language toolbar - Page 493

Changing style sheets depending on useragent

- by John Vasiliou

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Looking for a book which teaches how to write applications (as opposed to writng code)

- by rumtscho

I am not a developer. I have coded for fun and for university projects in several languages, and during an internship, I have written code which is still in use by a department of Fortune Global 500 company. I also have extensive theoretical knowledge of software engineering - process models, architecture, project management, UI construction using Lauesen's virtual windows... you get the picture. But I am not involved with software development in my job. I recently decided to start coding for fun again, and now I have some free days to do it. But this time, I want to do it right. I want to write a real, useful application, install it on my devices and use it myself. Also I want to publish it for others to use, should they want to do so. I am vaguely aware that there is more to writing an application than to writing code. There is stuff like version control software, decision for the right IDE, having a suite of unit tests, producing an installation package - and probably lots of other things I never thought of but which must be taken care of in a proper application, as opposed to a bunch of classes I am running from my IDE. All this is stuff I should know before I start, but I have not learned it. Coding books touch on some of the subjects like IDE choice, but don't go into detail, and are not exhaustive. Theoretical software engineering textbooks are even less helpful. So is there a book which teaches exactly that? I know that I can find information on each of these topics on the Internet, but I'd rather have a systematic book exhaustively listing all the things I should take care of if I want to create a good application, and offering the currently accepted solutions for them. In the best case, it will be language- and platform independent, but if you know of a good book focusing on a specific platform, I would like to know about it too. I know I want a lot, but given how important such knowledge is and how many people need it, surely somebody must have written such a book?

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Best practices to deal with "slightly different" branches of source code

- by jedi_coder

This question is rather agnostic than related to a certain version control program. Assume there is a source code tree under certain distributed version control. Let's call it A. At some point somebody else clones it and gets its own copy. Let's call it B. I'll call A and B branches, even if some version control tools have different definitions for branches (some might call A and B repositories). Let's assume that branch A is the "main" branch. In the context of distributed version control this only means that branch A is modified much more actively and the owner of branch B periodically syncs (pulls) new updates from branch A. Let's consider that a certain source file in branch B contains a class (again, it's also language agnostic). The owner of branch B considers that some class methods are more appropriate and groups them together by moving them inside the class body. Functionally nothing has changed - this is a very trivial refactoring of the code. But the change gets reflected in diffs. Now, assuming that this change from branch B will never get merged into branch A, the owner of branch B will always get this difference when pulling from branch A and merging into his own workspace. Even if there's only one such trivial change, the owner of branch B needs to resolve conflicts every time when pulling from branch A. As long as branches A and B are modified independently, more and more conflicts like this appear. What is the workaround for this situation? Which workflow should the owner of branch B follow to minimize the effort for periodically syncing with branch A?

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C++ : Declaring the array size with a non-constant variable

- by Jérôme

I always thought that when declaring an array in C++, the size has to be a constant integer value. For instance : int MyArray[5]; // correct or const int ARRAY_SIZE = 6; int MyArray[ARRAY_SIZE]; // correct but int ArraySize = 5; int MyArray[ArraySize]; // incorrect Here is also what is explained in The C++ Programming Language, by Bjarne Stroustrup : The number of elements of the array, the array bound, must be a constant expression (§C.5). If you need variable bounds, use a vector(§3.7.1, §16.3). For example: void f(int i) { int v1[i]; // error : array size not a constant expression vector<int> v2[i]; // ok } But to my big surprise, the code above does compile fine on my system ! Here is what I tried to compile : void f(int i) { int v2[i]; } int main() { int i = 3; int v1[i]; f(5); } I got no error ! I'm using GCC v4.4.0. Is there something I'm missing ?

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How would I sort files to directories based on filenames?

