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• Parallelism in Python

  - by fmark

  What are the options for achieving parallelism in Python? I want to perform a bunch of CPU bound calculations over some very large rasters, and would like to parallelise them. Coming from a C background, I am familiar with three approaches to parallelism: Message passing processes, possibly distributed across a cluster, e.g. MPI. Explicit shared memory parallelism, either using pthreads or fork(), pipe(), et. al Implicit shared memory parallelism, using OpenMP. Deciding on an approach to use is an exercise in trade-offs. In Python, what approaches are available and what are their characteristics? Is there a clusterable MPI clone? What are the preferred ways of achieving shared memory parallelism? I have heard reference to problems with the GIL, as well as references to tasklets. In short, what do I need to know about the different parallelization strategies in Python before choosing between them?

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• GCC, functions, and pointer arguments, warning behaviour

  - by James Morris

  I've recently updated to a testing distribution, which is now using GCC 4.4.3. Now I've set everything up, I've returned to coding and have built my project and I get one of these horrible messages: *** glibc detected *** ./boxyseq: free(): invalid pointer: 0x0000000001d873e8 *** I absolutely know what is wrong here, but was rather confused as to when I saw my C code where I call a function which frees a dynamically allocated data structure - I had passed it an incompatible pointer type - a pointer to a completely different data structure. warning: passing argument 1 of 'data_A_free' from incompatible pointer type note: expected 'struct data_A *' but argument is of type 'struct data_B *' I'm confused because I'm sure this would have been an error before and compilation would never have completed. Is this not just going to make life more difficult for C programmers? Can I change it back to an error without making a whole bunch of other warnings errors too? Or am I loosing the plot and it's always been a warning?

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• PerformanceCounterCategory.Exists is very slow if category doesn't exists

  - by Shrike

  I have kind of library which uses a bunch of its own perf counters. But I want my library works fine even if that perf counters weren't installed. So I've created wrappers aroung PerformanceCounter and on the first use check if PerfCounter exists or not. If they exists then I'm using native PerformanceCounter instead I'm using a wrapper which do nothing. So to check perf counter existence I use PerformanceCounterCategory.Exists The problem is that if there isn't such category then PerformanceCounterCategory.Exists call takes (on my machine) about 10 seconds! Needless to say it's too slow. What can I do? Code to try it by youself: using System; using System.Diagnostics; class Program { static void Main(string[] args) { var ts = Stopwatch.StartNew(); var res = PerformanceCounterCategory.Exists("XYZ"); Console.WriteLine(ts.ElapsedMilliseconds); Console.WriteLine("result:" + res); } }

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• How can I instantiate a base class and then convert it to a derived class?

  - by Eric

  I was wondering how to do this, consider the following classes public class Fruit { public string Name { get; set; } public Color Color { get; set; } } public class Apple : Fruit { public Apple() { } } How can I instantiate a new fruit but upcast to Apple, is there a way to instantiate a bunch of Fruit and make them apples with the name & color set. Do I need to manually deep copy? Of course this fails Fruit a = new Fruit(); a.Name = "FirstApple"; a.Color = Color.Red; Apple wa = a as Apple; System.Diagnostics.Debug.Print("Apple name: " + wa.Name); Do I need to pass in a Fruit to the AppleCTor and manually set the name and color( or 1-n properties) Is there an better design to do this?

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• Regex Searching in Emacs

  - by Inaimathi

  I'm trying to write some Elisp code to format a bunch of legacy files. The idea is that if a file contains a section like "<meta name=\"keywords\" content=\"\\(.*?\\)\" />", then I want to insert a section that contains existing keywords. If that section is not found, I want to insert my own default keywords into the same section. I've got the following function: (defun get-keywords () (re-search-forward "<meta name=\"keywords\" content=\"\\(.*?\\)\" />") (goto-char 0) ;The section I'm inserting will be at the beginning of the file (or (march-string 1) "Rubber duckies and cute ponies")) ;;or whatever the default keywords are When the function fails to find its target, it returns Search failed: "[regex here]" and prevents the rest of evaluation. Is there a way to have it return the default string, and ignore the error?

