: Note: those hidden directories could contain user data as well, it's not all just configuration/settings.
Answer: It is also possible with Ubuntu Tweak


How can I say "We shall want for nothing" in Latin?

Question: Could someone advise how I might translate "We shall want for nothing" into Latin? I'm struggling to find an online translator that isn't gibberish and my own attempt is even worse!
Comment: Welcome to the site! Feel free to include your own attempt, as it can be helpful in developing a translation that captures what you want to express.
Comment: The reason the online translators don't do a good job is that "we shall want for nothing" is not idiomatic modern English. Start by finding an equivalent more colloquial phrase, for example "we'll have everything we need".
Answer: This feels like it's intended as a callback to Psalm 23, which in the KJV starts:

The Lord is my shepherd; I shall not want.

The corresponding line in the Vulgate (where it's Psalm 22):

Dominus regit me, et nihil mihi deerit.

This literally means "[the Lord rules me, and] nothing (nihil) will-be-absent (deerit) for-me (mihi)". Pluralising mihi gives:

Nihil nōbīs dēerit.
Answer: I agree that Psalm 23 is a natural choice, but if you wanted to avoid the religious connection, another common word for this is egere, and if you check under its entry on Lewis and Short (skip the crappy online translators!), you can see some more synonyms:

ĕgĕo , ŭi, 2 (
I.part. fut. egitura, Tert. adv. Marc. 4, 24), v. n. cf. Gr. ἀχήν, poor; root αχ-, αγχ, in ἄχος, ἄγχω, etc.; Lat. angustus, angina, to be needy (for syn. cf.: indigeo, careo, vaco).

Note that egeo most often takes an ablative, though sometimes it can take a genitive and even once noted for taking an accusative in Plautus.
To translate the phrase as you put it gives you

nulla re egebimus

which is a phrase that is attested in Cicero's Paradoxes of the Stoics:

nihil adpetunt, nulla re egent, nihil sibi deesse sentiunt, nihil requirunt.

In fact, besides the fact that they're present and third person, all four of the above phrases work well for your original phrase.

nihil adpetemus, nulla re egebimus, nihil nobis deesse sentiemus, nihil requiremus.

One more possibility is using desiderare in a metaphorical sense:

II. With predominant idea of lacking, wanting, to miss any thing: "ex me audies, quid in oratione tua desiderem," Cic. Rep. 2, 38: "si non est, nolis esse neque desideres," Plaut. Bacch. 4, 8, 73: "quid a peritioribus rei militaris desiderari videbatur," Caes. B. C. 3, 61, 3 et saep.—Esp. with quominus: "praeter quercum Dodonaeam nihil desideramus, quo minus Epirum ipsum possidere videamur," Cic. Att. 2, 4, 5. —

Nihil desideramus is almost exactly what you need, except that it's present tense. Changing it to future gives you:

nihil desiderabimus
Answer:
Nihilō carēbimus.

Carēbimus literally means "we shall lack", or "we shall want" in the sense of "want" in your phrase. Another form of the same verb is caret, meaning "it lacks", which is where we get the English word "caret", the little mark ^ that indicates a place in some text where something is missing.
This verb takes an object in the ablative case, which nicely corresponds to "for nothing" in your sentence. Nihilum means "nothing"; nihilō is its ablative case.
Here are definitions for both words on Wiktionary, with pronunciation and grammar: careō ("I lack") and nihilum.

I am not an expert on Latin, so please wait at least a day before accepting this answer. The words nihil and nihilum have some quirks in Latin, and I may have unwittingly stepped into one. Also, as pithy as this wording is, there might be an even nicer established wording from the Latin literature that expresses the same thought.
Comment: Sure enough, Cairnarvon found a well known sentence in the literature to echo. That might be the better choice, depending on exactly what meaning you want to convey.
Comment: FWIW your translation looks perfectly acceptable to me, and I upvoted it.
Comment: The only quibble I have is that I don't see *nihilo* used as an object of *careo* (did a quick PHI search, so perhaps there's something later?). Even in that Cicero passage I cited, he uses *nulla re* with *egeo* instead of *nihilo* (despite using *nihil* for the other three verbs). For some reason, it just feels off to me to have the ablative of *nihilo* here in this way, but I can't quite say why.
Comment: @cmw I have much less experience with Latin, and I have the same feeling: that _nihilo_ is probably somehow unidiomatic. My intuitions have been wrong before, though (including [just a few days ago](https://latin.stackexchange.com/questions/19328/in-regards-to-est-and-errare-humanum-est#comment40813_19330)).


wsl (ubuntu) to Ganache GUI on windows 10 Network Connection Error

Question: Am using wsl(ubuntu) on windows 10.
Problem is, am trying to deploy.js file with node,  here's the error result --> ("  reason: 'could not detect network',   code: 'NETWORK_ERROR',   event: 'noNetwork'  using  ganache network)
Answer: Activate your Ganache, and navigate to "contract"...scroll-down and click-on "link truffle projects , accept the allow your system to have access to Ganache on the pop-up and afterward follow the same procedure and continue by clicking on link-truffle projects then go  to server and finally change server host-name to "vEHTHERNET(WSL)".
Answer: ✅ANSWERED
to use Ganache on both Windows and WSL from ganache GUI of Windows.
1: goto your ganache setting > Server > HOSTNAME.
2: Set Value to 0.0.0.0 - All Interfaces

now in WSL web3 app, you need to provide specific Ip Address instead of 127.0.0.1
so to find that address use this command (bash command in ubuntu)
grep -m 1 nameserver /etc/resolv.conf | awk '{print $2}'

and use that IP to write your RPC URL for WSL based connection like this
let provider = new ethers.providers.JsonRpcProvider(http://172.19.192.1:7545);


Is this means that hard drive will fail?

Question: My question is: does this CrystalDiskInfo mean that my hard drive is going to die?
Comment: "Please clarify your specific problem or add additional details to highlight exactly what you need." - The single sentence contained in this question is confusing and does not make sense grammatically.
Comment: @BrunoBalas It is hard to know exactly how close a drive is to dying at any given point. That said, if you are the least bit concerned about the data on that drive, heed any warnings and back your data up now. At the very worst, when the drive does die (maybe tomorrow, maybe months from now) you will have it.
Answer: A sector is called 'pending' if when the sector was read by the drive, the drive detected that the sector was partially corrupted, but the drive was able to recover the data successfully.
(If the sector was totally corrupted, the drive would have reported a bad sector to the OS, and the OS would have reported some sort of error).
Simply misreading a sector doesn't give the drive enough information to know what to do with the sector - it needs to decide if the actual sector on the platter is faulty, or if there was some sort of temporary problem. So the sector is listed as 'pending further analysis'. There are 3 main things that might have happened:

Data mis-read due to freak problem (e.g. vibration causing head to go out of alignment, power glitch, overheating, etc.).
Data mis-saved due to freak problem (same as 1).
Bad sector.

Next time the sector is accessed, the drive will perform additional analysis until it reaches a diagnosis (e.g. fault 2 - read data, correct it using ECC and then re-save; or fault 3 - reallocate sector). Once a diagnosis is reached the sector is taken off the pending list.
You can, if you want, run a 'surface scan' on the disk using Chkdsk. This will read every sector on the drive, and the appropriate repair mechanisms will be triggered once enough information has been gathered.


How much energy do you need to levitate/counteract gravity without any displacement or change in height?

