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• I need an abstract field !

  - by Jules Olléon

  I know abstract fields do not exist in java. I also read this question but the solutions proposed won't solve my problem. Maybe there is no solution, but it's worth asking :) Problem I have an abstract class that does an operation in the constructor depending on the value of one of its fields. The problem is that the value of this field will change depending on the subclass. How can I do so that the operation is done on the value of the field redefined by the subclass ? If I just "override" the field in the subclass the operation is done on the value of the field in the abstract class. I'm open to any solution that would ensure that the operation will be done during the instantiation of the subclass (ie putting the operation in a method called by each subclass in the constructor is not a valid solution, because someone might extend the abstract class and forget to call the method). Also, I don't want to give the value of the field as an argument of the constructor. Is there any solution to do that, or should I just change my design ?

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• How do you unit test a method containing a LINQ expression?

  - by Phil.Wheeler

  I'm struggling to get my head around how to accommodate a mocked method that only accepts a Linq expression as its argument. Specifically, the repository I'm using has a First() method that looks like this: public T First(Expression<Func<T, bool>> expression) { return All().Where(expression).FirstOrDefault(); } The difficulty I'm encountering is with my MSpec tests, where I'm (probably incorrectly) trying to mock that call: public abstract class with_userprofile_repository { protected static Mock<IRepository<UserProfile>> repository; Establish context = () => { repository = new Mock<IRepository<UserProfile>>(); repository.Setup<UserProfile>(x => x.First(up => up.OpenID == @"http://testuser.myopenid.com")).Returns(GetDummyUser()); }; protected static UserProfile GetDummyUser() { UserProfile p = new UserProfile(); p.OpenID = @"http://testuser.myopenid.com"; p.FirstName = "Joe"; p.LastLogin = DateTime.Now.Date.AddDays(-7); p.LastName = "Bloggs"; p.Email = "[email protected]"; return p; } } I run into trouble because it's not enjoying the Linq expression: System.NotSupportedException: Expression up = (up.OpenID = "http://testuser.myopenid.com") is not supported. So how does one test these sorts of scenarios?

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• Lua task scheduling

  - by Martin

  I've been writing some scripts for a game, the scripts are written in Lua. One of the requirements the game has is that the Update method in your lua script (which is called every frame) may take no longer than about 2-3 milliseconds to run, if it does the game just hangs. I solved this problem with coroutines, all I have to do is call Multitasking.RunTask(SomeFunction) and then the task runs as a coroutine, I then have to scatter Multitasking.Yield() throughout my code, which checks how long the task has been running for, and if it's over 2 ms it pauses the task and resumes it next frame. This is ok, except that I have to scatter Multitasking.Yield() everywhere throughout my code, and it's a real mess. Ideally, my code would automatically yield when it's been running too long. So, Is it possible to take a Lua function as an argument, and then execute it line by line (maybe interpreting Lua inside Lua, which I know is possible, but I doubt it's possible if all you have is a function pointer)? In this way I could automatically check the runtime and yield if necessary between every single line.

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• DropDownListFor and relating my lambda to my ViewModel

  - by Daniel Harvey

  After googling for a while I'm still drawing a blank here. I'm trying to use a ViewModel to pull and provide a dictionary to a drop down list inside a strongly typed View: <%@ Page Title="" Language="C#" MasterPageFile="~/Views/Shared/Site.Master" Inherits="Notebook.ViewModels.CorporationJoinViewModel" %> ... <%: Html.DropDownListFor(c => c.CorpDictionary.Keys, new SelectList(Model.CorpDictionary, "Value", "Key"))%> I'm getting the error CS1061: 'object' does not contain a definition for 'CorpDictionary' and no extension method 'CorpDictionary' accepting a first argument of type 'object' could be found and the relevant bit of my ViewModel public class CorporationJoinViewModel { DB _db = new DB(); // data context public Dictionary<int, string> CorpDictionary { get { Dictionary<int, string> corporations = new Dictionary<int, string>(); int x = 0; foreach (Corporation corp in _db.Corporations) { corporations.Add(x, corp.name); } return corporations; } } I'll admit i have a pretty magical understanding of how linq is finding my ViewModel object from that lambda, and the error message is making me think it's not. Is my problem the method I'm using to pass the data? What am I missing here?