- by gnomed

I have a huge number of files to sort all named in some terrible convention. Here are some examples: (4)_mr__mcloughlin____.txt 12__sir_john_farr____.txt (b)mr__chope____.txt dame_elaine_kellett-bowman____.txt dr__blackburn______.txt These names are supposed to be a different person (speaker) each. Someone in another IT department produced these from a ton of XML files using some script but the naming is unfathomably stupid as you can see. I need to sort literally tens of thousands of these files with multiple files of text for each person; each with something stupid making the filename different, be it more underscores or some random number. They need to be sorted by speaker. This would be easier with a script to do most of the work then I could just go back and merge folders that should be under the same name or whatever. There are a number of ways I was thinking about doing this. parse the names from each file and sort them into folders for each unique name. get a list of all the unique names from the filenames, then look through this simplified list of unique names for similar ones and ask me whether they are the same, and once it has determined this it will sort them all accordingly. I plan on using Perl, but I can try a new language if it's worth it. I'm not sure how to go about reading in each filename in a directory one at a time into a string for parsing into an actual name. I'm not completely sure how to parse with regex in perl either, but that might be googleable. For the sorting, I was just gonna use the shell command: `cp filename.txt /example/destination/filename.txt` but just cause that's all I know so it's easiest. I dont even have a pseudocode idea of what im going to do either so if someone knows the best sequence of actions, im all ears. I guess I am looking for a lot of help, I am open to any suggestions. Many many many thanks to anyone who can help. B.

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Javascript get image height works in all browsers but firefox...please help?

- by Calvin

So I am fairly new to Javascript (a lot of experience in PHP), but I have this basic script that checks a link that redirects to a URL to see what the image height is. If it is a certain height then it adds one to a variable, otherwise nothing. I would easily do this in PHP but the images are on other servers and not my own so it doesn't work. Anyways, ehre is the script. Let me know if you have any tips. Works well and tested in Chrome, Safari, Opera, and IE. <script language='JavaScript'> window.onload = function() { var nstar = 0, urls = []; urls[0] = "http://optout.imiclk.com/cgi/nai_status.cgi?nocache="; urls[1] = "http://www.adbrite.com/mb/nai_optout_check.php?nocache="; urls[2] = "http://events.adchemy.com/visitor/auuid/nai-status?nocache="; function getImgSize(imgSrc){ var newImg = new Image(); newImg.src = imgSrc; return{height:newImg.height, width:newImg.width} } for(i=0,length=urls.length;i<length;i++){ if(getImgSize(urls[i]).height==43){nstar++;} } document.getElementById('tracknum').innerHTML = "<b>" + nstar + "</b>"; } </script>

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Optimize INSERT / UPDATE / DELETE operation

- by clime

I wonder if the following script can be optimized somehow. It does write a lot to disk because it deletes possibly up-to-date rows and reinserts them. I was thinking about applying something like "insert ... on duplicate key update" and found some possibilities for single-row updates but I don't know how to apply it in the context of INSERT INTO ... SELECT query. CREATE OR REPLACE FUNCTION update_member_search_index() RETURNS VOID AS $$ DECLARE member_content_type_id INTEGER; BEGIN member_content_type_id := (SELECT id FROM django_content_type WHERE app_label='web' AND model='member'); DELETE FROM watson_searchentry WHERE content_type_id = member_content_type_id; INSERT INTO watson_searchentry (engine_slug, content_type_id, object_id, object_id_int, title, description, content, url, meta_encoded) SELECT 'default', member_content_type_id, web_member.id, web_member.id, web_member.name, '', web_user.email||' '||web_member.normalized_name||' '||web_country.name, '', '{}' FROM web_member INNER JOIN web_user ON (web_member.user_id = web_user.id) INNER JOIN web_country ON (web_member.country_id = web_country.id) WHERE web_user.is_active=TRUE; END; $$ LANGUAGE plpgsql; EDIT: Schemas of web_member, watson_searchentry, web_user, web_country: http://pastebin.com/3tRVPPVi. (content_type_id, object_id_int) in watson_searchentry is unique pair in the table but atm the index is not present (there is no use for it). This script should be run at most once a day for full rebuilds of search index.

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In C, when do structure names have to be included in structure initializations and definitions?

- by Tyler

I'm reading The C Programming Language by K&R and in the section on structures I came across these code snippets: struct maxpt = { 320, 200 }; and /* addpoints: add two points */ struct addpoint(struct point p1, struct point p2) { p1.x += p2.x; p1.y += p2.y; return p1; } In the first case, it looks like it's assigning the values 320 and 200 to the members of the variable maxpt. But I noticed the name of the struct type is missing (shouldn't it be "struct struct_name maxpt = {320, 200}"? In the second case, the function return type is just "struct" and not "struct name_of_struct". I don't get why they don't include the struct names - how does it know what particular type of structure it's dealing with? My confusion is compounded by the fact that in previous snippets they do include the structure name, such as in the return type for the following function, where it's "struct point" and not just "struct". Why do they include the name in some cases and not in others? /* makepoint: make a point from x and y components */ struct point makepoint(int x, int y) { struct point temp; temp.x = x; temp.y = y; return temp; }