Question: I know you can't have work without any displacement, so I was kind of wondering as to what keeps, for example, a man on a jetpack, off the ground but with no more change in height from the initial height he was on? Is this still a form of energy or something else because if he burns fuel to keep himself off the ground, doesn't that mean energy is  being used?
Comment: in the case of a jetpack the energy goes into accelerating the exhaust down quickly, while no work is being done on the user the exhaust is being accelerated to a high speed. similarly if you used a jetpack on the ground pointed horizontally it would accelerate air but if you had a firm grip on the ground then no work would be done on you. the displacement in the case of the exhaust is the length of the engine it travels through.
Answer: A table can forever keep an apple "levitated" above the ground with it's normal force. That requires no energy. No work is done.
A force does not spend energy to fight against another force.
The force may cost energy to be produced, though. This is a separate issue. The jetpack spends fuel to produce an up-drift force, the human body spends nutrition to extend/contract muscles to produce the "holding"-force to hold a milk can, but the table spends nothing to produce it's normal force.
The jetpack falls down after a while and you feel tired after a while, not because work was done on the objects, but because work was done inside those "machines" (jetpack and body) that produce the forces. The table never gets tired. It never spends any work.
The issue is clearly not about holding anything. It takes no energy to hold stuff. You are correct that no work is done on the levitating man, if he undergoes no displacement. Work may be done inside the "machine" that produces the force, but that is internal.
Comment: Comments are not for extended discussion; this conversation has been [moved to chat](https://chat.stackexchange.com/rooms/78215/discussion-on-answer-by-steeven-how-much-energy-do-you-need-to-levitate-countera).
Answer: To hover in the air conservation of momentum dictates that to keep a 100 kg object hovering we must "throw" 100 kg down towards the earth at a velocity of 9.8 m/s for every second we want to hover. The kinetic energy required to accelerate 100 kg to 9.8 m/s is 4.8 kilojoules. So a propeller that grabs 100 kg of air per second would need 4.8 kilojoules per second or 4.8 kilowatts (a watt is joules per second).
We could also propel twice the mass of air at half the speed to hover. Since kinetic energy is the square of the velocity propelling 200 kg down at 4.9 m/s would use 2.4 kilowatts or half the energy. So bigger is better and there is no theoretical limit to how low your energy consumption can go. Some sort of futurist tractor beam that can push or pull a very large mass of air would use almost no power. With our current technology and materials a very large open blade (i.e. a helicopter), large ducted fan or high-bypass turbofan are your best options as they will move the maximum amount of air with the minimum amount of energy.
If you want a more traditional jet pack where all of the reaction mass is kept on board then you're getting into rocketry and don't care about energy efficiency. You just care about the specific impulse (energy density) of the fuel. Also, even with the best rocket fuels your maximum flight times will be measured in seconds.
Comment: The claim in the first sentence is simply wrong as written. Firstly it is the acceleration that you impart to the matters not a speed—a pointless distinction if you carry your own reaction mass but an important one if you use ambient mass as a jet does. Secondly, you can accelerate a lot of mass slightly or a little mass a lot. You're on the right track, of course, but have run too much ahead with simplifications .
Comment: If you have a tractor beam, don't waste its energy moving air, have it push against a tree or something. The tree doesn't move (except negligibly) so it doesn't do any work.
Answer: Your question is actually profound in a subtle way.  The key to understanding this is that the man has a force being applied to him by gravity that is pulling him down.  In order for him to stay aloft at a constant height, there must be a force that acts in the opposite direction and counteracts the force of gravity.
In your example, that counteracting force is supplied by the jetpack.  So the jetpack must continually produce an acceleration upwards of equivalent to the weight of the man (and the jetpack.)  But why is that different than when the man is standing on the ground?  The earth's gravity is still acting on you but you don't have to continually burn fuel to stay in place.  Consider the same standing on a large spring.  When he first get's on it, he will move towards the earth and compress the spring until it pushes back enough to stop his motion.  The spring's upward force is being supplied by weight of the man in a reflective manner.  Essentially the ground does the same thing.  The elasticity of the surface creates a mechanical equilibrium.  Newtonian models don't really describe how materials produce elastic force.  It is simply assumed.
Comment: "*Newtonian models don't really describe how materials produce elastic force. It is simply assumed.*" This is not correct. Elasticity with Young's modulus, bulk modulus, spring stiffness constants etc. is a typical mechanical discipline described with laws like Hooke's and similar. The elastic effect is just usually ignored when talking about a very stiff surface - yes, at the atomic scale, some slight displacement of atoms does happen, but so fast and so little that is practically is instantaneous. Equilibrium is reached in an instant, so we can simplify the picture by ignoring that.
Comment: @Steeven I think you misunderstand.
Answer: The jetpack must create an upward force equal to that of gravity. For that reason, it expels some mass downward. Below, I calculate the work done by the jetpack assuming all mass expelled from it leaves at velocity $v$ which depends on the details of the jetpack's construction.
Suppose the jetpack of mass $m(t)$, where $t$ is the time loses $\delta m(t)$ mass over a short time $\delta t$, then its momentum changes by $\delta p = \delta m \cdot v$, where $v(t)$ is the velocity by which matter is pushed down from the jetpack. To counterbalance the gravity, you have to have
$m(t)g=-\frac{\delta m}{\delta t} v$, where the sign comes from the fact that you lose mass from the jetpack as you try levitating. In the limit of very short time, the equation becomes:
$\dfrac{dm(t)}{m(t)}=-\dfrac{v}{g}dt$
This equation is solvable, and one can obtain the mass that needs to be lost to keep flying with the jetpack. Suppose you need to find the work done by flying it, that has to be equal to the kinetic energy of the expelled gas over some time. Since we assumed all mass is expelled at $v$ from the jetpack, the work should be $W(t)=\dfrac{(m_0-m(t))v^2}{2}$, with $m_0$ being the initial mass of man+jetpack.
Answer: As @Steeven explains, no energy is required in principle. However, you will find that 'hovering' does require energy. How much depends on how you hover.
The basics are very simple. Gravity exerts a constant force $F$ on the levitating object. To counteract that force $F$, you can either place the object on a table, or impart momentum on an external reaction mass like air (helicopter), or propel some of your own mass (rocket).
The force generated by imparting momentum on an external reaction mass is
$$ F \propto \dot{m}v$$
with $\dot{m}$ the reaction mass flow and $v$ the reaction mass velocity. To do this, you require a certain amount of power,
$$ P \propto \dot{m}v^2$$
From this, it is immediately obvious that you want to have a very large mass flow and a very low reaction mass velocity. This is why helicopters are more efficient than jetpacks (and turbofans more efficient than turbojets).
In rocket science, this still holds, but since you need to store all your mass $m$ on board before starting your hover, it is preferable to expend a lot of energy to minimise the mass flow. This is why jetpacks are still more preferable than rocket suits for hovering on Earth.


Why are the Tags not in alphabetical order ?

Question: As a newcomer I need to look in various mixed up boxes with no logical order .Why not have a switch to create alphabetical access to new information ?
Comment: http://meta.stackexchange.com/questions/77808/does-it-matter-the-order-you-tag-your-question And I concur, this is a perfect question about the site - so it should be over on meta.
Answer: There is a very logical order to the tags. The tags that appear first are the most popular and used most often. It's so when a user looks at a question they are more likely to see a tag that they know if the can answer the question or not or even want to look at the question or not.
With only up to 5 tags being on a question, the sorting isn't too important and either way would most likely be ok. However, one drawback that alphabetically sorting can lead is having tags that are never used being much more prominent than tags used all the time. For example, if we were to tag alphabetically accent guitar rhythm on a question, the most popular tag on the site that may be a deciding factor to whether or not someone looks at the questions is sandwiched in the middle of the tags and on a quick read may be glossed over.
When you look at tags on the tag page you have the option of sorting alphabetically, but it's not extremely practical for searching for what you want.
Answer: A quick follow up to Dom's post.
One of the great ways Stack Exchange uses tags is that you don't need to hunt around for one that fits best. Just start typing the main topic of your question and the most common tags will automagically appear. For me this works way better than any other I have seen.
When browsing to find information, again, I don't need to look alphabetically. I either search all posts by age to get the newest stuff, or I use the word describing exactly what I want, eg fingerstyle


What are auxlangs?

Question: I often hear certain conlangs (e.g. Esperanto) referred to as auxlangs. What exactly is an auxlang? How can I tell if a conlang is an auxlang? What are some examples of auxlangs?
Answer: Auxlang is a short word for "auxiliary language", i.e., a language that is designed and promoted as a bridge language between people of different languages.
There are International Auxiliary Languages (IALs) aiming at the whole world as a target, and Zonal Auxiliary Languages aiming at a group of linguistically or culturally related people.
Examples for international auxiliary languages are Volapük, Esperanto, or Interlingua; examples for zonal languages are Interslavic or Afrihili.
Answer: According to Wikipedia, auxlangs are languages intended to facilitate international communication. Some notable examples include Esperanto and Interlingua. Of these, Esperanto is the most well-known and successful. Zamenhof created it in the 1880s.
Edit: As pointed out in the comments by @RadovanGarabik, these are only some notable examples of constructed auxlangs. See the comments for examples of natural ones.
Comment: By far more notable auxlangs are English, Latin, French, Chinese, Greek, to name just a few. Of course, these are not conlangs.
Comment: @RadovanGarabík, based on time period, of course. Looking at the world today, it's English and then anything else.
Comment: @KeithMorrison I'm sure there are regions where this doesn't apply. E.g. in Central and Eastern Europe, more people will be able to speak Russian than English, especially among the older population. But I agree that globally and among younger people it would probably be English.
Comment: @OliverMason, let's put it this way: if you were told you were going to be placed in a populated area chosen at random somewhere on Earth, and you were only allowed to speak in one language, what language would provide you the highest probability of finding someone who could understand you in the shortest amount of time?