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• Help needed with Javascript Variable Scope / OOP and Call Back Functions

  - by gargantaun

  I think this issue goes beyond typical variable scope and closure stuff, or maybe I'm an idiot. Here goes anyway... I'm creating a bunch of objects on the fly in a jQuery plugin. The object look something like this function WedgePath(canvas){ this.targetCanvas = canvas; this.label; this.logLabel = function(){ console.log(this.label) } } the jQuery plugin looks something like this (function($) { $.fn.myPlugin = function() { return $(this).each(function() { // Create Wedge Objects for(var i = 1; i <= 30; i++){ var newWedge = new WedgePath(canvas); newWedge.label = "my_wedge_"+i; globalFunction(i, newWedge]); } }); } })(jQuery); So... the plugin creates a bunch of wedgeObjects, then calls 'globalFunction' for each one, passing in the latest WedgePath instance. Global function looks like this. function globalFunction(indicator_id, pWedge){ var targetWedge = pWedge; targetWedge.logLabel(); } What happens next is that the console logs each wedges label correctly. However, I need a bit more complexity inside globalFunction. So it actually looks like this... function globalFunction(indicator_id, pWedge){ var targetWedge = pWedge; someSql = "SELECT * FROM myTable WHERE id = ?"; dbInterface.executeSql(someSql, [indicator_id], function(transaction, result){ targetWedge.logLabel(); }) } There's a lot going on here so i'll explain. I'm using client side database storage (WebSQL i call it). 'dbInterface' an instance of a simple javascript object I created which handles the basics of interacting with a client side database [shown at the end of this question]. the executeSql method takes up to 4 arguments The SQL String an optional arguments array an optional onSuccess handler an optional onError handler (not used in this example) What I need to happen is: When the WebSQL query has completed, it takes some of that data and manipulates some attribute of a particular wedge. But, when I call 'logLabel' on an instance of WedgePath inside the onSuccess handler, I get the label of the very last instance of WedgePath that was created way back in the plugin code. Now I suspect that the problem lies in the var newWedge = new WedgePath(canvas); line. So I tried pushing each newWedge into an array, which I thought would prevent that line from replacing or overwriting the WedgePath instance at every iteration... wedgeArray = []; // Inside the plugin... for(var i = 1; i <= 30; i++){ var newWedge = new WedgePath(canvas); newWedge.label = "my_wedge_"+i; wedgeArray.push(newWedge); } for(var i = 0; i < wedgeArray.length; i++){ wedgeArray[i].logLabel() } But again, I get the last instance of WedgePath to be created. This is driving me nuts. I apologise for the length of the question but I wanted to be as clear as possible. END ============================================================== Also, here's the code for dbInterface object should it be relevant. function DatabaseInterface(db){ var DB = db; this.sql = function(sql, arr, pSuccessHandler, pErrorHandler){ successHandler = (pSuccessHandler) ? pSuccessHandler : this.defaultSuccessHandler; errorHandler = (pErrorHandler) ? pErrorHandler : this.defaultErrorHandler; DB.transaction(function(tx){ if(!arr || arr.length == 0){ tx.executeSql(sql, [], successHandler, errorHandler); }else{ tx.executeSql(sql,arr, successHandler, errorHandler) } }); } // ---------------------------------------------------------------- // A Default Error Handler // ---------------------------------------------------------------- this.defaultErrorHandler = function(transaction, error){ // error.message is a human-readable string. // error.code is a numeric error code console.log('WebSQL Error: '+error.message+' (Code '+error.code+')'); // Handle errors here var we_think_this_error_is_fatal = true; if (we_think_this_error_is_fatal) return true; return false; } // ---------------------------------------------------------------- // A Default Success Handler // This doesn't do anything except log a success message // ---------------------------------------------------------------- this.defaultSuccessHandler = function(transaction, results) { console.log("WebSQL Success. Default success handler. No action taken."); } }

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• masking a UIImage

  - by iworkinprogress

  I'm working on an app that can change the borders or a rectangular UIImage. The borders will vary, but will look like the UIImage was cut out with scissors, or something to that affect. What is the best way to do this? My first thought is to prep a bunch of transparent PNGs with the correct border effect I'm looking for, and then somehow use that as a mask for my UIImage. Is this the right path? Or is there a more flexible programmatic way to do this?

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• Is it really wrong to version documents using CouchDB's default behaviour?