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• Understanding OOP Principles in passing around objects/values

  - by Hans

  I'm not quite grokking a couple of things in OOP and I'm going to use a fictional understanding of SO to see if I can get help understand. So, on this page we have a question. You can comment on the question. There are also answers. You can comment on the answers. Question - comment - comment - comment Answer -comment Answer -comment -comment -comment Answer -comment -comment So, I'm imagining a very high level understanding of this type of system (in PHP, not .Net as I am not yet familiar with .Net) would be like: $question = new Question; $question->load($this_question_id); // from the URL probably echo $question->getTitle(); To load the answers, I imagine it's something like this ("A"): $answers = new Answers; $answers->loadFromQuestion($question->getID()); // or $answers->loadFromQuestion($this_question_id); while($answer = $answers->getAnswer()) { echo $answer->showFormatted(); } Or, would you do ("B"): $answers->setQuestion($question); // inject the whole obj, so we have access to all the data and public methods in $question $answers->loadFromQuestion(); // the ID would be found via $this->question->getID() instead of from the argument passed in while($answer = $answers->getAnswer()) { echo $answer->showFormatted(); } I guess my problem is, I don't know when or if I should be passing in an entire object, and when I should just be passing in a value. Passing in the entire object gives me a lot of flexibility, but it's more memory and subject to change, I'd guess (like a property or method rename). If "A" style is better, why not just use a function? OOP seems pointless here. Thanks, Hans

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• Please explain JSONP

  - by Cheeso

  I don't understand jsonp. I understand JSON. I don't understand JSONP. Wikipedia is the top search result for JSONP. It says JSONP or "JSON with padding" is a JSON extension wherein a prefix is specified as an input argument of the call itself. Huh? What call? That doesn't make any sense to me. JSON is a data format. There's no call. The 2nd search result is from some guy named Remy, who writes JSONP is script tag injection, passing the response from the server in to a user specified function. I can sort of understand that, but it's still not making any sense. What is JSONP, why was it created (what problem does it solve), and why would I use it? Addendum: I've updated Wikipedia with a clearer and more thorough description of JSONP, based on jvenema's answer. Thanks, all.

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• Instantiate type variable in Haskell

  - by danportin

  EDIT: Solved. I was unware that enabling a language extension in the source file did not enable the language extension in GHCi. The solution was to :set FlexibleContexts in GHCi. I recently discovered that type declarations in classes and instances in Haskell are Horn clauses. So I encoded the arithmetic operations from The Art of Prolog, Chapter 3, into Haskell. For instance: fac(0,s(0)). fac(s(N),F) :- fac(N,X), mult(s(N),X,F). class Fac x y | x -> y instance Fac Z (S Z) instance (Fac n x, Mult (S n) x f) => Fac (S n) f pow(s(X),0,0) :- nat(X). pow(0,s(X),s(0)) :- nat(X). pow(s(N),X,Y) :- pow(N,X,Z), mult(Z,X,Y). class Pow x y z | x y -> z instance (N n) => Pow (S n) Z Z instance (N n) => Pow Z (S n) (S Z) instance (Pow n x z, Mult z x y) => Pow (S n) x y In Prolog, values are insantiated for (logic) variable in a proof. However, I don't understand how to instantiate type variables in Haskell. That is, I don't understand what the Haskell equivalent of a Prolog query ?-f(X1,X2,...,Xn) is. I assume that :t undefined :: (f x1 x2 ... xn) => xi would cause Haskell to instantiate xi, but this gives a Non type-variable argument in the constraint error, even with FlexibleContexts enabled.

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• What are the most interesting equivalences arising from the Curry-Howard Isomorphism?