What are auxlangs?


Summing attribute values from many points geocoded on same location using ArcGIS Desktop?

Question: I have geocoded a batch of addresses and many of these are geocoded on the same location. Now I want to keep just one point on each location but sum the field values of all points from the same location. How I can do that using ArcGIS?
Comment: Try dissolve tool
Comment: Use the dissolve tool as @BERA suggested, the answer of converting it to a raster is unnecessary.
Comment: Yes, dissolve tool will solve this on the best way, but @BERA should give the answer, not a comment.
Answer: Try the dissolve tool:

Aggregates features based on specified attributes

And:

The attributes of the features that become aggregated by dissolve can
  be summarized or described using a variety of statistics. The
  statistic used to summarize attributes is added to the output feature
  class as a single field with the naming standard of statistic type +
  underscore + input field name.


A word to describe someone who has moved back to live in his/her home country?

Question: If a person was born and brought up in country A, at some point went to live in country B (as an adult) for a few years and then moved back to country A, is there a single word to describe that person's situation?
Comment: He/She is **back home**!
Answer: The OED has the noun repatriate to mean someone who has been restored to their (own or original) country.  Examples given there include:

The majority of these repatriates have the choice of living on totally inadequate means or entering the workhouse.
Seven ships have been named as bringing repatriates home to Britain.
Mrs Meir, the Prime Minister, and··the Defence Minister, were among those at Lod airport today to give the repatriates a heroes' welcome.
Comment: I don't believe it has that connotation at all.
Comment: @camden_kid That would be closer to a [deportee](http://oxforddictionaries.com/definition/english/deportee). ODO has a helpful entry for [repatriate](http://oxforddictionaries.com/definition/english/repatriate).
Comment: @coleopterist Though that would be out of a country. You'd have to mangle some Latin and get an adportee. :P
Comment: I'm hesitant about marking this as the answer because it doesn't 'feel' right but also the three examples indicate external forces influencing the return of the repatriates.
Comment: @camden_kid If you look at the examples for the verb, whence the noun derives, there is no theme of some sort of **forced** repatriation.  It might be that they had been in exile, or rescued, or many other things.
Answer: Remigrant comes to mind.
Merriam-Webster:

REMIGRANT: a migrant who returns
Comment: *Remigrant*. Does not appear in Oxford and Cambridge dictionaries.
Answer: It is not often used as a noun, (at least not compared to (a) its opposite, expatriate, nor (b) the verb form repatriate, [the important pronunciation difference being, I think--and there may be UK/N Am differences here--that the verb has stress on the second syllable and pronounces it--re-PAY-tri-ate, and the noun, while also stressing the second syllable, pronouns it--re-PA-tri-ate.  Be that as it may, the best noun I can come up with is repatriate.
I am Canadian, and for example, a decade ago, an expatriate Canadian, Michael Ignatieff, moved back to Canada after a twenty or thirty year absence living and working in America and the U.K., in order to become the leader of the Canadian Liberal Party.  You could say he repatriated himself, or that he became a repatriate.  (Ignatieff's story did not end well; after losing two federal Canadian elections, Ignatieff resigned from politics and currently serves as a professor at the University of Toronto.  His extended expatriate status was a huge issue in both of the elections he lost; it is an illustrative example of how to use these terms.)
Comment: +1 for the first 8 words, which are significantly missing from the top answer.
Answer: How about repat? It would be the opposite of expat.
Comment: Not bad :-) I prefer it to repatriate but I'm not sure it's a word in the dictionary.
Answer: Returned emigrant, which does imply someone away for years, in double figures perhaps.


IP Resource Errors While Building a Windows Failover Cluster on Azure

Question: I have setup a failover cluster in Microsoft Azure on four VMs. I have two nodes in one subnet/region, one node in another subnet/region and a final node in a third subnet/region. The cluster passed the validation wizard and was successfully built, but the two nodes in other subnets will not go online, reporting "Failed to bring the resource 'IP Address x.x.x.x' online".

Azure Vnet Setup

Vnet 1 - South Central US - 10.16.16.0/16 - Nodes 1 & 2 - Online
Vnet 2 - West US - 10.116.16.0/16 - Node 3 - Offline
Vnet 3 - East US - 10.216.16.0/16 - Node 4 - Offline

All three IP Address resources are DHCP. I've attempted to make them static but they still failed while coming online. If I click "Information Details" on the error I get:
Error code: 0x80071397
The operation failed because either the specified cluster node is not the owner of the resource, or the node is not a possible owner of the resource.
It seems to be an issue with the way Azure DHCP works, but I'm stalled. There must be a way to get his working. What am I missing?
Answer: only one can be online at a time. if you go to the list of nodes, and turn off the node where its ip is the one being used by the cluster (10.16.16), then another ip address in your list will come online.


How do I execute/run Ruby code in Sublime Text 2 (Windows)?

Question: I have tried using ctrl + B, but all I get in return is  [Finished in 0.3s]. Do I need to download a package to be able  to use hotkeys? If so, does anyone know where I can find the package I need with good instructions?
Thank you.
Answer: Control+b runs the file you are currently viewing in ST2's irc (interactive ruby console).
Control+` (thats the tick/tilda button) opens the irc for your currently viewed project.
These are good for linux, osx, and windows, assuming no other packages or key bindings override these default commands.
Also, note that the console is your systems Python in OSX, and a Python interpreter in Windows and linux.
Good, searchable documentation is found here, one of the three suggested links found from ST2's website about documentation here.


Does your land property give you the right to anything below your property like an oil reserve?

Question: Does your land property gives you the right to anything below your property like an oil reserve? I am wondering if the state has the rights to it, or you have the rights to it in the United States. I am also wondering if you can only claim the part directly under it or it works in some other way. Also, does it make a difference if your land covers the entire oil under your land or not?
Comment: Unless you're specifically asking about federal law, asking about "in the United States" is almost always too broad. You can't even say that the entire US is based on the common-law tradition.
Comment: In Hawaii, ALL mineral rights are reserved to the state gov't
Answer: That is going to depend on the deed.  In some cases a deed will include both surface rights and mineral rights. In other cases mineral rights are owned by some different entity, or by the state.
If someone else owns the mineral rights, they can exploit those without the permission of the surface owner, and in some jurisdictions at lest they have a right to come onto the property and dig or drill there, even against the surface owner's wishes.  In  the US that varies by state, and also by the terms of the deed.
If the surface owner also owners mineral rights, no one can dig ir drill from that land without permission, but I am not sure on the rules for 'slant digging".
Comment: Slant or off set drilling and production is generally legal in the US. It is rare for a surface deed to include mineral rights in TX. The mineral rights generally belong to the previous owner of the surface , generally not the state.
Comment: A number of states allow "forced pooling" for oil and gas. Because of well-spacing requirements, the party drilling the well must control the rights for the entire area associated with the well location. This means that a single small landowner could veto the exploitation of the minerals on the entire region. (In NM that's 640 acres for natural gas.) Since it's in the state's interest to collect the royalties on the gas, the state has  "forced pooling" laws that forces you to join the pooling of rights to drill the well. You may receive lower royalties than those who joined voluntarily.
Comment: and it is also possible that you have mineral rights but they only extend say a few hundred meters down. As most oil and gas deposits are much deeper you won't own those.
Answer: Mineral rights are joined to the surface rights until they are severed.
Sometimes this is done by deed from a private owner, which shows up in the chain of title in public records.
In parts of the United States (mostly in the Rocky Mountains and places to the west of them), the United States government reserved mineral rights to itself before transferring the surface rights to private ownership.
Comment: The original deed for my property (in NM), issued under the Homestead Act of 1906, only reserves coal rights for the US gov't. Other minerals remained with the homesteader. And that was good for the homesteader, because there was oil and natural gas beneath the property.
Comment: @Llaves Most of the variation in the U.S. involves different homestead acts and different general rules of reserving mineral rights in other property to which patents from the federal government were issued. There were about half a dozen iterations.