  - by Tomas Sedovic

  This is one of those "I know I shouldn't do this but it's oh so convenient." questions. Sorry about that. I plan to use CouchDB for storing a bunch of documents and keeping their entire revision history. CouchDB does the versioning automatically, but it is strongly discouraged for programmer's use: "You cannot rely on document revisions for any other purpose than concurrency control." From what I've found on the CouchDB wiki, the versions can get deleted either during compaction or during replication. As far as I can tell, Compaction must always be triggered manually and Replication occurs only when there's more than one database server. The question is: if I won't run compaction and will use only single database instance for my documents, can I just use CouchDB's document versioning and expect it to work? What other problems I might run into? E.g. does not running compaction hurt the performance or consume significantly more disk space (than if I did handle the versioning manually)?

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• How do you use kate? Tips/Tricks/Workflow

  - by Roman A. Taycher

  We all seen a bunch of these? Mostly for IDE's but also for vim and emacs. Kate is (only) a text editor (an awesome one) but it has a ton of options plus a number of plugins, so its hard to know all of it well. How do use the Kate text editor? Please share your workflow and help me and others learn some of the cool tricks you use. I'll start I use the built in terminal a lot opening files quickly, and using it as an enhanced haskell repl with ghci (since ghci doesn't allow you all to just put in all kinds of haskell code). Also use split views to quickly compare files (especially different versions of the same file). Also the auto-complete maybe simple(more use for saving typing time then remembering functions) but it works really well for that. Also if You highlight something and hit a start [/{/( it puts it in between brackets rather then replacing it with a bracket(why the hell do a lot of IDEs not have this feature).

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• TestCase scripting framework

  - by Mikey

  Hey guys, For our webapp testing environment we're currently using watin with a bunch of unit tests, and we're looking to move to selenium and use more frameworks. We're currently looking at Selenium2 + Gallio + Xunit.net, However one of the things we're really looking to get around is compiled testcases. Ideally we want testcases that can be edited in VS with intellisense, but don't require re-compilling the assembly every single time we make a small change, Are there any frameworks likely to help with this issue? Are there any nice UI tools to help manage massive ammount of testcases? Ideally we want the testcase writing process to be simple so that more testers can aid in writing them. cheers

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• jQuery: How to select the next element with name-xyz?

  - by Shpigford

  I'm trying to build a really simple generic toggle functionality where the toggling "switch" has a class of .toggle and then I want it to toggle() the next element that has the class .toggle-content. Example HTML: <p> <a href="#" class="toggle">Toggle the thing</a> </p> <p class="toggle-content hidden">I'm totally hidden right now</p> So right now I'd toggle that with: $(".toggle").click(function() { $(this).parent().next('.toggle-content').toggle(); }); The problem is if the .toggle class is any deeper in the DOM, I have to keep tacking on more parent()'s depending on how deep it is/isn't. So how can I just select the next instance of .toggle-content without having use a bunch of parent()'s and next()'s?

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• Embedding Youtube Videos on user's wall with the Javascript API

  - by dragonmantank

  I'm attempting to embed a Youtube video on a user's wall using the Javascript API. I've poured over a bunch of different tutorials and came up with this: var data = { method: 'feed', link: current_video_url, // Link to the vide on our site source: current_video_src, // Link to the Youtube video, http://youtube.com/v/[hash] picture: current_picture_url, // Thumbnail from youtube title: current_video_title, // Title from our page caption: current_video_description // Text from our page } FB.ui(data); Running the Facebook Debugger shows that the current_video_url link is being properly parsed for OG tags and is available as an embedded video. I ran a debugger and all the variables are being properly set in the data variable. This worked fine for about 2 days, and then we pushed the site live. We set up a new AppID in Facebook but now all the videos are just thumbnails instead of being embedded in the user's wall. I thought maybe this is a problem with the live site but now the dev site is broken too. What's the best way to get this to work consistently?

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• Zip up web page groups to view in browsers

  - by Arlen Beiler

  There should be a standard for saving viewing bunches of webpages as a website. For instance, say I have a whole bunch of pages, such as I get from the WordPress plugin "Really Static" which saves an entire site, and I have all the links start with a slash. Now, I can't really use those links if I am reading it from the file system. If there would be a standard where we could zip up files, give them a unique extension (like "hzip" for html zip), and open the file with any browser, which would display it as though the root of that file were the root of the pages. "http://example.com" The links would all work right. This would facilitate sharing and copying groups of webpages.

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• Recursive wildcards in GNU make?

