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  • Python list comprehension overriding value

    - by Joschua
    Hi, folks have a look at the following piece of code, which shows a list comprehension.. >>> i = 6 >>> s = [i * i for i in range(100)] >>> print(i) When you execute the code example in Python 2.6 it prints 99, but when you execute it in Python 3.x it prints 6. What were the reason for changing the behaviour and why is the output 6 in Python 3.x? Thank you in advance!

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  • In Javascript, a function starts a new scope, but we have to be careful that the function must be in

    - by Jian Lin
    In Javascript, I am sometimes too immerged in the idea that a function creates a new scope, that sometimes I even think the following anonymous function will create a new scope when it is being defined and assigned to onclick: <a href="#" id="link1">ha link 1</a> <a href="#" id="link2">ha link 2</a> <a href="#" id="link3">ha link 3</a> <a href="#" id="link4">ha link 4</a> <a href="#" id="link5">ha link 5</a> <script type="text/javascript"> for (i = 1; i <= 5; i++) { document.getElementById('link' + i).onclick = function() { var x = i; alert(x); return false; } } </script> but in fact, the anonymous function will create a new scope, that's right, but ONLY when it is being invoked, is that so? So the x inside the anonymous function is not created, no new scope is created. When the function was later invoked, there is a new scope alright, but the i is in the outside scope, and the x gets its value, and it is all 6 anyways. The following code will actually invoke a function and create a new scope and that's why the x is a new local variable x in the brand new scope each time, and the invocation of the function when the link is clicked on will use the different x in the different scopes. <a href="#" id="link1">ha link 1</a> <a href="#" id="link2">ha link 2</a> <a href="#" id="link3">ha link 3</a> <a href="#" id="link4">ha link 4</a> <a href="#" id="link5">ha link 5</a> <script type="text/javascript"> for (var i = 1; i <= 5; i++) { (function() { var x = i; document.getElementById('link' + i).onclick = function() { alert(x); return false; } })(); // invoking it now! } </script> If we take away the var in front of x, then it is a global x and so no local variable x is created in the new scope, and therefore, clicking on the links get all the same number, which is the value of the global x.

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  • F# ref-mutable vars vs object fields

    - by rwallace
    I'm writing a parser in F#, and it needs to be as fast as possible (I'm hoping to parse a 100 MB file in less than a minute). As normal, it uses mutable variables to store the next available character and the next available token (i.e. both the lexer and the parser proper use one unit of lookahead). My current partial implementation uses local variables for these. Since closure variables can't be mutable (anyone know the reason for this?) I've declared them as ref: let rec read file includepath = let c = ref ' ' let k = ref NONE let sb = new StringBuilder() use stream = File.OpenText file let readc() = c := stream.Read() |> char // etc I assume this has some overhead (not much, I know, but I'm trying for maximum speed here), and it's a little inelegant. The most obvious alternative would be to create a parser class object and have the mutable variables be fields in it. Does anyone know which is likely to be faster? Is there any consensus on which is considered better/more idiomatic style? Is there another option I'm missing?

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  • Multiple Timers with setTimeInterval

    - by visibleinvisibly
    I am facing a problem with setInterval being used in a loop. I have a function subscribeFeed( ) which takes an array of urls as input. It loops through the url array and subscribes each url to getFeedAutomatically() using a setInterval function. so if three URL's are there in the array, then 3 setInterval's will be called. The problem is 1)how to distinguish which setInterval is called for which URL. 2)it is causing Runtime exception in setInterval( i guess because of closure problem in javascript) //constructor function myfeed(){ this.feedArray = []; } myfeed.prototype.constructor= myfeed; myfeed.prototype.subscribeFeed =function(feedUrl){ var i=0; var url; var count = 0; var _this = this; var feedInfo = { url : [], status : "" }; var urlinfo = []; feedUrl = (feedUrl instanceof Array) ? feedUrl : [feedUrl]; //notifyInterval = (notifyInterval instanceof Array) ? notifyInterval: [notifyInterval]; for (i = 0; i < feedUrl.length; i++) { urlinfo[i] = { url:'', notifyInterval:5000,// Default Notify/Refresh interval for the feed isenable:true, // true allows the feed to be fetched from the URL timerID: null, //default ID is null called : false, position : 0, getFeedAutomatically : function(url){ _this.getFeedUpdate(url); }, }; urlinfo[i].url = feedUrl[i].URL; //overide the default notify interval if(feedUrl[i].NotifyInterval /*&& (feedUrl[i] !=undefined)*/){ urlinfo[i].notifyInterval = feedUrl[i].NotifyInterval; } // Trigger the Feed registered event with the info about URL and status feedInfo.url[i] = feedUrl[i].URL; //Set the interval to get the feed. urlinfo[i].timerID = setInterval(function(){ urlinfo[i].getFeedAutomatically(urlinfo[i].url); }, urlinfo[i].notifyInterval); this.feedArray.push(urlinfo[i]); } } // The getFeedUpate function will make an Ajax request and coninue myfeed.prototype.getFeedUpdate = function( ){ } I am posting the same on jsfiddle http://jsfiddle.net/visibleinvisibly/S37Rj/ Thanking you in advance