Does rdp'ing into the console session close any applications you launched during a physical log in previously?

Question: If I launch an application while physically being at the server and then some time later decide to remote into the same server's console session ( /admin flag) and then log out, will that log out close the application that I launched while physically being at the server?  This is usually something that I could test myself but in this scenario gaining physical access to the server is a rare opportunity.
Answer: Yes, logging out will close all the applications, same as logging out from the console does (by going Start > Log Out)
If you just close the RDP window (by clicking the x) that's the same as just walking away from the console - everything will continue to run.


Where to ask basic questions about machine learning?

Question: I am a beginner in machine learning.
I have very basic doubts and confusions. On which site can I ask such questions related to machine learning?
Comment: Related for expert questions on machine learning: [Which StackExchange website for Machine Learning and computational algorithms?](http://meta.stackexchange.com/questions/130524/which-stackexchange-website-for-machine-learning-and-computational-algorithms)
Answer: There are 3 production sites: stackoverflow, stats.se, and math.se. There is some overflow between the sites, but at one extreme if your question is purely about an aspect of coding, use stackoverflow, and at the other extreme if your question is purely about an aspect of the maths, using maths.se. So for example, if you can't figure out the right way to organize your data prior to calling a regression library, use stackoverflow, but if your question is about how to use calculus to figure out a gradient in order to efficiently minimize your errors then use maths.se.
Stats.se (cross-validated) overlaps with both sites. Questions there often have some sort of programming element to them (especially in R or other statistical languages), but in the context of a statistical or machine learning problem. It's also a good place to ask question on topics including methodology, how to choose features, interpreting results, and the mathematics behind the algorithms.
All of these sites welcome questions from beginners, so long as they meet the sites' standards -- which in general means looking for solutions first especially on the site itself, describing a specific on-topic problem, and, where appropriate, the steps you have taken, or would take, to attempt to solve it. To get a good idea of the kinds of questions and answers on the different sites, you can look at the machine-learning tag on each site.
I'm not familiar with Computer Science, as mentioned in TGMClans' answer. But Browsing through the Computer Science questions tagged machine learning seems to confirm the description of Computer Science questions given in this answer

As a rule of thumb, if your question depends on code, ask on Stack Overflow; if your question calls for mathematical notation, ask on Computer Science. Algorithms expressed in pseudocode straddle the border.

You do, however, need to choose just one site for each question: don't cross-post. See here for more information.
Finally, here is a quantitative overview of the number of questions on the different sites.

Sources: SO, math, stats, compsci
Note that "unanswered" is a little misleading as it includes question which have answers with no votes, and also questions which are essentially answered in comments (this is particularly common on stats.se).
(Here is a related question on maths.se meta: Which site is better to ask question about Machine Learning?)
Comment: April 2022 - there is very little on Cross-validated for applied ML. They dont even have a tag for `cnn`. If you need to talk to people who understands ML on high-level you are always at great disadvantage on Cross-validated because it is owned by people who only know statistics (and they dont care to learn anything else and attacking everyone who doesnt fit the *profile*). And SO is now completely unusable by the sheer amount of micro-groups of zealots trying to protect their sandcastles. Questions got closed or downvoted to oblivion for no good reason. So where do I ask about Bayesian CNNs ?
Answer: You can ask here Computer Science , CrossValidated and here on StackOverflow.
Comment: If you show that you have tried to solve your own problems first (show attempted solutions, show results of web searches), it should be OK. Take care that your questions can be answered objectively though; do not ask [Gorilla vs Shark questions](http://blog.stackoverflow.com/2011/08/gorilla-vs-shark/) on Stack Exchange.
Comment: I am quite low standard , i mean just beginner ,is it okay for me ?
Answer: Aspects of machine learning are now part of our Artificial Intelligence Stack Exchange, but check out the specifics of scope:

What topics can I ask about here?
If you have a question about...

social issues in a world where artificial intelligence is common,
conceptual aspects of AI, or
human factors in AI development

...and it is not about...

the implementation of machine learning, or
asking for a development tool or career path recommendation

...then you're in the right place to ask your question!


Painted Kitchen Cabinets

Question: I painted my kitchen cabinets. Worked well but over time they have become sticky in areas often touched; near knobs. I think it is a bit of grease from cooking but when I try to clean them, paint comes off and they are still sticky. The rest of cabinets are fine.
Comment: I have lightly sanded and repainted in these areas in the past but in time the problem returns.
Comment: If the cabinets are a laminate (plastic) or "faux wood" and not real wood from a tree the paint won't adhere well.
Comment: How long before the first peeling issue? And the type of paint used (oil or latex)?
Comment: It takes some practice, but a quick spray of oven cleaner, (the caustic stuff) followed up with simple green, and a light scrub (used green scrubby) with an immediate rinse with a rag makes grease come off easy.  We use such a combo on cabinets, plant shelves near kitchens, greasy fans in kitchens, etc.  Just be careful, because the over cleaner can and will take off the paint in large quantities if you leave it for any amount of time.  But the sodium hydroxide will react with the grease and make the grease literally just wipe off where you could not scrub it off before.
Comment: BTW, what is your question?  I did not see one.
Answer: If the paint is peeling off the wood surface then most likely there was a problem with how the cabinet was prepared for the paint or with the paint itself.  Painting any surface located in a kitchen takes more effort and attention to details when prepping it for paint. As you already know grease from cooking that becomes airborne in water vapor will adhere to wall surfaces and accumulate over time. For proper paint adhesion it must be removed through scrubbing with a de-greaser-type cleaner. Most commonly used is the cleaner labeled or containing "TSP" (Tri-Sodium Phosphate). Added to warm water and applied with a sponge (or sponge mop) the cleaner easily cuts through greasy build-up. Rinse the applicator often and then re-wipe with fresh clean water to remove any residue.  Since the cabinets are already peeling it will be hard to hide the edge of the paint with a new coat. You can try sanding the edge to "feather" or angle the transition so when another coat is applied it may be less noticeable. But I would guess since the wood hadn't been prepared you will continue to have peeling issues. It may be wise to start anew by sanding all the door faces to remove new paint, clean with TSP and prime and finish coat.
Answer: The paint isn't the main issue here. Repainting is just a temporary fix as the grease comes back. If it's upper cabinets as well, then your exhaust fan isn't working well enough and needs to be cleaned or replaced. However, I'd suggest just looking up some good degreasers to clean it off with. Dawn, magic erasers (could harm the paint), etc.

Painted Kitchen Cabinets


C++ users proposals

Question: I believe I have some good suggestions to improve the C++ language. Who should I contact to discuss this? Do I have the chance to be heard, especially if I'm not a renowned expert?
Comment: You should at least *know very well* the existing standards (up to the details, and the sections!) and that takes time.
Answer: According to this page on the C++ committee's official web site:

The public can submit proposed defect reports via the Internet news
  group comp.std.c++

You can access comp.std.c++ via groups.google.com, but it's better to get an account on an actual NNTP server (I use news.eternal-september.org, which is free of charge) and use a new client (Thunderbird does this; I use Gnus under Emacs myself).
I recommend browsing the group before posting, just to get a feel for how it works.
Note that revisions of the standard only come out every few years, and a new one was just published on 2011-09-11.  I don't know what, if any, work has started on a new revision.
Comment: Note that a *defect report* is used to report defects. Proposals for *additions* use a different route. However, a discussion in comp.std.c++ might be a good starting point to find prior attempts. After all, chances are that one is not the first to create this fancy new feature that should be added to C++.
Comment: The C++ standards committee is continuing their work: their next meeting is in Kona in February 2012. It's certainly extremely wise for the OP to become known on comp.std.c++ prior to submitting a language feature.
Answer: You have a 100% "chance to be heard," but you should be aware that the goals for the C and C++ language standards are quite conservative. There's billions of lines of C and C++ code out there and breaking changes are viewed with extreme reluctance. And, like any long-lived group, they probably are a little suspicious about new voices. As @Keith Thompson suggested, you should probably start with the NNTP groups, just to get a sense of the technical level of discussion.
Depending on how radical your changes are, the wisest thing to do might be to gather your changes and implement them under a new name and see if they gain any traction. Once they've been tested and their interaction with the broad world of C and C++ code is understood, you'd have a better chance of having them incorporated into the language.
(And note that the C committee doesn't have the same relationship with comp.std.c that the C++ committee has with comp.std.c++.  But comp.std.c.is still a good place to get feedback on any proposals for the C standard -- as long as you're aware that there's nothing official about it.)