  - by Tom

  I came upon the Curry-Howard Isomorphism relatively late in my programming life, and perhaps this contributes to my being utterly fascinated by it. It implies that for every programming concept there exists a precise analogue in formal logic, and vice versa. Here's an "obvious" list of such analogies, off the top of my head: program/definition | proof type/declaration | proposition inhabited type | theorem function | implication function argument | hypothesis/antecedent function result | conclusion/consequent function application | modus ponens recursion | induction identity function | tautology non-terminating function | absurdity tuple | conjunction (and) disjoint union | exclusive disjunction (xor) parametric polymorphism | universal quantification So, to my question: what are some of the more interesting/obscure implications of this isomorphism? I'm no logician so I'm sure I've only scratched the surface with this list. For example, here are some programming notions for which I'm unaware of pithy names in logic: currying | "((a & b) => c) iff (a => (b => c))" scope | "known theory + hypotheses" And here are some logical concepts which I haven't quite pinned down in programming terms: primitive type? | axiom set of valid programs? | theory ? | disjunction (or)

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• Generic function pointers in C

  - by Lucas

  I have a function which takes a block of data and the size of the block and a function pointer as argument. Then it iterates over the data and performes a calculation on each element of the data block. The following is the essential outline of what I am doing: int myfunction(int* data, int size, int (*functionAsPointer)(int)){ //walking through the data and calculating something for (int n = 0; n < size; n++){ data[n] = (*function)(data[n]); } } The functions I am passing as arguments look something like this: int mycalculation(int input){ //doing some math with input //... return input; } This is working well, but now I need to pass an additional variable to my functionpointer. Something along the lines int mynewcalculation(int input, int someVariable){ //e.g. input = input * someVariable; //... return input; } Is there an elegant way to achieve this and at the same time keeping my overall design idea?

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• Best approach to storing image pixels in bottom-up order in Java

  - by finnw

  I have an array of bytes representing an image in Windows BMP format and I would like my library to present it to the Java application as a BufferedImage, without copying the pixel data. The main problem is that all implementations of Raster in the JDK store image pixels in top-down, left-to-right order whereas BMP pixel data is stored bottom-up, left-to-right. If this is not compensated for, the resulting image will be flipped vertically. The most obvious "solution" is to set the SampleModel's scanlineStride property to a negative value and change the band offsets (or the DataBuffer's array offset) to point to the top-left pixel, i.e. the first pixel of the last line in the array. Unfortunately this does not work because all of the SampleModel constructors throw an exception if given a negative scanlineStride argument. I am currently working around it by forcing the scanlineStride field to a negative value using reflection, but I would like to do it in a cleaner and more portable way if possible. e.g. is there another way to fool the Raster or SampleModel into arranging the pixels in bottom-up order but without breaking encapsulation? Or is there a library somewhere that will wrap the Raster and SampleModel, presenting the pixel rows in reverse order? I would prefer to avoid the following approaches: Copying the whole image (for performance reasons. The code must process hundreds of large (= 1Mpixels) images per second and although the whole image must be available to the application, it will normally access only a tiny (but hard-to-predict) portion of the image.) Modifying the DataBuffer to perform coordinate transformation (this actually works but is another "dirty" solution because the buffer should not need to know about the scanline/pixel layout.) Re-implementing the Raster and/or SampleModel interfaces from scratch (but I have a hunch that I will be unable to avoid this.)

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• Reference-type conversion operators: asking for trouble?

  - by Ben

  When I compile the following code using g++ class A {}; void foo(A&) {} int main() { foo(A()); return 0; } I get the following error messages: > g++ test.cpp -o test test.cpp: In function ‘int main()’: test.cpp:10: error: invalid initialization of non-const reference of type ‘A&’ from a temporary of type ‘A’ test.cpp:6: error: in passing argument 1 of ‘void foo(A&)’ After some reflection, these errors make plenty of sense to me. A() is just a temporary value, not an assignable location on the stack, so it wouldn't seem to have an address. If it doesn't have an address, then I can't hold a reference to it. Okay, fine. But wait! If I add the following conversion operator to the class A class A { public: operator A&() { return *this; } }; then all is well! My question is whether this even remotely safe. What exactly does this point to when A() is constructed as a temporary value? I am given some confidence by the fact that void foo(const A&) {} can accept temporary values according to g++ and all other compilers I've used. The const keyword can always be cast away, so it would surprise me if there were any actual semantic differences between a const A& parameter and an A& parameter. So I guess that's another way of asking my question: why is a const reference to a temporary value considered safe by the compiler whereas a non-const reference is not?

