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• Static error page served by nginx when my application is down

  - by dreeves

  If my (Rails) application is down or undergoing database maintenance or whatever, I'd like to specify at the nginx level to server a static page. So every URL like http://example.com/* should server a static html file, like /var/www/example/foo.html. Trying to specify that in my nginx config is giving me fits and infinite loops and whatnot. I'm trying things like location / { root /var/www/example; index foo.html; rewrite ^/.+$ foo.html; } How would you get every URL on your domain to serve a single static file?

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• ssh sessions in xterms freeze for many minutes whenever they disconnect

  - by dreeves

  If I ssh to a remote machine and then lose internet connectivity, the session freezes. I can't control-c or otherwise abort and go back to my local xterm or terminal prompt but if I wait several minutes it will do so. There must be some way to force it to abort the remote ssh session when connectivity is lost. I'm on a Mac but I believe this happens on cygwin or linux as well.

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• True middle click on Mac trackpad under Leopard.

  - by dreeves

  The answers here http://superuser.com/questions/13351/how-to-middle-click-on-new-macbooks seem to give solutions that work for older versions of OSX, and one works for Snow Leopard. But none of them seem to work for Leopard. Does anyone have a solution to generate a true middle click without an external mouse, under Leopard?

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• Preview.app only auto-refreshes if you alt-tab to it.

  - by dreeves

  I'm working on a program that generates an image. I can keep the image open in Preview and when the image changes, Preview refreshes it, but only when I alt-tab (sorry, command-tab) to it. I'd like to keep the image in view and have it auto-refresh without touching it. Is that possible? (One ridiculous way I've found to do this is with a script that converts the image to a PDF file which I can keep open in Skim.app (a PDF reader) which has that ability.)

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• Android: Haptic feedback: onClick() event vs hapticFeedbackEnabled in the view

  - by dreeves

  If you want a button to provide haptic feedback (ie, the phone vibrates very briefly so you can feel that you really pushed the button), what's the standard way to do that? It seems you can either explicitly set an onClick() event and call the vibrate() function, giving a number of milliseconds to vibrate, or you can set hapticFeedbackEnabled in the view. The documentation seems to indicate that the latter only works for long-presses or virtual on-screen keys: http://developer.android.com/reference/android/view/View.html#performHapticFeedback(int) If that's right, then I need to either make my button a virtual on-screen key or manually set the onClick() event. What do you recommend? Also, if I want the vibrating to happen immediately when the user's finger touches the button, as opposed to when their finger "releases" the button, what's the best way to accomplish that? Related question: http://stackoverflow.com/questions/2228151/how-to-enable-haptic-feedback-on-button-view

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• Sample uniformly at random from an n-dimensional unit simplex.

  - by dreeves

  Sampling uniformly at random from an n-dimensional unit simplex is the fancy way to say that you want n random numbers such that they are all non-negative, they sum to one, and every possible vector of n non-negative numbers that sum to one are equally likely. In the n=2 case you want to sample uniformly from the segment of the line x+y=1 (ie, y=1-x) that is in the positive quadrant. In the n=3 case you're sampling from the triangle-shaped part of the plane x+y+z=1 that is in the positive octant of R3: (Image from http://en.wikipedia.org/wiki/Simplex.) Note that picking n uniform random numbers and then normalizing them so they sum to one does not work. You end up with a bias towards less extreme numbers. Similarly, picking n-1 uniform random numbers and then taking the nth to be one minus the sum of them also introduces bias. Wikipedia gives two algorithms to do this correctly: http://en.wikipedia.org/wiki/Simplex#Random_sampling (Though the second one currently claims to only be correct in practice, not in theory. I'm hoping to clean that up or clarify it when I understand this better. I initially stuck in a "WARNING: such-and-such paper claims the following is wrong" on that Wikipedia page and someone else turned it into the "works only in practice" caveat.) Finally, the question: What do you consider the best implementation of simplex sampling in Mathematica (preferably with empirical confirmation that it's correct)? Related questions http://stackoverflow.com/questions/2171074/generating-a-probability-distribution http://stackoverflow.com/questions/3007975/java-random-percentages

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• Scrape HTML tables from a given URL into CSV