C++ users proposals


how to record wind?

Question: hello all,
excuse my noob'iness, but i really don't know how to record wind...i know one technique with contact mics, but other than that how do you go about recording wind?
p.s.: sorry for my bad english :)
Answer: It's not the wind itself that makes sound, but what it goes through/against. You don't need to have your mic in the wind, just aiming it at interesting sounding objects that create friction (wires, trees, windows, etc.) is probably what you're after.
A common practice to re-create wind is to have fabric pulled against a carpeted floor. Ben Burtt did that to create the sound of wind in Wall-E.
Check out these videos where he talks about it:
http://www.youtube.com/watch?v=8A6z8QkVHk4
[youtube]8A6z8QkVHk4[/youtube]
and:
http://www.youtube.com/watch?v=Jl7lFPZeIOE&feature=related
[youtube]Jl7lFPZeIOE[/youtube]
Comment: Blow Tools library is good to reference for ideas of what types of wind interactions you can record too.
Comment: ok i never thought about it this way, thanks for your answer ;)
Answer: I'll touch on two different subjects than have been mentioned so far.
Having clean wind sounds is important, but I think it's equally so to have "dirty" wind recordings. Sometimes you need that "exposed to the elements capsule" sound. If you have separate clean and dirty recordings, you have a lot more control over designing the intensity of the wind sound for a scene.
Another note, omni-directional microphones are far less susceptible to blowing out a signal in wind. A spaced pair can do wonders for recording ambiences with wind.
Comment: @Shaun, cool tip re. omnis, will have to try that.
Answer: Look at objects as woodwind instruments (or stringed instruments in the case of tensioned wires). Sometimes really small holes and gaps that are vertical can sound cool when the wind comes right through them (like the hatchback of my car), but just like a flute, sometimes rounder or larger gaps sound best when the wind is coming at more of a perpendicular 90-degree angle (like knot holes in the side of a rickety old barn).
Another trick is to hold a sheet of foam over an air conditioning unit. You can "play" howling wind sounds by opening and closing gaps in the foam sheet. With a tool like iZotope RX you can even remove most of the machinery noise easily (what you can't remove can be easily masked by the whistling or other sound layers). Totally works!
If you want to get the sound of wind in a natural environment without overloading your mic capsules, use your eyes. Wind flows around objects and creates eddies, like water. You can often spot small areas where the wind is lessened or even still, even on the most blustery of days, if you have the patience. This is easiest to spot in areas with good plant coverage like lawns or long grass.
Comment: @NoiseJockey, genius.
Comment: Well, hang on, credit where credit's due: While the first paragraph is my own observations, I forget where I read the second paragraph as advice (but I've done it and it totally works), and the third is advice given to me by Gordon Hempton (which I've also done and it totally works, although patience is required).
Comment: @Noisejockey: Regarding option 3: How much background noise do you get if you use this method? I am looking for a way to regard the "pressure on your eyes in windy situations" and this sounds like a good idea, but i'm wondering what type of environment you tend to record these 'eddies'?
Comment: @Arnoud: It's so variable that it's hard to say. One doesn't ever want to record wind itself: All one can do is record wind IN or THROUGH something else. Finding these "wind eddies" is just an aid for positioning your mics to get that sound without being buffeted directly by the wind itself. Unless it's a wide open desert, wind eddies/buffers are everywhere. What matters is if they're close enough to what you want to record, which doesn't always happen!
Comment: @Noisejockey: I just noticed i made a type instead of 'regard' i mean't 'record' (don't know what i was thinking typing that!). Thanks for your answer though!
Answer: You'll always have far more control if you create your own. Get yourself a synth with a noise generator, you can model the wind by controlling various parameters. It's especially easy if you have a midi keyboard with assignable controls, you'll be able to make any wind noise you want. If you want an example of what's possible, check out The Flying Duchess here:
http://www.animation2011.com/
It's a short animation I did the sound for, which features that synthesised wind. Also, apparently in the Terminator, a wind noise that they used was just some dude blowing over the top of a microphone.
Answer: try big condencer stereo/mic/s with a large diaphragm + anti-wind foam cap (kind of Softie Windshield) + HQ low-noise preamp. The better way - is to sit at home close to fireplace and to emulate any wind on the planet by white noise generator in every synth or use some ready samples )
Comment: ahh i'm always forgetting some obvious stuff, thanks for reminding noise generators ;)
Answer: If you're lucky enough to have an old house with leaky windows (wait... did I say lucky?), when there is a wind storm, they generally whine and whistle just like you'd expect.
Comment: Speaking of which, listen to the window of my house when I used to live in Edinburgh: http://www.soundplusdesign.com/?p=370 Madness!
Comment: That's stellar.  Is the low rumble a contact mic, or did the window really buzz, too?  As for my window, it woke me up a few weeks ago, and instead of grabbing my recorder, I figured the snoring dogs would drown it out (even though it woke me up, not them), and went back to sleep. Excuses, excuses.
Answer: I had been having a lot of trouble recording wind and outdoor ambiences on windy days with my Zoom H4n and the foam windsock it came with, but once I got a new windscreen it made a huge difference. I got the Redhead windscreen designed for my H4n and it was totally worth the $40 or so it cost. Getting a good stereo image for something like wind isn't always needed but it's good to have and a portable recorder with a stero pair of mics that you can always have with you is the best way to capture wind when it happens.
As far as using it for sound design goes I use the sample feature in Absynth and play the wind with a midi controller. Also automating an EQ with a narrow Q boosted or cut can also add some customized movement and expression to an uneventful wind recording.
Answer: For particularly blustery wind, there's always the vehicular "create your own" option.  To capture howling, whistly wind with a raw and violent low end component, grab your rig plus a metal colander and bug a friend with a convertible to drive you around some relatively car-free roads at decent speed.  Miking the inside of the colander can yield great results.  A colleague and I did this a few years back for Medal of Honor: Airborne and used it in some of the freefall/airdrop wind, open door in the plane, and so on.

how to record wind?


Lighter-Than-Air-Bridge II: Tethered Gondolas

Question:
Would something like this be feasible? Span of bridge is approximately 5km. Height above canyon floor is approximately 800 meters (1/2 mile). Bridge structurally close to neutral. Tethered dirigibles carry extra load. Tube trams carry passengers. Structure is flexible, actively conformal and segmented. Site is fairly sheltered. One end is urban area. Other end is wilderness resort / preserve. Climate mild - like Pacific Northwest possibly. Atmosphere somewhat more dense than earth. Winds tend to flow mostly down-canyon or up canyon, perpendicular to structure. I am an artist, not an engineer or anything.
Comment: What holds the bridge up, and what are the "gondolas?"
Comment: The bridge is supported by the cables. So it is a sort of suspension bridge. The gondolas are the blimps that carry heavier loads. They are travelling back and forth along a sort of track. I am working on a better drawing.
Comment: In your new drawing you might want to use the same language as in your question text ("tethered dirigibles"), which would make it totally clear what they are.  Also clarify whether the balloons support the bridge, vice-versa, or neither.  Another question, what is meant by "structurally neutral?"  Other than those points I think I understand what you're getting at.
Comment: By structurally neutral I mean that the bridge by itself is self-supporting. But the balloons support any extra weight that would be transporting a load across the bridge. So the balloons do not support the bridge, only the added loads.
Answer: A self-supported cable of constant density forms a curve known as a catenary, which is close to a parabola.  (In fact a parabola is formed when the weight is distributed evenly horizontally like a suspension bridge, instead of evenly along the cable.)
I won't go into all the mechanics, but there are two important equations we need.  First, the shape of the caternary is described by:
$$
y=a\left[\cosh\left(\frac x a\right)-1\right]
$$
Where $a$ is some characteristic length.  We can describe the tension $T$ in the cable as:
$$
T=\lambda ga\cosh\left(\frac x a\right)=\lambda g(y+a)
$$
Where $\lambda$ is the cable's mass per unit length, and $g$ is the gravitational acceleration.
All we need to know is how wide the span is and how much the cable sags.  In the best-case scenario, the cable will be allowed to sag all the way to the bottom of the canyon; we can use this as a lower bound on the required material strength.
Assuming that $y=800~\text{m}$ and $x=2.5~\text{km}$ (and implicitly $g=9.8~\text{m}/\text{s}^2$), we get:
$$
a = 4.03~\text{km} \\
\frac T \lambda = 47\,400~\text{m}^2/\text{s}^2=47.4~\text{kN}\cdot\text{m}/\text{kg}
$$
The ratio $T/\lambda$ is the required specific strength of our bridge material (I'll use the symbol $\varsigma$ to denote it).  At first this looks pretty good, since steel has a strength of around $250~\text{kN}\cdot\text{m}/\text{kg}$.  However, this doesn't account for the whole bridge, only the cables.
We can figure out the payload fraction $\zeta$ (the fraction of the weight of our bridge that is not cable) with this formula:
$$
\zeta = 1-\text{SF}\times\frac{\varsigma_\text{requirement}}{\varsigma_\text{material}}
$$
Note $\text{SF}$, the safety factor.  It ensures that we have some margin, so we don't fall apart at the slightest touch.  Based on some quick research, typical actual safety factors for bridges are around 4, but I'll use a design SF of 5.  The maximum payload fraction for steel cables over this canyon is therefore
$$
\zeta = 1-5\times\frac{47~\text{kN}\cdot\text{m}/\text{kg}}{250~\text{kN}\cdot\text{m}/\text{kg}}=6.3\%
$$
This means that pretty much all of the weight of the bridge will have to be dedicated to the supporting cable(s).
Plotting the shape of the bridge reveals another problem: the slope at the ends is pretty steep.