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Get Form Input via Ajax

- by user3651491

I have a jqgrid plugin which I call via Ajax. I have index.php and a getGridData.php. How will I pass form input in getGridData.php via ajax and use it in getGridData.php? I tried serialize but I can't pass or access it on getGridData.php. I need it as parameters for mysql. Here's my code. <script language="javascript" type="text/javascript"> function jgGrid() { $(document).ready(function () { $("#grid").jqGrid({ url: "inc/Controller/getGridData.php"+$("#thisForm").serialize(), data : formData, datatype: "json", mtype: "POST", colNames: ["SiteID", "TerminalID", "TransactionType", "Amount", "ServiceStatus"], colModel: [ { name: "SiteID"}, { name: "TerminalID"}, { name: "TransactionType"}, { name: "Amount"}, { name: "ServiceStatus"}, ], pager: "#pager", rowNum: 10, rowList: [10,20], sortname: "SiteID", sortorder: "asc", height: 'auto', viewrecords: true, gridview: true, caption: "" }); }); } </script> getGridData.php include('../Model/Queries.php'); $cardnumber = $_POST['cardnumber']; $transact_type = $_POST['transact_type']; $fromdate = $_POST['fromdate']; $todate = $_POST['todate']; $loyalty = new Queries(); $get_mid = $loyalty->loyaltyConn($cardnumber); $somedata = json_encode($loyalty->nposConn($get_mid, $transact_type, $fromdate, $todate)); echo $somedata;

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Redirect after refreshing update panel

- by teebot.be

Hello, Do you think it's possible to refresh an update panel and immediately after redirecting the response (for instance a download) ? I tried this: an invisible button - as an asyncpostbacktrigger a download button - when it is clicked the onclientclick clicks the invisible button the click event on the invisible button refreshes the update panel then the download button click event launches the download (normal postback which launches the download) However for some reason when the invisible button is clicked by the download button client script, it doesn't refresh the update panel.. Do you have an idea why it doesn't work? Or do you have other and cleaner techniques? Here's how the elements are declared: <asp:Button runat="server" ID="ButtonInvisible" Text="" Click="RefreshDisplay" /> <asp:Button runat="server" ID="ButtonDownload" Text="Download" OnClientClick="clickInvisible(this.id)" Click="Download" /><Triggers> <asp:AsyncPostBackTrigger ControlID="ButtonInvisible" /></Triggers> //the javascript <script type="text/javascript" language="javascript"> function clickInvisible(idButton) { document.getElementById('ButtonInvisible').click(); }</script> ' //the methods Download(object source, EventArgs e){Response.Redirect("test.txt")} RefreshDisplay(object source, EventArgs e){ ButtonCancel.Enabled = false;}

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Sockets and COBOL

- by kati

I have received a job at a hospital which still uses COBOL for all organizational work, the whole (now 20 Terabyte) database (Which was a homebrew in, guess what, COBOL) is filled with the data of every patient since the last 45 (or so) years. So that was my story. Now to my question: Currently, all sockets were (from what I've seen) implemented by COBOL programs writing their data into files. These files then were read out by C++ programs (That was an additional module added in the late 1980s) and using C++ sockets sent to the database. Now this solution has stopped working as they are moving the database from COBOL to COBOL, yes - they didn't use MySQL or so - they implemented a new database - again in COBOL. I asked the guy that worked there before me (hes around 70 now) why the hell someone would do that and he told me that he is so good at COBOL that he doesn't want to write it in any other language. So far so good now my question: How can I implement socket connections in COBOL? I need to create an interface to the external COBOL database located at, for example, 192.168.1.23:283.

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Custom stream wrappers, what could they be useful for in web applications?

- by michael

I suppose the concept is language agnostic, but I don't know what it's called in other languages. In PHP they're Stream Wrappers. In short, a wrapper class that allows manipulation of a streamable resource (resource that can be read to/written to/seek into, such as a file, a db, an url). For example, in a template engine (a view), upon including a template file such as: include "view.wrapper://path/to/my/template/file.phtml"; my custom wrapper, declared elsewhere and associated with "view.wrapper", would first intercepts the file to replace such things as short tags (<?=) with a more verbose counterpart (<?php echo). This allows developers to use short tags in views, even if the server isn't set to allow it. It can also be applied to the preprocessing of views pseudo syntax such as {@myVar} (e.g. replacing it with $this->myVar). This is only one application of custom stream wrappers, but the feature seems powerful enough to make me think that there are others that could make life a lot simpler for developers. What have you built, or thought about building, custom stream wrappers for? where have you seen some interesting implementations? I'm particularly interested in their applications in web development.