  - by dreeves

  I seek a tool that can be run on the command line like so: tablescrape 'http://someURL.foo.com' [n] If n is not specified and there's more than one HTML table on the page, it should summarize them (header row, total number of rows) in a numbered list. If n is specified or if there's only one table, it should parse the table and spit it to stdout as CSV or TSV. Potential additional features: To be really fancy you could parse a table within a table, but for my purposes -- fetching data from wikipedia pages and the like -- that's overkill. The Perl module HTML::TableExtract can do this and may be good place to start for writing the tool I have in mind. An option to asciify any unicode. An option to apply an arbitrary regex substitution for fixing weirdnesses in the parsed table. Related questions: http://stackoverflow.com/questions/259091/how-can-i-scrape-an-html-table-to-csv http://stackoverflow.com/questions/1403087/how-can-i-convert-an-html-table-to-csv http://stackoverflow.com/questions/2861/options-for-html-scraping

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• Mathematica regular expressions on unicode strings.

  - by dreeves

  This was a fascinating debugging experience. Can you spot the difference between the following two lines? StringReplace["–", RegularExpression@"[\\s\\S]" -> "abc"] StringReplace["-", RegularExpression@"[\\s\\S]" -> "abc"] They do very different things when you evaluate them. It turns out it's because the string being replaced in the first line consists of a unicode en dash, as opposed to a plain old ascii dash in the second line. In the case of the unicode string, the regular expression doesn't match. I meant the regex "[\s\S]" to mean "match any character (including newline)" but Mathematica apparently treats it as "match any ascii character". How can I fix the regular expression so the first line above evaluates the same as the second? Alternatively, is there an asciify filter I can apply to the strings first? PS: The Mathematica documentation says that its string pattern matching is built on top of the Perl-Compatible Regular Expressions library (http://pcre.org) so the problem I'm having may not be specific to Mathematica.

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• How do I convert a date/time to epoch time in Perl?

  - by dreeves

  Given a date/time as an array of (year, month, day, hour, minute, second), how would you convert it to epoch time, i.e. the number of seconds since 1970-01-01 00:00:00 GMT? Bonus question: If given the date/time as a string, how would you first parse it into the (y,m,d,h,m,s) array?

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• Curve fitting: Find the smoothest function that satisfies a list of constraints.

  - by dreeves

  Consider the set of non-decreasing surjective (onto) functions from (-inf,inf) to [0,1]. (Typical CDFs satisfy this property.) In other words, for any real number x, 0 <= f(x) <= 1. The logistic function is perhaps the most well-known example. We are now given some constraints in the form of a list of x-values and for each x-value, a pair of y-values that the function must lie between. We can represent that as a list of {x,ymin,ymax} triples such as constraints = {{0, 0, 0}, {1, 0.00311936, 0.00416369}, {2, 0.0847077, 0.109064}, {3, 0.272142, 0.354692}, {4, 0.53198, 0.646113}, {5, 0.623413, 0.743102}, {6, 0.744714, 0.905966}} Graphically that looks like this: We now seek a curve that respects those constraints. For example: Let's first try a simple interpolation through the midpoints of the constraints: mids = ({#1, Mean[{#2,#3}]}&) @@@ constraints f = Interpolation[mids, InterpolationOrder->0] Plotted, f looks like this: That function is not surjective. Also, we'd like it to be smoother. We can increase the interpolation order but now it violates the constraint that its range is [0,1]: The goal, then, is to find the smoothest function that satisfies the constraints: Non-decreasing. Tends to 0 as x approaches negative infinity and tends to 1 as x approaches infinity. Passes through a given list of y-error-bars. The first example I plotted above seems to be a good candidate but I did that with Mathematica's FindFit function assuming a lognormal CDF. That works well in this specific example but in general there need not be a lognormal CDF that satisfies the constraints.

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• How do I convert a date/time to epoch time (aka unix time -- seconds since 1970) in Perl?

  - by dreeves

  Given a date/time as an array of (year, month, day, hour, minute, second), how would you convert it to epoch time, i.e., the number of seconds since 1970-01-01 00:00:00 GMT? Bonus question: If given the date/time as a string, how would you first parse it into the (y,m,d,h,m,s) array?

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• String munging in Objective-C with NSAttributedString.

  - by dreeves

  I have an NSAttributedString s and an integer i and I'd like a function that takes s and i and returns a new NSAttributedString that has a (stringified) i prepended to s. It looks like some combination of -stringWithFormat:, -initWithString:, and -insertAttributedString: would do it but I'm having trouble piecing it together without a lot of convolution and temporary variables. More generally, pointers to guides on making sense of NSAttributedString and NSMutableAttributedString would be awesome.