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  • how to keep the value of a variable in a closure

    - by Florian Fida
    i need to create multiple javascript functions which have a static id inside, so the function itself knows what data to process. Here is some code: (function(){ function log(s){ if(console && console.log) console.log(s); else alert(s); } var i = 10; while (i--){ window.setTimeout(function(){ // i need i to be 10, 9, 8... here not -1 log(i); },500); } })(); The problem ist that i allways gets updated by the loop, and i need to prevent this. Thanks in advance for any help, comments or tips!

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  • modified closure warning in ReSharper

    - by Sarah Vessels
    I was hoping someone could explain to me what bad thing could happen in this code, which causes ReSharper to give an 'Access to modified closure' warning: bool result = true; foreach (string key in keys.TakeWhile(key => result)) { result = result && ContainsKey(key); } return result; Even if the code above seems safe, what bad things could happen in other 'modified closure' instances? I often see this warning as a result of using LINQ queries, and I tend to ignore it because I don't know what could go wrong. ReSharper tries to fix the problem by making a second variable that seems pointless to me, e.g. it changes the foreach line above to: bool result1 = result; foreach (string key in keys.TakeWhile(key => result1)) Update: on a side note, apparently that whole chunk of code can be converted to the following statement, which causes no modified closure warnings: return keys.Aggregate( true, (current, key) => current && ContainsKey(key) );

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  • accessing my public methods from within my namespace

    - by Derek Adair
    I am in the process of making my own namespace in JavaScript... (function(window){ (function(){ var myNamespace = { somePublicMethod: function(){ }, anotherPublicMethod: function(){ } } return (window.myNamespace = window.my = myNamespace) }()); })(window); I'm new to these kinds of advanced JavaScript techniques and i'm trying to figure out the best way to call public methods from within my namespace. It appears that within my public methods this is being set to myNamespace. Should I call public methods like... AnotherPublicMethod: function(){ this.somePublicMethod() } or... AnotherPublicMethod: function(){ my.somePublicMethod(); } is there any difference?

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  • Javascript OOP - accessing the inherited property or function from a closure within a subclass

    - by Ali
    Hi All, I am using the javascript inheritance helper provided here: http://ejohn.org/blog/simple-javascript-inheritance/ I have the following code, and I have problem accessing the inherited property or function from a closure within a subclass as illustrated below. I am new to OOP javascript code and I appreciate your advice. I suppose within the closure, the context changes to JQuery (this variable) hence the problem. I appreciate your comments. Thanks, -A PS - Using JQuery 1.5 var Users = Class.extend({ init: function(names){this.names = names;} }); var HomeUsers = Users.extend({ work:function(){ // alert(this.names.length); // PRINTS A // var names = this.names; // If I make a local alias it works $.map([1,2,3],function(){ var newName = this.names.length; //error this.names is not defined. alert(newName); }); } }); var users = new HomeUsers(["A"]); users.work();

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  • Is it possible to create a throttle function that can take in as parameters another function (that also has parameters), and the time delay

    - by Stan Quinn
    So I've already written a function that works (based on underscores throttle) for functions that don't take in a parameter, but I'd like to make it generic enough to pass in a function with a variable number of parameters. Here's what I have: (function () { var lastTime = new Date().getTime(); function foo() { var newTime = new Date().getTime(); var gap = newTime - lastTime; // Travels up scope chain to use parents lastTime. Function has access to variables declared in the same scope console.log('foo called, gap:' + gap); lastTime = newTime; // Updates lastTime //console.log(x); //x++; } var throttle = function(func, wait) { var result; var timeout = null; // flag updated through closure var previous = 0; // time last run updated through closure return function() { //func, wait, timeout, previous available through scope var now = new Date().getTime(); var remaining = wait - (now - previous); if (remaining <= 0) { clearTimeout(timeout); timeout = null; previous = now; result = func.apply(this, arguments); //func is available through closure } return result; }; }; document.addEventListener("scroll", throttle(foo, 1000)); //document.addEventListener("scroll", throttle(foo(5), 2000)); }()); But I'd like to modify foo to foo(x) and get this to work (function () { var lastTime = new Date().getTime(); function foo(x) { var newTime = new Date().getTime(); var gap = newTime - lastTime; // Travels up scope chain to use parents lastTime. Function has access to variables declared in the same scope console.log('foo called, gap:' + gap); lastTime = newTime; // Updates lastTime console.log(x); x++; } var throttle = function(func, wait) { var result; var timeout = null; // flag updated through closure var previous = 0; // time last run updated through closure return function() { //func, wait, timeout, previous available through scope var now = new Date().getTime(); var remaining = wait - (now - previous); if (remaining <= 0) { clearTimeout(timeout); timeout = null; previous = now; result = func.apply(this, arguments); //func is available through closure } return result; }; }; document.addEventListener("scroll", throttle(foo(5), 2000)); }());