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Unable to `submit()` an html form after intercepting the submit with javascript.

- by Ross Rogers

I'm trying to intercept the submission of a form in order to change the value of my keywords label. I have the following code: <HTML> <FORM name="searchForm" method="get" action="tmp.html" > <input type="label" name="keywords" /> <input type="button" name="submit" value="submit" onclick="formIntercept();"/> </FORM> <SCRIPT language="JavaScript"> document.searchForm.keywords.focus(); function formIntercept( ) { var f = document.forms['searchForm']; f.keywords.value = 'boo'; f.submit(); }; </SCRIPT> </HTML> When I run this in chrome and click the submit button the keywords label changes to boo, but the javascript console says: Uncaught TypeError: Property 'submit' of object <#an HtmlFormElement> is not a function. How can I submit the form with the manipulated keywords?

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redirecting _top page from asp:login control in iframe onloggedin

- by jumpdart

So yeah, Im building a little authenticated content(.NET app) to a large html site managed by another group. They are only comfortable with managing html so all my app content will be contained in iframes. Everything is working fine as far as navigation and calling services and whatnot but i cant bust out of the frame with my asp:login control. Im trying to register some JS on logged in but with no success. Thanks protected void login_LoggedIn(object sender, EventArgs e) { StringBuilder strScript = new StringBuilder(); strScript.Append("<script language='javascript'>"); string sHome = ConfigurationManager.AppSettings["AppHomePageURL"].ToString(); //strScript.AppendFormat("window.navigate('{0}');", sHome); //strScript.AppendFormat("parent.location.href='{0}';", sHome); //strScript.AppendFormat("window.open('{0}', '_top', '', false);", sHome); strScript.AppendFormat("top.location.href='{0}';", sHome); strScript.Append("WTF_let_me_outa_here();"); strScript.Append("</script>"); ClientScript.RegisterClientScriptBlock(typeof(Page), "LoginGO", strScript.ToString()); }

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Explaining a Ruby code snippet

- by Michael Foukarakis

I'm in that uncomfortable position again, where somebody has left me with a code snippet in a language I don't know and I have to maintain it. While I haven't introduced Ruby to myself some parts of it are quite simple, but I'd like to hear your explanations nonetheless. Here goes: words = File.open("lengths.txt") {|f| f.read }.split # read all lines of a file in 'words'? values = Array.new(0) words.each { |value| values << value.to_i } # looked this one up, it's supposed to convert to an array of integers, right? values.sort! values.uniq! diffs = Array.new(0) # this looks unused, unless I'm missing something obvious sum = 0 s = 0 # another unused variable # this looks like it's computing the sum of differences between successive # elements, but that sum also remains unused, or does it? values.each_index { |index| if index.to_i < values.length-1 then sum += values.at(index.to_i + 1) - values.at(index.to_i) end } # could you also explain the syntax here? puts "delta has the value of\n" # this will eventually print the minimum of the original values divided by 2 puts values.at(0) / 2 The above script was supposed to figure out the average of the differences between every two successive elements (integers, essentially) in a list. Am I right in saying this is nowhere near what it actually does, or am I missing something fundamental, which is likely considering I have no Ruby knowledge?

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Calling a method from another view in objective-c. (iphone sdk).

- by MarcZero

Hello. I am currently creating a multi-view game on the iPhone platform. I have my main view start to play some background music upon loading. I then go to another view and start the game. I am trying to get the background music from the original view to stop once I start the game. I am having trouble getting the stop playing music message to my original view. Here is the relevant info: The main view where the sound is played from is just a subView of the programViewController class called *viewController. The heading is in the programAppDelegate class. The sound is done using the AVAudioPlayer class and plays fine when the program starts up and when I navigate to other subviews that are added on by going through the menu system. In the view that I want to start the game, I attempt to call the instance of the class to turn of the player but anytime I use this format of code: [viewController #######]; It gives a build error of "viewController undeclared" no matter what I put after the "viewController" in the message.I have attempted to import the programAppDelegate.h file but it still gives the same error. I realize this might be a simple misunderstanding of the objective-c language but I cannot find any info on this issue. I am a recent convert from Java so I am trying to wrap my head around everything. Thank you for your time!