  - by Roger Lipscombe

  It's been a while since I've used make, so bear with me... I've got a directory, flac, containing .FLAC files. I've got a corresponding directory, mp3 containing MP3 files. If a FLAC file is newer than the corresponding MP3 file (or the corresponding MP3 file doesn't exist), then I want to run a bunch of commands to convert the FLAC file to an MP3 file, and copy the tags across. The kicker: I need to search the flac directory recursively, and create corresponding subdirectories in the mp3 directory. And I want to use make to drive this.

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• How to calculate order (big O) for more complex algorithms (ie quicksort)

  - by bangoker

  I know there are quite a bunch of questions about big O notation, I have already checked Plain english explanation of Big O , Big O, how do you calculate/approximate it?, and Big O Notation Homework--Code Fragment Algorithm Analysis?, to name a few. I know by "intuition" how to calculate it for n, n^2, n! and so, however I am completely lost on how to calculate it for algorithms that are log n , n log n, n log log n and so. What I mean is, I know that Quick Sort is n log n (on average).. but, why? Same thing for merge/comb, etc. Could anybody explain me in a not to math-y way how do you calculate this? The main reason is that Im about to have a big interview and I'm pretty sure they'll ask for this kind of stuff. I have researched for a few days now, and everybody seem to have either an explanation of why bubble sort is n^2 or the (for me) unreadable explanation a la wikipedia Thanks!

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• How to deal with many to many relationships with NSFetchedResultsController?

  - by Phil Yates

  OK so I have two entities in my data model (let's say entityA and entityB), both of these entities have a to-many relationship to each other. I have setup a NSFetchedResultsController to fetch a bunch of entityA. Now I'm trying to have the section names for the tableview be the title of entityB. sectionNameKeyPath:@"entityB.title" Now this causes a problem, where by the section name returned from that relationship appears to be ({title1}) or ({title1,title2...titleN}) obviously depending on how many different entityB's are involved. This doesn't look great in a tableview and doesn't group the objects as I would like. What I would like is a section per entityB title with entityA appearing under each section, under multiple sections if necessary. I'm at a loss as how I am supposed to achieve this whether I need to update the predicate to get the entity to appear multiple times or whether I need to update the section and header functions to do some processing as the controller loops through the objects. Any help is appreciated :) Thanks

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• jQuery image grid effect

  - by anon

  I have an image sitting on a page that I want to create a grid type overlay (that covers the image with a black fill) which will be partitioned into 50x50 pixels (what ever size, tbh) squares. The squares on the grid will then flip over, one at a time, in random positions revealing the image below it. The only way I can think of accomplishing this would be to create a whole bunch of grid squares and overlay them on the image with jQuery, then flip each image square individually. This, though, would be a pain in the ass. Doing this all dynamically in jQuery is what I'm hoping to accomplish. Any ideas?

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• IoC from start to finish

  - by Dave

  I'm quite sure that IoC is the way to go for my application. There are a ton of articles and even questions here on SO that discuss the different containers. I've read several blogs today with partial examples. I am personally leaning towards starting with the CommonServiceLocator and Unity as two way to solve the same problem -- I just need a bunch of assemblies to get data from a database, which I assume is what needs to be injected everywhere. I've yet to find any sites that really take a problem from beginning to end, with concrete code examples. For example, I've yet to find one that discusses an IServiceLocator and how to actually register it (or do whatever is required to make it known). What are your favorite posts / articles / SO questions that can take a noob from start to finish with the implementation?

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• glibc backtrace - can't redirect output to file

  - by Jason Antman

  Hi, I'm in the process of debugging a C program (that I didn't write). I have all of the internal debugging tools (a whole bunch of printf's) enabled, and I wrote a small PHP script that uses proc_open() and just grabs both stdout and stderr, and time-coordinates them in one file. At the moment, the binary is dieing with a realloc() error that's caught by glibc, and a glibc backtrace is printed, beginning with: *** glibc detected *** /sbin/rsyslogd: realloc(): invalid next size: 0x00002ace626ac910 *** Here's the thing I don't understand: I've confirmed that the PHP script is catching both stdout and stderr from the binary's process and writing them to the correct files, but this backtrace is still printed to the console. Where is this coming from? Is there some magical output channel other than stdout and stderr? Any ideas on how I go about capturing this backtrace to a file, or sending it out with stderr? Thanks, Jason

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• .NET DataGridView rows' default alignment all set to left

  - by Xster

  DataGridView gets bound to DataTable. I then try to set some columns' DefaultCellStyle.Alignment to right. It doesn't work. After a bunch of tinkering, it turns out that every single row's DefaultCellStyle.Alignment is set to left and overrides it. DataGridView.DefaultCellStyle or RowsDefaultCellStyle doesn't do anything. Makes sense since a particular row's DefaultCellStyle is more specific but what gives. Why are they all set to something and not NotSet? How do I get rid of it instead of iterating through all rows?