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• Simple perl program failing to execute

  - by yves Baumes

  Here is a sample that fails: #!/usr/bin/perl -w # client.pl #---------------- use strict; use Socket; # initialize host and port my $host = shift || 'localhost'; my $port = shift || 55555; my $server = "10.126.142.22"; # create the socket, connect to the port socket(SOCKET,PF_INET,SOCK_STREAM,(getprotobyname('tcp'))[2]) or die "Can't create a socket $!\n"; connect( SOCKET, pack( 'Sn4x8', AF_INET, $port, $server )) or die "Can't connect to port $port! \n"; my $line; while ($line = <SOCKET>) { print "$line\n"; } close SOCKET or die "close: $!"; with the error: Argument "10.126.142.22" isn't numeric in pack at D:\send.pl line 16. Can't connect to port 55555! I am using this version of Perl: This is perl, v5.10.1 built for MSWin32-x86-multi-thread (with 2 registered patches, see perl -V for more detail) Copyright 1987-2009, Larry Wall Binary build 1006 [291086] provided by ActiveState http://www.ActiveState.com Built Aug 24 2009 13:48:26 Perl may be copied only under the terms of either the Artistic License or the GNU General Public License, which may be found in the Perl 5 source kit. Complete documentation for Perl, including FAQ lists, should be found on this system using "man perl" or "perldoc perl". If you have access to the Internet, point your browser at http://www.perl.org/, the Perl Home Page. While I am running the netcat command on the server side. Telnet does work.

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• Two-way data binding of controls in a user control inside a FormView

  - by Sandor Drieënhuizen

  I'm trying to perform two-way data binding on the controls in my user control, which is hosted inside a FormView template. FormView: <asp:ObjectDataSource runat="server" ID="ObjectDataSource" TypeName="WebApplication1.Data" SelectMethod="GetItem" UpdateMethod="UpdateItem"> </asp:ObjectDataSource> <asp:FormView runat="server" ID="FormView"> <ItemTemplate> <uc:WebUserControl1 runat="server"></uc:WebUserControl1> </ItemTemplate> <EditItemTemplate> <uc:WebUserControl1 runat="server"></uc:WebUserControl1> </EditItemTemplate> </asp:FormView> User control: <%@ Control Language="C#" ... %> <asp:TextBox runat="server" ID="TitleTextBox" Text='<%# Bind("Title") %>'> </asp:TextBox> The binding works fine when the FormView is in View mode but when I switch to Edit mode, upon calling UpdateItem on the FormView, the bindings are lost. I know this because the FormView tries to call an update method on the ObjectDataSource that does not have an argument called 'Title'. I tried to solve this by implementing IBindableTemplate to load the controls that are inside my user control, directly into the templates (just like I had entered them declaratively like in the code above). However, when calling UpdateItem in edit mode, the container that gets passed into the ExtractValues method of the template, does not contain the TextBox anymore. It did in view mode! I have found some questions on SO that relate to this problem but they are rather dated and don't provide straight forward answers. How do you think I could solve this problem? It seems to be such a simple requirement but apparently it's more like opening a can of worms...

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• Powershell Regex help in extracting text between strings

  - by vivekeviv

  i Have an arguments like the one below which i pass to powershell script -arg1 -abc -def -arg2 -ghi -jkl -arg3 -123 -234 Now i need to extract three strings without any whitespace string 1: "-abc -def" string 2: "-ghi -jkl" string 3: "-123 -234" i figured this expression could do it. But this doesnt seem to work. $args -match '-arg1(?'arg1'.*?) -arg3(?'arg3'.*?) -arg3(?'arg3'.*)'. THis should return $matches['arg1'] etc. So whats wrong in above expression. Why do i get an error as shown below runScript.ps1 -arg1 -abc -def -arg2 -ghi -jkl -arg3 -123 -234 Unexpected token 'arg1'.?) -arg2 (?'arg2'.?) -arg3 (?'arg3'.)'' in expression or statement. At G:\powershell\tools\powershell\runTest.ps1:1 char:71 + $args -match '-arg1 (?'arg1'.?) -arg2 (?'arg2'.?) -arg3 (?'arg3'.)' <<<< + CategoryInfo : ParserError: (arg1'.?) -arg2...g3 (?'arg3'.)':String) [], ParseException + FullyQualifiedErrorId : UnexpectedToken and also the second question is how do i make arg1 or arg2 or arg3 optional? The argument to script can be -arg2 -def -ghi. I'll take some default values for arg(1|2|3) that is not mentioned. Thanks