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Generic delegate instances

- by Luc C

I wonder if C# (or the underlying .NET framework) supports some kind of "generic delegate instances": that is a delegate instance that still has an unresolved type parameter, to be resolved at the time the delegate is invoked (not at the time the delegate is created). I suspect this isn't possible, but I'm asking it anyway... Here is an example of what I'd like to do, with some "???" inserted in places where the C# syntax seems to be unavailable for what I want. (Obviously this code doesn't compile) class Foo { public T Factory<T>(string name) { // implementation omitted } } class Test { public void TestMethod() { Foo foo = new Foo(); ??? magic = foo.Factory; // No type argument given here yet to Factory! // What would the '???' be here (other than 'var' :) )? string aString = magic<string>("name 1"); // type provided on call int anInt = magic<int>("name 2"); // another type provided on another call // Note the underlying calls work perfectly fine, these work, but i'd like to expose // the generic method as a delegate. string aString2 = foo.Factory<string>("name 1"); int anInt2 = foo.Factory<int>("name 2"); } } Is there a way to actually do something like this in C#? If not, is that a limitation in the language, or is it in the .NET framework?

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Guidance on E-commerce in a Python Framework

- by Ant

I am looking for guidance in making decisions on using Python for E-commerce. My reasoning to use Python in the first place is i do a lot of 3D so it's a real help to use Python for web server side as well as 3D so I do not need to learn a new language. When I say E-commerce i mean the logic behind displaying products and a check out system that is Robust....and yes I really want to build this myself so I can change it. I do not want to use an out the box solution that costs a lot of money every month. I want control of server side, data bases, e-commerce etc. My questions are: What Frameworks are out there? I looked at previous questions on stackoverflow but only found old posts about Sachmo and Get paid. What is the learning path, I do not want to make a mistake that costs six months of studying? Python then Django? How long if I spent 10 hours a day studying this would it take to get up and running? I write javaScript badly, so I am not a complete noob, I understand the basics of programming languages. many thanks in advance

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How can I integrate advanced computations into a database field?

- by ciclistadan

My biological research involves the measurement of a cellular structure as it changes length throughout the course of observation (capturing images every minute for several hours). As my data sets have become larger I am trying to store them in an Access database, from which I would like to perform various queries about their changes in size. I know that the SELECT statement can incorporate some mathematical permutations, but I have been unable to incorporate many of my necessary calculations (probably due to my lack of knowledge). For example, one calculation involves determining the rate of change during specifically defined periods of growth. This calculation is entirely dependent on the raw data saved in the table, therefore I didn't this it would be appropriate to just calculate it in excel prior to entry into the field. So my question is, what would be the most appropriate method of performing this calculation. Should I attempt to string together a huge SELECT calculation in my QUERY, or is there a way to use another language (I know perl?) which can be called to populate the new query field? I'm not looking for someone to write the code, just where is it appropriate to incorporate each step. Also, I am currently using Office Access but would be interested in any mySQL answers as I may be moving to this platform at a later date. Thanks all!

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How is covariance cooler than polymorphism...and not redundant?

- by P.Brian.Mackey

.NET 4 introduces covariance. I guess it is useful. After all, MS went through all the trouble of adding it to the C# language. But, why is Covariance more useful than good old polymorphism? I wrote this example to understand why I should implement Covariance, but I still don't get it. Please enlighten me. using System; using System.Collections.Generic; using System.Linq; using System.Text; namespace Sample { class Demo { public delegate void ContraAction<in T>(T a); public interface IContainer<out T> { T GetItem(); void Do(ContraAction<T> action); } public class Container<T> : IContainer<T> { private T item; public Container(T item) { this.item = item; } public T GetItem() { return item; } public void Do(ContraAction<T> action) { action(item); } } public class Shape { public void Draw() { Console.WriteLine("Shape Drawn"); } } public class Circle:Shape { public void DrawCircle() { Console.WriteLine("Circle Drawn"); } } public static void Main() { Circle circle = new Circle(); IContainer<Shape> container = new Container<Circle>(circle); container.Do(s => s.Draw());//calls shape //Old school polymorphism...how is this not the same thing? Shape shape = new Circle(); shape.Draw(); } } }

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Need help in javascript

- by raja

Hi: I am getting the error for the below code. Please help me out. <html><head> <title>(Type a title for your page here)</title> <script language=JavaScript> function check_length(my_formm,fieldName) { alert(fieldName); alert(document.my_formm.fieldName.value); } </script> </head> <body> <form name=my_form method=post> <input type="text" onKeyPress=checkCompanyName(); onChange=check_length("my_form","my_text"); name=my_text rows=4 cols=30 value=""> <br> <input size=1 value=50 name=text_num> Characters Left </form> </body> </html>

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Choose 'better' or more familiar technologies for a new project?