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• Iterating Oracle collections of objects with out exploding them

  - by Scott Bailey

  I'm using Oracle object data types to represent a timespan or period. And I've got to do a bunch of operations that involve working with collections of periods. Iterating over collections in SQL is significantly faster than in PL/SQL. CREATE TYPE PERIOD AS OBJECT ( beginning DATE, ending DATE, ... some member functions...); CREATE TYPE PERIOD_TABLE AS TABLE OF PERIOD; -- sample usage SELECT <<period object>>.contains(period2) FROM TABLE(period_table1) t The problem is that the TABLE() function explodes the objects into scalar values, and I really need the objects instead. I could use the scalar values to recreate the objects but this would incur the overhead of re-instantiating the objects. And the period is designed to be subclassed so there would be additional difficulty trying to figure out what to initialize it as. Is there another way to do this that doesn't destroy my objects?

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• Merge overlapping triangles into a polygon

  - by nornagon

  I've got a bunch of overlapping triangles from a 3D model projected into a 2D plane. I need to merge each island of touching triangles into a closed, non-convex polygon. The resultant polygons shouldn't have any holes in them (since the source data doesn't). Many of the source triangles share (floating point identical) edges with other triangles in the source data. What's the easiest way to do this? Performance isn't particularly important, since this will be done at design time.

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• JavaScript Metaprogramming: Reduce boilerplate of adding functions to a function queue

  - by thurn

  I'm working with animation in JavaScript, and I have a bunch of functions you can call to add things to the animation queue. Basically, all of these functions look like this: function foo(arg1, arg2) { _eventQueue.push(function() { // actual logic } } I'm wondering now if it would be possible to cut down on this boilerplate a little bit, though, so I don't need that extra "_eventQueue" line in the function body dozens of times. Would it be possible, for example, to make a helper function which takes an arbitrary function as an argument and returns a new function which is augmented to be automatically added to the event queue? The only problem is that I need to find a way to maintain access to the function's original arguments in this process, which is... complicated.

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• Error running celeryd

  - by Eric Palakovich Carr

  I'm posting this question (and answer) so if anybody else has this problem in the future, you'll be able to google it. If you are trying to run celeryd in Django like so: python manage.py celeryd You can receive the following error immediately after it has started: celery@eric-desktop-dev has started. Traceback (most recent call last): File "manage.py", line 11, in <module> execute_manager(settings) <... snip ...> File "/usr/local/lib/python2.6/dist-packages/amqplib-0.6.1-py2.6.egg/amqplib/client_0_8/connection.py", line 134, in __init__ self._x_start_ok(d, login_method, login_response, locale) File "/usr/local/lib/python2.6/dist-packages/amqplib-0.6.1-py2.6.egg/amqplib/client_0_8/connection.py", line 704, in _x_start_ok args.write_longstr(response) File "/usr/local/lib/python2.6/dist-packages/amqplib-0.6.1-py2.6.egg/amqplib/client_0_8/serialization.py", line 352, in write_longstr self.write_long(len(s)) TypeError: object of type 'NoneType' has no len() A rather cryptic error message, with no real clue as to where to go to fix the problem. See below for the answer so you don't waste a bunch of time on this error like I did today :)

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• Implementing Role based Helpers

  - by Cynics

  So my question is how would you implement your handwritten Helpers based on the role of current user. Would it be efficient to change the behaviour at request time? e.g. the Helper somehow figures out the role of user, and include the proper SubModule? module ApplicationHelper module LoggedInHelper # Some functions end module GuestHelper # The Same functions end # If User is Guest then include GuestHelper # If User is LoggedIn then include LoggedInHelper end Is it efficient this way? is it rails way? I've got a whole bunch of function that act like this, and I don't want to wrap every single one of them in an if statement def menu_actions if current_user.nil? # User is guest { "Log in" => link_to "Login", "/login" } else # User is Logged In { "Log out" => link_to "Logout", "/logout" } end end Thank you for your time and thoughts.

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