Assuming a more "reasonable" value for the sag of $125~\text{m}$ (obtained by limiting the grade at the ends to 10%) gives us a shape like this:

Which is probably more like what you had in mind.  However, in order to raise the bridge we have to increase the tension.  The numbers we get now are:
$$
a = 25~\text{km} \\
\varsigma = 247~\text{kN}\cdot\text{m}/\text{kg} \\
\zeta_\text{steel} < 0
$$
The required strength is just about the limiting strength of a steel cable.  While a steel cable could support itself, it would do so just barely.  Any nick in the cable, or a strong gust of wind, and it would snap.
However, Kevlar has 10× the strength-to-weight ratio of steel, and is not an implausible material to use: it's a manmade polymer, and common production processes can spin strands of unlimited length.  You could lay the cable in the same manner used to lay the steel cables for the Golden Gate bridge, but spinning the Kevlar strands as you go.
$$
\varsigma_\text{Kevlar} = 2514~\text{kN}\cdot\text{m}/\text{kg} \\
\zeta_\text{Kevlar} = 51\%
$$
With a conservative safety factor, the total weight of the bridge, cars, and passengers/cargo should be no more than that of the cables themselves.  I would suggest a thin, aerodynamic, flexible composite shell surrounding the cables and tram cars to keep live loads from the wind low, and to keep the cables spaced correctly.  You wouldn't be able to have a hard, rigid deck like modern bridges, so think of it like a high-tech rope bridge.

As for the balloons, I'm guessing they're essentially like modern blimps, but tethered to the bridge so that they can simply pull themselves along.  Although this is more energy-efficient, it won't be much faster since you don't want to load the bridge too much.  Plus you will still have to give the balloons maneuvering capabilities to avoid each other when they pass in opposite directions (or build two parallel bridges spaced a couple hundred meters apart).  Unless you need high volume or high speed, the bridge won't really add much to the balloons operating by themselves as aerial ferries.
So yes, such a bridge is possible, as long as you can use advanced materials (and are careful to avoid aeroelastic flutter!).
Comment: Interesting question; I haven't had to pull out the catenary equations since freshman statics!
Comment: I'm not sure why you wouldn't just scrap the balloon support and just get a stronger cable.  The cable needs to be a bit stronger than minimum to hold strain from winds, and the force pulling/pushing the gondola.  Or if you have great, safe lighter than air craft, why tether them to the ground?
Comment: @Oldcat I assumed no support from the balloons (as per the question), and included a safety factor in my calculations.  However, now that you mentioned it, I actually [looked up](http://ascelibrary.org/doi/pdf/10.1061/%28ASCE%291084-0702%281997%292%3A4%28157%29) safety factors for real suspension bridges, which are higher than I originally thought, so I'll go ahead and redo the numbers.  (I'm used to the aerospace industry where SF is commonly 1.25 to 1.5 due to tight controls.)
Comment: What a great answer. Thanks much. I am not wedded to the blimps.


Is it safe to use a stainless steel pan after making salt and vinegar brine in it?

Question: I used my stainless steel pan to dissolve salt in a 30% vinegar/salt solution to kill weeds outside.  After the salt dissolved I let the solution cool.  This stripped the shiny surface of the pan.  The surface still feels smooth like the normal areas.  Is this pan still safe to use for foods?
Comment: aside: don't bother using vinegar as a weed killer, they come back in no time, so you would need to spray every few days to be as effective as a generic roundup knock off.
Answer: Your pan is fine, the vinegar and salt just dulled it visually but it is safe to use.


what do you think would be a reasonable amount of time to write 60k words book?

Question: As my first time submitting a book proposal, it is required to indicate the amount of time expected to deliver the first draft of the book. I am not sure what to say because I have no idea about what the editors would consider enough time to write a book of 60k words. As I would really appreciate your insights, what do you think would be a reasonable amount of time to write 60k words book?
Comment: From scratch? Starting from your PhD thesis (not uncommon in some fields, primarily humanities)? Working on it full-time, or only nights and weekends? Way too many parameters.
Comment: Twice the time you think is reasonable.
Comment: A year, at least, based on my experience...
Comment: @henning: I suspect "twice the time ..." even applies to books [like this](https://academia.stackexchange.com/a/167400/49593)!
Answer: As @JonCuster comments, you don't provide enough context for a reasonable answer. Is this just something you want to write but haven't started? Do you have fragments or drafts already down on (metaphorical) paper?
My experience writing mathematics textbooks is that each one takes me several years, starting with rough notes prepared for a course. I never wrote a formal book proposal since each book was essentially done before I had a publisher.
I wrote a draft of a 60K book with no technical content while on sabbatical one year. I never finished it (and probably never will).
Answer: Guessing that it comes to 150 or so pages, but hard to be exact. I was once told (by someone in the "know") that a good writing plan is to "make one page of progress per day". That doesn't mean write one page and then quit for the day, but that your draft is one page longer at the end of each day. It also doesn't mean that the days will be short or easy. Some will be very long and hard with revisions.
But if you can manage that, then about five months.
I'm pretty sure that the intent was not scientific writing but that the creative bit was also done along with the writing, as opposed to just having a complete outline. But I don't think that my experience writing programing textbooks is too far different.


How can I convert JPG into PDF easily?

Question: Is there any way to batch convert .jpg files into single .pdf files?
Comment: This is a dupe question http://apple.stackexchange.com/questions/11163/how-do-i-combine-two-or-more-images-to-get-a-single-pdf-file/11169#11169
Answer: Never forget your little fellow Automator, sitting in a hard drive near you :)
Launch Automator, create a new workflow. Now you need to add two actions:

In the "Files & Folders" select Ask for Finder Items and drag it.
In the PDFs Category, find "New PDF from Images" and also drag it.