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  • If the "with" statement in Javascript creates a new scope, why does the following code not work as e

    - by Jian Lin
    If the "with" statement in Javascript creates a new scope, shouldn't clicking on the links show a different x which are in different scopes? It doesn't. <a href="#" id="link1">ha link 1</a> <a href="#" id="link2">ha link 2</a> <a href="#" id="link3">ha link 3</a> <a href="#" id="link4">ha link 4</a> <a href="#" id="link5">ha link 5</a> <script type="text/javascript"> for (i = 1; i <= 5; i++) { with({foo:"bar"}) { var x = i; document.getElementById('link' + i).onclick = function() { alert(x); return false; } } } </script>

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  • How to access a method of a closure's parent object?

    - by Bytecode Ninja
    I have defined a class named MyClass and I have defined two methods myMethod1 and myMethod2 for it: function MyClass() {} MyClass.prototype.myMethod1 = function() {...}; MyClass.prototype.myMethod2 = function() {...}; Inside myMethod1, I use jQuery and there's a callback closure defined there: MyClass.prototype.myMethod2 = function() { $.jQuery({success: function(data) { this.myMethod2(); }, ...}); } Now the problem is that this no longer is referring to MyClass. The question is how can I refer to it? At the moment I have assigned it to a variable named thisObj and access it this way: MyClass.prototype.myMethod2 = function() { var thisObj = this; $.jQuery({success: function(data) { thisObj.myMethod2(); }, ...}); } Is there a better way to access MyClass.this from the closure nested in myMethod2? Thanks in advance.

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  • Accessing loop iteration in a sub-function?

    - by DisgruntledGoat
    I'm using the Google Maps API to plot several points on a map. However, in the click event function below, i is always set to 4, i.e. its value after iterating the loop: // note these are actual addresses in the real page var addresses = new Array( "addr 1", "addr 2", "addr 3", "addr 4" ); for (var i = 0; i < addresses.length; i++) { geocoder.getLatLng(addresses[i], function(point) { if (point) { var marker = new GMarker(point); map.addOverlay(marker); map.setCenter(point, 13); GEvent.addListener(marker, "click", function() { // here, i=4 marker.openInfoWindowHtml("Address: <b>" + addresses[i] + "</b>"); }); } }); } So when the marker displays it's using addresses[4] which is undefined. How do I pass the correct value of i to the function?

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  • keeping track of multiple runs of the same function, part 2

    - by qwertymk
    This is related to this Anyway what I need is actually something slightly different I need some way of doing this: function run(arg) { this.ran = this.ran || false; if (!this.ran) init; /* code */ this.ran = true; } This works fine, I just want to make sure that this code works even when this in case it was called with call() or apply() Check this out for what I'm talking about, All of the calls after the first one should all be true, no matter the context

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  • Closure vs Anonymous function (difference?)

    - by Maxim Gershkovich
    Hi, I have been unable to find a definition that clearly explains the differences between a closure and an anonymous function. Most references I have seen clearly specify that they are distinct "things" yet I can't seem to get my head around why. Could someone please simplify it for me? What are the specific differences between these two language features? Which one is more appropriate in what scenarios?

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  • Why lambdas seem broken in multithreaded environments (or how closures in C# works).

    Ive been playing around with some code for.NET 3.5 to enable us to split big operations into small parallel tasks. During this work I was reminded why Resharper has the Access to modified closure warning. This warning tells us about a inconsistency in handling the Immutable loop variable created in a foreach loop when lambdas [...]...Did you know that DotNetSlackers also publishes .net articles written by top known .net Authors? We already have over 80 articles in several categories including Silverlight. Take a look: here.