- by John

I am looking to start work on a brand-new project, something I've been thinking about for a while as my first independent sellable project. It's broadly speaking a web-based service application, and my first choice, server-language is quite easy... I know Java pretty well from working on Java web-apps in the past. However my experience doing web-apps involved JSP, Servlets and JSTL... I know the ideas behind newer technologies like Hibernate/Spring but have never used them. So we wrote our own DAOs, handled AJAX by writing special mini-JSP pages that generated XML/JSON pages, etc. I'm not hugely into the idea that Spring/Hibernate are the 'only' or 'right' way to do any Java web-project, but they are widely used. On the other hand, not only would trying to learn these increase initial development time, but I'd be using my learning attempts to build a production system. I remember one of Joel's early articles said (I'll paraphrase since I can't find it) "regardless what's cool, always use the technologies that the lead developer (or dev team?) knows best" I wondered what people thought about that? ps: should this be CW?

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Framework or CMS for php web development?

- by rocknroll

Hi all, I am a C++,Unix developer who has never dabbled in web development apart from creating simple HTML pages. I am going to change that and develop a website at a personal level soon. I am going to use php,mysql on a linux machine. In this regard I am browsing through relevant literature. The language isn't a problem but reading about CMS's and frameworks is confusing. And since I am new to web development, the number of CMS's and Frameworks are overwhelming. ? My question is do I need to have knowledge of one or more CMS' and/or Frameworks like Drupal,Joomla,Zend,Wordpress etc. If yes which is the best open source CMS' and/or Framework suggested for a newbie. ?? If the answer to the above question is yes, does the choice changes if one delves into the realms of commercial web development? Note:- I will be developing on a Linux machine, using open source tools. Thanks in advance

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Is there command-line tool to extract typedef, structure, enumeration, variable, function from a C or C++ file?

- by FooF

I am desiring a command-line tool to extract a definition or declaration (typedef, structure, enumeration, variable, or function) from a C or C++ source file. Also a way to replace an existing definition/declaration would be handy (after transforming the extracted definition by a user-submitted script). Is there such generic tool available, or is some resonably close approximation of such a tool? Scriptability and ability to hook-up with user created scripts or programs is of importance here, although I am academically curious of GUI programs too. Open source solutions for Unix/Linux camp are preferred (although I am curious of Windows and OS X tools too). Primary language interests are C and C++ but more generic solution would be even better (I think we do not need super accurate parsing capabilities for finding, extracting and replacing a definition in a program source file). Sample Use Cases (extra - for the curious mind): Given deeply nested structs and variable (array) initializations of these types, suppose there is a need to change a struct definition by adding or reordering fields or rewriting the variable/array definitions in more readable format without introducing errors resulting from manual labor. This would work by extracting the old initializations, then using a script/program to write the new initializations to replace the old ones. For implementing a code browsing tool - extract a definition. Decorative code generation (e.g. logging function entries / returns). Scripted code structuring (e.g. extract this and that thing and put in different place without change - version control commit comment could document the command to perform this operation to make it evident and verifiable that nothing changed). Note about tags: More accurate tag than code-generation would be code-transformation but it does not exist.

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I can't seem to figure out type variables mixed with classes.

- by onmach

I pretty much understand 3/4 the rest of the language, but every time I dip my feet into using classes in a meaningful way in my code I get permantently entrenched. Why doesn't this extremely simple code work? data Room n = Room n n deriving Show class HasArea a where width :: (Num n) => a -> n instance (Num n) => HasArea (Room n) where width (Room w h) = w So, room width is denoted by ints or maybe floats, I don't want to restrict it at this point. Both the class and the instance restrict the n type to Nums, but it still doesn't like it and I get this error: Couldn't match expected type `n1' against inferred type `n' `n1' is a rigid type variable bound by the type signature for `width' at Dungeon.hs:11:16 `n' is a rigid type variable bound by the instance declaration at Dungeon.hs:13:14 In the expression: w In the definition of `width': width (Room w h) = w In the instance declaration for `HasArea (Room n)' So it tells me the types doesn't match, but it doesn't tell me what types it thinks they are, which would be really helpful. As a side note, is there any easy way to debug an error like this? The only way I know to do it is to randomly change stuff until it works.

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