Your workflow should look like this:

Now read the options for each Action and customize accordingly. Investigate similar actions (especially in the Files & Folders category) and perhaps the Photos if you want to transform your images from JPG to something else. In fact, go ahead and look all over automator, there are a lot of interesting things there. :)
When you're satisfied with your Workflow, click "Run" on the top right corner. Watch the magic happens and depending upon the number of items and your CPU power, you might go grab a coffee/tea.
Comment: @user19087 `brew install imagemagick` and then do something like `for pic in *.jpg; do convert $pic $pic.pdf; done`
Comment: it does only generate a single pdf which contains all pictures stored in the folder. How to create a single pdf of each picture file in the folder? Regards
Chris
Answer: I'm more at ease with commandline tools myself. ImageMagick is great for this. You can install IM through Homebrew:
brew install imagemagick

After that you can simply do this in a Terminal:
convert *.jpg output.pdf

and if the resulting PDF is a bit too big you can try:
convert -quality 60 *.jpg output.pdf

Of course ImageMagick also works on other Unix systems, and even on cygwin.
Comment: In case PDF is generated with rotated images, use `convert -quality 60 -rotate 90 *.jpg output.pdf`
Comment: This worked good for me but it does convert them all into one PDF file. If you want a PDF for every jpg you need to have a loop in your command like you suggested in the comment above.
Answer: Do not install anything else. macOS already has what you need. Just use "sips"
In my example a file is called "file.jpg" and I want to convert it to "file.pdf". Accordingly, I enter this command here.
sips --setProperty format pdf file.jpg --out file.pdf

Now I want to convert all files with the extension ".jpg" to ".pdf". So what do i do I'm making a "for" loop that allows me to easily convert all files from ".jpg" to ".pdf". You can also change the "for" loop to convert png to jpg, for example. No problem at all, just replace formats.
for i in *.jpg; do sips --setProperty format pdf "${i}" --out "${i%.*}.pdf"; done

There is a hidden program in the image capture. This program enables you to convert images to PDF. Since the results are not nearly as good as I described with "sips" it is still there and it is good to know.
It doesn't really have a graphical user interface, but it can still be started via GUI. But I'll show it with the terminal
The program I'm talking about is
/System/Library/Image\ Capture/Automatic\ Tasks/MakePDF.app

Change to the directory with the terminal where the executable file is located.
cd /System/Library/Image\ Capture/Automatic\ Tasks/MakePDF.app/Contents/MacOS

Now we have to start it with our file that we have as a PDF
./MakePDF /Path/to/image

Note:
I recommend that you select "Adjust to 8x10" on the shelf. This setting remains persistent. You don't have to keep changing it.
Comment: Consider something like: `for f in *.[jJ][pP][gG]; do sips -s format pdf "${f}" -o "${f%.*}.pdf"; done`
Answer: I don't know if this works with .jpg files but the easiest way I know to convert text is to select print in the file menu, then in the lower left of the print menu select save as PDF.
Answer: I've combined @Martin Marconcini suggestion to use Automator with the only answer actually answering the question about "batch converting .jpg files into single .pdf files". Note there that plural form was used both for source and destination.
@BabyBoy suggested to use command line tool sips in 2021, but it's been with Macs since OS X 10.3 Panther released back in 2003, and sips addresses the many or plural part of the posed question.
The script:
for sourceFile in "$@"
do
    sips --setProperty format pdf "${sourceFile}" --out "${sourceFile%.*}.pdf"
done

An Automator Quick Action

Make sure you have set "Pass input" to "as arguments" as per above.
"$@" here represents the selected documents/images as input.
The sips line sets the format of the sourceFile getting processed by using keys of which pdf is one and --out specifies the source name for naming the resulting pdf. Source and destination path is the same.
sips can do a lot more. Run man sips in Terminal to read about this.
To run after having saved as Quick Action

Select source files in Finder
Right-click/Ctrl-click
Go down to Quick Action in the popupmenu and choose the script

Thanks to both @BabyBoy and @Martin Marconcini for pointing to a great solution. It's great when you can just use what you already got.
Answer: Preview has "Export as PDF" It's simple to use, and has always worked for me:
Comment: The OP explicitly asked "Is there any way to batch convert .jpg files into single .pdf files?" and you cannot batch covert to PDF in Preview!

How can I convert JPG into PDF easily?


Why do sites offer an MD5 hash of a file?

Question: Why do sites offer an MD5 hash of a file?  How does that help you verify the integrity/source of the file?  Wouldn't paying attention to your URL be enough security?
Comment: This is not a programming question.  Maybe better for Server Fault?  The idea of the hash is that you can better detect corruption of the file -- either deliberate or bit rot.
Comment: Every 2^10 to 2^12 bit is messed up, data is sent in packages and thy also contain hash (if it does not match package is resent.), because of this normal people do not even notice this. On heavy network load or couple more reasons failing rate increases. MD5 shows up little changes what passed the tests (with big difference).
Answer: It helps to verify that once you are done downloading the file, that you have the same file that is sitting on their server.
After running your local file through a hash function, if the slightest difference is found between your copy and theirs, it will result in you getting a different hash than what they have posted.
Answer: An MD5 hash doesn't protect you if the site gets hacked because the MD5 sum an be changed as well by the hacker. A GPG signature (in a separate file) would protect against this if you have a copy of the public key that you've gotten from somewhere else, or verified that it's the correct key by checking the signatures on the key.
What an MD5 hash is good for is making sure the file didn't get corrupted in the download process. TCP checksums don't all of the errors that could occur while your file is in transit (though those errors are rare), and I have seen an ISO image get corrupted during the download process before.
Answer: Security is one (important) thing, but I think it is also to make sure you downloaded the file in its entirety.
Comment: That's also checked when hashing the file, but filesize is much easier to use for that purpose.
Answer: Well in theory both files should produce the same hash if they are exactly the same (though collisions do occur).
You can provide it easily with a server side language, such as ol' trusty PHP...
<code>echo md5_file('my_file.zip');
</code>
Comment: Although that'll regenerate the MD5 hash for each pageview (if you don't have a cache in place), and generating an MD5 hash for a big file (hundreds of megabytes, for example a Linux distribution) can take a few minutes - more than PHP will usually swallow, and you don't want your users to wait for minutes for a page to generate. There's md5 commandline tools available for all OSes, better to use those and 'statically' add those to the list 'o downloads (or the public ftp folder).
Comment: @Cthulhu Yep, I was trying to keep the example relevant - but you will want to cache that hash, or better still calculate it once on acquiring the file - or have a background process do it.
Answer: It offers a defence against a possible man-in-the-middle attack. Suppose a malicious party had somehow been able to hijack the request for the file, but had not been able to alter the text of the web page itself. The MITM would be able to get you to download a different sequence of bytes, but would not be able to make the hash of his file match the hash quoted on the qweb page, that of the real file.
Comment: You only get protected against a MITM attack if you're sure you've obtained the reference hash from a trusted source.
Comment: @Bruno: However, though not often the case nowadays with higher connection speeds, you could get the hash from the official site and the file on local media from someone else (i.e. a *real* man in the middle ;).  This is also the principle behind BitTorrent.
Answer: There are several reasons:

a Man-in-the-Middle attack could possible alter your download and inject a virus or trojan or other malicious code in the download
It's also possible that some corruption occurred in the bitstream while downloading and the downloaded file might be broken.
There might be a proxy server involved which might cutoff the connection early.
Some older browser couldn't download files bigger than 2GB, so downloading bigger files would also mean a corrupted download.
It also allows a user to check if the "same" file on a mirror server is really the same file and not an altered version.
Answer: Sometimes, files transferred may be corrupt, either intentionally by an attacker or due to a faulty connection.
Cryptographic hash functions (such as MD5) are designed to change if there's a change in the original file. As such, you can detect such alterations.
Note that, if you want protection against intentional modification (made by an attacker), you need a way to verify that the reference hash you obtained is correct and from a trusted source (e.g. signature via PGP or X.509 certificates for example). You may also need a better hash algorithm (e.g. SHA-1, SHA-256), as collisions attacks have been demonstrated against MD5.


Older gas valve leaks when open

Question: I have an older home that has a gas fire starter and the valve is mounted in the brick.  The pipe in the box was capped and we have no idea how long the fireplace was not used before we bought the house.  Maybe 30+ years.  We installed a vent-free gas log set and when we open the valve immediately smelled gas coming from the packing around the valve stem.  The valve has a packing nut holding the stem in but I can't find a single online reference to this particular valve. The sleeve that threads onto the valve that the key goes thru is approximately twice as large as a new 1/2" valve.  We'd like to avoid at almost all costs having to disrupt the brick to repair the valve.  Anyone recognize this valve?
Comment: I really can not recommend that amateurs do _anything_ with gas lines. The consequences of a mistake are unacceptable. It _is_ worth hiring a pro.
Comment: What is on the other side of those blocks?  Blocks are usually 8 or 10 inches thick so that hole goes most of way though.  Will need to find a gas shut off valve before doing any work on repairing/replacing that valve.
Comment: under that hex nut is the exterior seal (o-ring), that is not the shut off seal. Replacing it might stop the leak
Answer: You should try to repack the valve yourself. First, with the gas supply isolated (shut off), try just tightening the packing nut and see if the leak stops. If it still leaks, you should be able to remove that packing nut and repack the packing gland.
On a valve that old it is likely that it is not an o-ring but likely some other composite material. You can get a twist of teflon packing string and put a loop or two on top of the existing packing and crank that nut down again, and try it.
If it still leaks, use tools to remove all the old packing, buy a few different sizes and shapes of packing washers (trim them with a knife to shape them if needed), throw a couple turns of the teflon string on top, and try again.
Answer: Most valves have two seals.
One inside at the bottom of the valve for open/close.
The second one on top to seal the shaft and stop the leaks to the outside.
In your case the top one is leaking after not been used for very long time.
You can try just tightening the hex nut (gently) and see if the leak stops,.
If not, you need to replace it (but closing the gas main first, then removing the hex nut and replacing the seal. Do not over-tighten it, it only needs to be compressed.