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• Slurping a file into a string in Perl

  - by dreeves

  Yes, There's More Than One Way To Do It™, but what is the most canonical, most efficient, and most concise methods to read an entire file into a string? (The latter two are perhaps at odds.)

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• Lexical and dynamic scoping in Mathematica: Local variables with Module, With, and Block

  - by dreeves

  The following code returns 14 as you'd expect: Block[{expr}, expr = 2 z; f[z_] = expr; f[7]] But if you change that Block to a Module then it returns 2*z. It seems to not matter what other variables besides expr you localize. I thought I understood Module, Block, and With in Mathematica but I can't explain the difference in behavior between Module and Block in this example. Related resources: Tutorial on Modularity and the Naming of Things from the Mathematica documentation Excerpt from a book by Paul R. Wellin, Richard J. Gaylord, and Samuel N. Kamin Explanation from Dave Withoff on the Mathematica newsgroup

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• Rot13 for numbers.

  - by dreeves

  EDIT: Now a Major Motion Blog Post at http://messymatters.com/sealedbids The idea of rot13 is to obscure text, for example to prevent spoilers. It's not meant to be cryptographically secure but to simply make sure that only people who are sure they want to read it will read it. I'd like to do something similar for numbers, for an application involving sealed bids. Roughly I want to send someone my number and trust them to pick their own number, uninfluenced by mine, but then they should be able to reveal mine (purely client-side) when they're ready. They should not require further input from me or any third party. (Added: Note the assumption that the recipient is being trusted not to cheat.) It's not as simple as rot13 because certain numbers, like 1 and 2, will recur often enough that you might remember that, say, 34.2 is really 1. Here's what I'm looking for specifically: A function seal() that maps a real number to a real number (or a string). It should not be deterministic -- seal(7) should not map to the same thing every time. But the corresponding function unseal() should be deterministic -- unseal(seal(x)) should equal x for all x. I don't want seal or unseal to call any webservices or even get the system time (because I don't want to assume synchronized clocks). (Added: It's fine to assume that all bids will be less than some maximum, known to everyone, say a million.) Sanity check: > seal(7) 482.2382 # some random-seeming number or string. > seal(7) 71.9217 # a completely different random-seeming number or string. > unseal(seal(7)) 7 # we always recover the original number by unsealing.

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• Convert array to CSV/TSV-formated string in Python.

  - by dreeves

  Python provides csv.DictWriter for outputting CSV to a file. What is the simplest way to output CSV to a string or to stdout? For example, given a 2D array like this: [["a b c", "1,2,3"], ["i \"comma-heart\" you", "i \",heart\" u, too"]] return the following string: "a b c, \"1, 2, 3\"\n\"i \"\"comma-heart\"\" you\", \"i \"\",heart\"\" u, too\"" which when printed would look like this: a b c, "1,2,3" "i ""heart"" you", "i "",heart"" u, too" (I'm taking csv.DictWriter's word for it that that is in fact the canonical way to output that array as CSV. Excel does parse it correctly that way, though Mathematica does not. From a quick look at the wikipedia page on CSV it seems Mathematica is wrong.) One way would be to write to a temp file with csv.DictWriter and read it back with csv.DictReader. What's a better way? TSV instead of CSV It also occurs to me that I'm not wedded to CSV. TSV would make a lot of the headaches with delimiters and quotes go away: just replace tabs with spaces in the entries of the 2D array and then just intersperse tabs and newlines and you're done. Let's include solutions for both TSV and CSV in the answers to make this as useful as possible for future searchers.

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• Import and Export for CSV are both broken in Mathematica

  - by dreeves

  Consider the following 2 by 2 array: x = {{"a b c", "1,2,3"}, {"i \"comma-heart\" you", "i \",heart\" u, too"}} If we Export that to CSV and then Import it again we don't get the same thing back: Import[Export["tmp.csv", d]] Looking at tmp.csv it's clear that the Export didn't work, since the quotes are not escaped properly. According to the RFC which I presume is summarized correctly on Wikipedia's entry on CSV, the right way to export the above array is as follows: a b c, "1,2,3" "i ""heart"" you", "i "",heart"" u, too" Importing the above does not yield the original array either. So Import is broken as well. I've reported these bugs to [email protected] but I'm wondering if others have workarounds in the meantime. One workaround is to just use TSV instead of CSV. I tested the above with TSV and it seems to work (even with tabs embedded in the entries of the array).