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• Find all substrings of a string - StringIndexOutOfBoundsException

  - by nazar_art

  I created class Word. Word has a constructor that takes a string argument and one method getSubstrings which returns a String containing all substring of word, sorted by length. For example, if the user provides the input "rum", the method returns a string that will print like this: r u m ru um rum I want to concatenate the substrings in a String, separating them with a newline ("\n"). Then return the string. Code: public class Word { String word; public Word(String word) { this.word = word; } /** * Gets all the substrings of this Word. * @return all substrings of this Word separated by newline */ public String getSubstrings() { String str = ""; int i, j; for (i = 0; i < word.length(); i++) { for (j = 0; j < word.length(); j++) { str = word.substring(i, i + j); str += "\n"; } } return str; } But it throws exception: java.lang.StringIndexOutOfBoundsException: String index out of range: -1 at java.lang.String.substring(String.java:1911) I stuck at this point. Maybe, you have other suggestions according this method signature public String getSubstrings(). How to solve this issue?

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• tikz: set appropriate x value for a node

  - by basweber

  This question resulted from the question here I want to produce a curly brace which spans some lines of text. The problem is that I have to align the x coordinate manually, which is not a clean solution. Currently I use \begin{frame}{Example} \begin{itemize} \item The long Issue 1 \tikz[remember picture] \node[coordinate,yshift=0.7em] (n1) {}; \\ spanning 2 lines \item Issue 2 \tikz[remember picture] \node[coordinate, xshift=1.597cm] (n2) {}; \item Issue 3 \end{itemize} \visible<2->{ \begin{tikzpicture}[overlay,remember picture] \draw[thick,decorate,decoration={brace,amplitude=5pt}] (n1) -- (n2) node[midway, right=4pt] {One and two are cool}; \end{tikzpicture} } % end visible \end{frame} which produces the desired result: The unsatisfying thing is, that I had to figure out the xshift value of 1.597cm by trial and error (more or less) Without xshift argument the result is: I guess there is an elegant way to avoid the explicit xshift value. The best way would it imho be to calculate the maximum x value of two nodes and use this, (as already suggested by Geoff) But it would already be very handy to be able to explicitly define the absolute xvalues of both nodes while keeping their current y values. This would avoid the fiddly procedure of adapting the third post decimal position to ensure that the brace looks vertical.

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• I'm having trouble with using std::stack to retrieve the values from a recursive function.

  - by Peter Stewart

  Thanks to the help I received in this post: http://stackoverflow.com/questions/2761918/how-do-i-use-this-in-a-member-function I have a nice, concise recursive function to traverse a tree in postfix order: void Node::postfix() { if (left != __nullptr) { left->postfix(); } if (right != __nullptr) { right->postfix(); } cout<<cargo<<"\n"; return; }; Now I need to evaluate the values and operators as they are returned. My problem is how to retrieve them. I tried the std::stack: #include <stack> stack <char*> s; void Node::postfix() { if (left != __nullptr) { left->postfix(); } if (right != __nullptr) { right->postfix(); } s.push(cargo); return; }; but when I tried to access it in main() while (!s.empty()) { cout<<s.top<<"\n"; s.pop; } I got the error: 'std::stack<_Ty::top': function call missing argument list; use '&std::stack<_Ty::top' to create a pointer to member' I'm stuck. Another question to follow shortly.

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• Adding a font for use in ReportLab

  - by Jimmy McCarthy

  I'm trying to add a font to the python ReportLab so that I can use it for a function. The function is using canvas.Canvas to draw a bunch of text in a PDF, nothing complicated, but I need to add a fixed width font for layout issues. When I tried to register a font using what little info I could find, that seemed to work. But when I tried to call .addFont('fontname') from my Canvas object I keep getting "PDFDocument instance has no attribute 'addFont'" Is the function just not implemented? How do I get access to fonts other than the 10 or so default ones that are listed in .getAvailableFonts? Thanks. Some example code of what I'm trying to make happen: from reportlab.pdfgen import canvas c = canvas.Canvas('label.pdf') c.addFont('TestFont') #This throws the error listed above, regardless of what argument I use (whether it refers to a font or not). c.drawString(1,1,'test data here') c.showPage() c.save() To register the font, I tried from reportlab.lib.fonts import addMapping from reportlab.pdfbase import pdfmetrics pdfmetrics.registerFont(TTFont('TestFont', 'ghettomarquee.ttf')) addMapping('TestFont', 0, 0, 'TestFont') where 'ghettomarquee.ttf' was just a random font I had lying around.