Older gas valve leaks when open


Tool for creating one pager

Question: I wonder if there is a specific tool for creating one pager cv's like the one bellow, besides photoshop or other high professional skill tools.

http://images-cdn.9gag.com/photo/a09NWOZ_700b_v1.jpg
Comment: Even if you can pull off a well-designed resume, you probably shouldn't. Google around for things like "HR fancy resume sins problem" and look for HR people telling you what not to do. It is a hard fact that many HR departments are snowed under with resumes and sometimes they cull the herd based on somewhat irrational or rapid processes. "Standing out" may be another way of saying "easy target." That particular resume you linked? A hot mess that demands cognitive effort to decipher. It goes into the round file..
Comment: Theoretically you could make this with Microsoft Office software, but the "high professional skill tools" are typically the tool of choice because they generally make things easier for you. In any case, what have you tried?
Comment: You can use any software. But note that these aren't usually what many would call 'well designed' resumes. They may be trendy, but not easy to pull off successfully.
Answer: I agree with Johannes- what you are looking to do can be accomplished with Microsoft Word or any other document editor that can layout text and images like OpenOffice. The amount of skill to create a page like your example would be about the same in any software.
Also, Photoshop would not be the software to use anyhow. Photoshop is an image editor or a digital layout tool. If you plan on printing it, you would use InDesign or any other document editor.
Comment: Even notepad! Though its not for non-experts!

Tool for creating one pager


What is the difference between "(no default, initially xxx)" and "(default xxx)" in the TikZ manual?

Question:
This is not the first time I encounter this problem. Are there any differences between "(no default, initially xxx)" and "(default xxx)"? Thank you.
Comment: If my answer fulfills your request, consider to accept it please.
Comment: @ChristianHupfer:Sorry, I have no experience with `tcolorbox` and I have not fully understood your explanation. Life is busy now. Please give me some time.
Comment: I just used `tcolorbox` as  a package that has `pgfkeys` and `tikz` syntax -- since you provided only a screen shot of code that does not compile for me, I switched to working code.
Answer: The pgfkeys documentation gives some clue: .initial is the value stored to the key at the time of definition, .default is the value that is used, if the user does not specify a value for the key
Example with tcolorbox (which uses pgfkeys and tikz in the backend)
The key lower separated is a boolean key, which is set to true in the beginning (initially), but it is sufficient to say lower separated only as the key, omitting =true, this way it is not necessary to remember the default value.
Now, the colframe key option is initially set to black!75!white, so this color will be used if colframe is not specified otherwise, but it has no default value, so just saying colframe is no valid syntax then.
The example from the O.P. does not compile for me, but initially lineto means that the key is set to lineto from the beginning and is used this way, but no default means, that it is not possible to say just pre, i.e a value must be specified with this key then.
We see the same behaviour in almost any key-value interface codes.
\documentclass{article}

\usepackage[most]{tcolorbox}

\begin{document}

\begin{tcolorbox}[lower separated]

Foo

\tcblower

foobar
\end{tcolorbox}

\end{document}

Answer: To complement Christian's answer, it may help to distinguish "code keys" (which have a /.code handler) and "value keys" (which merely store a value). Indeed, a single key can be both (and more), but the key point is that the /.default handler takes care of default arguments for /.codehandlers, while the /.initial handler takes care of the initial value (stored in \pgfkeys@<key> and accessed through e.g. \pgfkeysvalueof and \pgfkeyssetvalue). This answer may help to grasp the distinction.
Anyway, let us consider an example, with only pgfkeys:
\documentclass{article}
\usepackage{pgfkeys}
\begin{document}

\pgfkeys{/code key with initial/.initial={initial value}, /code key with initial/.code={code key with initial(#1)}}
\pgfkeys{/code key with default/.default={default value}, /code key with default/.code={code key with default(#1)}}

\pgfkeys{/value key with initial/.initial={initial value}}
\pgfkeys{/value key with default/.default={default value}}

\begin{enumerate}
    \item \pgfkeys{/code key with initial}
    % => code key with initial()
    \item \pgfkeys{/code key with default}
    % => code key with default(default value)
    \item \pgfkeys{/code key with initial=argument}
    % => code key with initial(argument)
    \item \pgfkeys{/code key with default=argument}
    % => code key with default(argument)
    \item \pgfkeysvalueof{/value key with initial}
    % => initial value
    \item \pgfkeysvalueof{/value key with default}
    % => (nothing!)
    \item \pgfkeyssetvalue{/value key with initial}{new value} \pgfkeysvalueof{/value key with initial}
    % => new value
    \item \pgfkeyssetvalue{/value key with default}{new value} \pgfkeysvalueof{/value key with default}
    % => new value
\end{enumerate}

\end{document}

I defined two "code keys" so that they merely print their name and argument. When there is no argument, the default value is used instead (when defined) [and the initial value is not used at all]. When an argument is given, the default value is indeed not used.
Now, we can also use keys to store values, and retrieve them with \pgfkeysvalueof (among other possibilities). In this case, the value of the key is set to the value given to /.initial when the key is defined, as explained by Christian, and the default value has no effect.
Lastly, we could write a /.code handler for a key using the value of the same key, but then you enter a world of doom because you overload the default \pgfkeys@<key>/.@cmd (set by /.code) which would (temporarily! see the documentation of /.initial and this answer) set the value of the key to something ...
Concerning your example, what the documentation calls a "default value" is in fact an initial value, as we can check on the source, tikzlibrarydecorations.code.tex, where we find /pgf/decoration/pre/.initial=lineto. This key is used several lines after through \pgfkeysvalueof.
Here, using pre=[something] in tikz will modify the value of the key only locally (through the default handler) because everything is executed inside a group. We can reproduce this with a simple pgfkeys example,
\documentclass{article}
\usepackage{pgfkeys}
\begin{document}

\def\pgfkeysingroup#1{
    \bgroup
    \pgfkeys{#1}
    \egroup
}

\pgfkeys{/val/.initial={initial}, /do/.code={val=\pgfkeysvalueof{/val}}}

\pgfkeysingroup{do}
% => val=initial
\pgfkeysingroup{val=argument,do}
% => val=argument
\pgfkeysingroup{do}
% => val=initial
\end{document}

Contrast with the following:
\documentclass{article}
\usepackage{pgfkeys}
\begin{document}

\pgfkeys{/val/.initial={initial}, /do/.code={val=\pgfkeysvalueof{/val}}}

\pgfkeys{do}
% => val=initial
\pgfkeys{val=argument,do}
% => val=argument
\pgfkeys{do}
% => val=argument
\end{document}


Resolve lack of access in Keychain Access

Question: I was trying to show a password in Keychain Access, so I put my password in to view it.  It then displayed this message: "The provided password did not unlock this keychain, or access has been limited to a particular user account."  My account is the administrator account, so I don't know what could be causing this.  Thanks.
Comment: do you have other users ?
Comment: @Buscar웃 Yeah, I do.
Comment: What keychain is the password stored in? Also, can you view its access controls (double-click it in Keychain Access, then click the Access Control tab)?
Comment: that would mean that the other user is still logged in, so log it out.
Comment: @Buscar웃 is there a way to log that account out without knowing the password?
Comment: not that I know of
Answer: You can log it out without the password. First press command+space and search activity monitor and press enter, then in the search bar in the corner search "loginwindow" without quotations and double click on the one that has the other users name by it. After that you should see a quit option. click it and it will give you 3 choices. Cancel, force quit, and quit. i recommend only force quitting if it isn't responding or if it doesn't work with regular quit. Also, this deletes any unsaved files and/or open tabs you have without saving them, so you might lose some data. About the Keychain password, i have the same question.