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• How do I compute the approximate entropy of a bit string?

  - by dreeves

  Is there a standard way to do this? Googling -- "approximate entropy" bits -- uncovers multiple academic papers but I'd like to just find a chunk of pseudocode defining the approximate entropy for a given bit string of arbitrary length. (In case this is easier said than done and it depends on the application, my application involves 16,320 bits of encrypted data (cyphertext). But encrypted as a puzzle and not meant to be impossible to crack. I thought I'd first check the entropy but couldn't easily find a good definition of such. So it seemed like a question that ought to be on StackOverflow! Ideas for where to begin with de-cyphering 16k random-seeming bits are also welcome...) See also this related question: http://stackoverflow.com/questions/510412/what-is-the-computer-science-definition-of-entropy

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• Running bundle install fails trying to remote fetch from rubygems.org/quick/Marshal...

  - by dreeves

  I'm getting a strange error when doing bundle install: $ bundle install Fetching source index for http://rubygems.org/ rvm/rubies/ree-1.8.7-2010.02/lib/ruby/site_ruby/1.8/rubygems/remote_fetcher.rb:304 :in `open_uri_or_path': bad response Not Found 404 (http://rubygems.org/quick/Marshal.4.8/resque-scheduler-1.09.7.gemspec.rz) (Gem::RemoteFetcher::FetchError) I've tried bundle update, gem source -c, gem update --system, gem cleanup, etc etc. Nothing seems to solve this. I notice that the URL beginning with http://rubygems.org/quick does seem to be a 404 -- I don't think that's any problem with my network, though if that's reachable for anyone else then that would be a simple explanation for my problem. More hints: If I just gem install resque-scheduler it works fine: $ gem install resque-scheduler Successfully installed resque-scheduler-1.9.7 1 gem installed Installing ri documentation for resque-scheduler-1.9.7... Installing RDoc documentation for resque-scheduler-1.9.7... And here's my Gemfile: source 'http://rubygems.org' gem 'json' gem 'rails', '>=3.0.0' gem 'mongo' gem 'mongo_mapper', :git => 'git://github.com/jnunemaker/mongomapper', :branch => 'rails3' gem 'bson_ext', '1.1' gem 'bson', '1.1' gem 'mm-multi-parameter-attributes', :git=>'git://github.com/rlivsey/mm-multi-parameter-attributes.git' gem 'devise', '~>1.1.3' gem 'devise_invitable', '~> 0.3.4' gem 'devise-mongo_mapper', :git => 'git://github.com/collectiveidea/devise-mongo_mapper' gem 'carrierwave', :git => 'git://github.com/rsofaer/carrierwave.git' , :branch => 'master' gem 'mini_magick' gem 'jquery-rails', '>= 0.2.6' gem 'resque' gem 'resque-scheduler' gem 'SystemTimer' gem 'capistrano' gem 'will_paginate', '3.0.pre2' gem 'twitter', '~> 1.0.0' gem 'oauth', '~> 0.4.4'

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• A database of questions with unambiguous numeric answers.

  - by dreeves

  I (and co-hackers) are building a sort of trivia game inspired by this blog post: http://messymatters.com/calibration. The idea is to give confidence intervals and learn how to be calibrated (when you're "90% sure" you should be right 90% of the time). We're thus looking for, ideally, thousands of questions with unambiguous numerical answers. Also, they shouldn't be too boring. There are a lot of random statistics out there -- eg, enclosed water area in different countries -- that would make the game mind-numbing. Things like release dates of classic movies are more interesting (to most people). Other interesting ones we've found include Olympic records, median incomes for different professions, dates of famous inventions, and celebrity ages. Scraping things like above, by the way, was my reason for asking this question: http://stackoverflow.com/questions/2611418/scrape-html-tables So, if you know of other sources of interesting numerical facts (in a parsable form) I'm eager for pointers to them. Thanks!

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• Equivalent of LaTeX's \label and \ref in HTML.