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  • Code and Slides: Techniques, Strategies, and Patterns for Structuring JavaScript Code

    - by dwahlin
    This presentation was given at the spring 2012 DevConnections conference in Las Vegas and is based on my Structuring JavaScript Code course from Pluralsight. The goal of the presentation is to show how closures combined with code patterns can be used to provide structure to JavaScript code and make it more re-useable, maintainable, and less susceptible to naming conflicts.  Topics covered include: Closures Using Object literals Namespaces The Prototype Pattern The Revealing Module Pattern The Revealing Prototype Pattern View more of my presentations here. Sample code from the presentation can be found here. Check out the full-length course on the topic at Pluralsight.com.

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  • Which Programming Languages Support the Following Features?

    - by donalbain
    My personal programming background is mainly in Java, with a little bit of Ruby, a tiny bit of Scheme, and most recently, due to some iOS development, Objective-C. In my move from Java to Objective-C I've really come to love some features that Objective-C has that Java doesn't. These include support for both static and dynamic typing, functional programming, and closures, which I'm trying to leverage in my code more often. Unfortunately there are trade-offs, including lack of support for generics and (on iOS at least) no garbage collection. These contrasts have lead me to start a search for some of the programming languages that support the following features: Object Oriented Functional Programming Support Closures Generics Support for both Static and Dynamic Typing Module Management to avoid classpath/dll hell Garbage Collection Available Decent IDE Support Admittedly some of these features(IDE support, Module Management) may not be specific to the language itself, but obviously influence the ease of development in the language. Which languages fit these criteria?

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  • Lambda Expressions and Memory Management

    - by Surya
    How do the Lambda Expressions / Closures in C++0x complicate the memory management in C++? Why do some people say that closures have no place in languages with manual memory management? Is there claim valid and if yes, what are the reasons behind it?

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  • JavaScript objects and Crockford's The Good Parts

    - by Jonathan
    I've been thinking quite a bit about how to do OOP in JS, especially when it comes to encapsulation and inheritance, recently. According to Crockford, classical is harmful because of new(), and both prototypal and classical are limited because their use of constructor.prototype means you can't use closures for encapsulation. Recently, I've considered the following couple of points about encapsulation: Encapsulation kills performance. It makes you add functions to EACH member object rather than to the prototype, because each object's methods have different closures (each object has different private members). Encapsulation forces the ugly "var that = this" workaround, to get private helper functions to have access to the instance they're attached to. Either that or make sure you call them with privateFunction.apply(this) everytime. Are there workarounds for either of two issues I mentioned? if not, do you still consider encapsulation to be worth it? Sidenote: The functional pattern Crockford describes doesn't even let you add public methods that only touch public members, since it completely forgoes the use of new() and constructor.prototype. Wouldn't a hybrid approach where you use classical inheritance and new(), but also call Super.apply(this, arguments) to initialize private members and privileged methods, be superior?

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  • Preffered lambda syntax?

    - by Roger Alsing
    I'm playing around a bit with my own C like DSL grammar and would like some oppinions. I've reserved the use of "(...)" for invocations. eg: foo(1,2); My grammar supports "trailing closures" , pretty much like Ruby's blocks that can be passed as the last argument of an invocation. Currently my grammar support trailing closures like this: foo(1,2) { //parameterless closure passed as the last argument to foo } or foo(1,2) [x] { //closure with one argument (x) passed as the last argument to foo print (x); } The reason why I use [args] instead of (args) is that (args) is ambigious: foo(1,2) (x) { } There is no way in this case to tell if foo expects 3 arguments (int,int,closure(x)) or if foo expects 2 arguments and returns a closure with one argument(int,int) - closure(x) So thats pretty much the reason why I use [] as for now. I could change this to something like: foo(1,2) : (x) { } or foo(1,2) (x) -> { } So the actual question is, what do you think looks best? [...] is somewhat wrist unfriendly. let x = [a,b] { } Ideas?

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  • Preferred lambda syntax?

    - by Roger Alsing
    I'm playing around a bit with my own C like DSL grammar and would like some oppinions. I've reserved the use of "(...)" for invocations. eg: foo(1,2); My grammar supports "trailing closures" , pretty much like Ruby's blocks that can be passed as the last argument of an invocation. Currently my grammar support trailing closures like this: foo(1,2) { //parameterless closure passed as the last argument to foo } or foo(1,2) [x] { //closure with one argument (x) passed as the last argument to foo print (x); } The reason why I use [args] instead of (args) is that (args) is ambigious: foo(1,2) (x) { } There is no way in this case to tell if foo expects 3 arguments (int,int,closure(x)) or if foo expects 2 arguments and returns a closure with one argument(int,int) - closure(x) So thats pretty much the reason why I use [] as for now. I could change this to something like: foo(1,2) : (x) { } or foo(1,2) (x) -> { } So the actual question is, what do you think looks best? [...] is somewhat wrist unfriendly. let x = [a,b] { } Ideas?

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