  - by dreeves

  I have an FAQ in HTML (example) in which the questions refer to each other a lot. That means whenever we insert/delete/rearrange the questions, the numbering changes. LaTeX solves this very elegantly with \label and \ref -- you give items simple tags and LaTeX worries about converting to numbers in the final document. How do people deal with that in HTML? ADDED: Note that this is no problem if you don't have to actually refer to items by number, in which case you can set a tag with <a name="foo"> and then link to it with <a href="#foo">some non-numerical way to refer to foo</a>. But I'm assuming "foo" has some auto-generated number, say from an <ol> list, and I want to use that number to refer to and link to it.

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• Named arguments in Mathematica.

  - by dreeves

  What's the best/canonical way to define a function with optional named arguments? To make it concrete, let's create a function foo with named arguments a, b, and c, which default to 1, 2, and 3, respectively. For comparison, here's a version of foo with positional arguments: foo[a_:1, b_:2, c_:3] := bar[a,b,c] Here is sample input and output for the named-arguments version of foo: foo[] --> bar[1,2,3] foo[b->7] --> bar[1,7,3] foo[a->6, b->7, c->8] --> bar[6,7,8] It should of course also be easy to have positional arguments before the named arguments.

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• Curve fitting: Find a CDF (or any function) that satisfies a list of constraints.

  - by dreeves

  I have some constraints on a CDF in the form of a list of x-values and for each x-value, a pair of y-values that the CDF must lie between. We can represent that as a list of {x,y1,y2} triples such as constraints = {{0, 0, 0}, {1, 0.00311936, 0.00416369}, {2, 0.0847077, 0.109064}, {3, 0.272142, 0.354692}, {4, 0.53198, 0.646113}, {5, 0.623413, 0.743102}, {6, 0.744714, 0.905966}} Graphically that looks like this: And since this is a CDF there's an additional implicit constraint of {Infinity, 1, 1} Ie, the function must never exceed 1. Also, it must be monotone. Now, without making any assumptions about its functional form, we want to find a curve that respects those constraints. For example: (I cheated to get that one: I actually started with a nice log-normal distribution and then generated fake constraints based on it.) One possibility is a straight interpolation through the midpoints of the constraints: mids = ({#1, Mean[{#2,#3}]}&) @@@ constraints f = Interpolation[mids, InterpolationOrder->0] Plotted, f looks like this: That sort of technically satisfies the constraints but it needs smoothing. We can increase the interpolation order but now it violates the implicit constraints (always less than one, and monotone): How can I get a curve that looks as much like the first one above as possible? Note that NonLinearModelFit with a LogNormalDistribution will do the trick in this example but is insufficiently general as sometimes there will sometimes not exist a log-normal distribution satisfying the constraints.

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• posmax: like argmax but gives the position(s) of the element x for which f[x] is maximal

  - by dreeves

  Mathematica has a built-in function ArgMax for functions over infinite domains, based on the standard mathematical definition. The analog for finite domains is a handy utility function. Given a function and a list (call it the domain of the function), return the element(s) of the list that maximize the function. Here's an example of finite argmax in action: http://stackoverflow.com/questions/471029/canonicalize-nfl-team-names/472213#472213 And here's my implementation of it (along with argmin for good measure): (* argmax[f, domain] returns the element of domain for which f of that element is maximal -- breaks ties in favor of first occurrence. *) SetAttributes[{argmax, argmin}, HoldFirst]; argmax[f_, dom_List] := Fold[If[f[#1]>=f[#2], #1, #2]&, First[dom], Rest[dom]] argmin[f_, dom_List] := argmax[-f[#]&, dom] First, is that the most efficient way to implement argmax? What if you want the list of all maximal elements instead of just the first one? Second, how about the related function posmax that, instead of returning the maximal element(s), returns the position(s) of the maximal elements?

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• What's a simple way to web-ify my command-line daemon?

  - by dreeves

  Suppose I have a simple daemon type script that I run on my webserver. I run it in a terminal, with gnu screen, so I can keep an eye on it. That works fine (incidentally, I use this trick). But now suppose I'd like to make a web page where I can keep an eye on my script's output. What's the easiest way to do that? Notes: This is mainly for myself and a couple co-hackers so if websockets is the answer and it only works on Chrome or something, that's acceptable. This question is asking something similar: http://stackoverflow.com/questions/1964494/how-to-make-all-connected-browsers. But I'm hoping for a simpler, quick-and-dirty solution, and especially a general way to quickly do this for any script I might want to keep an eye on from a browswer.

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