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• WPF Spellcheck Engine takes up too much memory.

  - by Matt H.

  Each datatemplate in my WPF ItemsControl contains FIVE custom bindable richtextbox controls. It is a data-driven app that for authoring multiple-choice questions -- The question and four answer choices must all support: 1) Spell check 2) Rich formatting (otherwise I'd use regular textboxes) The spell check object in .NET 4 has a Friend constructor that takes a single argument of owner As TextBoxBase This means every richtextbox in the ItemsControl has 5 Spellcheck objects! This is the problem -- every spell check engine is consuming about 500k memory. So after you favor in the spellcheck, bindings, additional controls in the DataTemplate, etc.. a single multi choice question consumes more than 3MB memory. Users with 100--200 questions will quickly see the App raise it's memory consumption to 500+ MB. Management is definitely not OK with this. Is there a way to minimimze this problem? The best suggestion I've heard is to enable/disable spellcheck if the richtextbox is in the ItemsControl's scrollviewer: I haven't gotten an answer to how to go about it: (http://stackoverflow.com/questions/2869012/possible-to-implement-an-isviewportvisible-dependencyproperty-for-an-item-in-an-i) Any good ideas?

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• Moving delegate-related function to a different thread

  - by Chris

  Hello everybody. We are developing a library in C# that communicates with the serial port. We have a function that is given to a delegate. The problem is that we want it to be run in a different thread. We tried creating a new thread (called DatafromBot) but keep using it as follows (first line): comPort.DataReceived += new SerialDataReceivedEventHandler(comPort_DataReceived); DatafromBot = new Thread(comPort_DataReceived); DatafromBot.Start(); comPort_DataReceived is defined as: Thread DatafromBot; public void comPort_DataReceived(object sender, SerialDataReceivedEventArgs e) { ... } The following errors occur: Error 3 The best overloaded method match for 'System.Threading.Thread.Thread(System.Threading.ThreadStart)' has some invalid arguments C:...\IR52cLow\CommunicationManager.cs 180 27 IR52cLow Error 4 Argument '1': cannot convert from 'method group' to 'System.Threading.ThreadStart' C:...\IR52cLow\CommunicationManager.cs 180 38 IR52cLow Any ideas of how we should convert this to get it to compile? Please note that comPort.DataReceived (pay attention to "." instead of "_") lies within a system library and cannot be modified. Thanks for your time! Chris

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• Why do the overloads of String.Format exist?

  - by GiddyUpHorsey

  I was using Reflector to look at the implementation of String.Format and had always been under the impression that the overloads of String.Format that took 1, 2 & 3 arguments were optimized versions of the method that takes an object array. However, what I found was that internally they create an object array and then call a method that takes an object array. 1 arg public static string Format(string format, object arg0) { if (format == null) { throw new ArgumentNullException("format"); } return Format(null, format, new object[] { arg0 }); } 2 args public static string Format(string format, object arg0, object arg1) { if (format == null) { throw new ArgumentNullException("format"); } return Format(null, format, new object[] { arg0, arg1 }); } 3 args public static string Format(string format, object arg0, object arg1, object arg2) { if (format == null) { throw new ArgumentNullException("format"); } return Format(null, format, new object[] { arg0, arg1, arg2 }); } Object array public static string Format(string format, params object[] args) { if ((format == null) || (args == null)) { throw new ArgumentNullException((format == null) ? "format" : "args"); } return Format(null, format, args); } Internally they all end up using the same code and so using the 1, 2 & 3 argument versions are no faster than the object array version. So my question is - why do they exist? When you use the object array version with a comma separated list of values, the compiler automatically converts the arguments into an object array because of the params/ParamArray keyword which is essentially what the 1, 2 & 3 versions do, so they seem redundant. Why did the BCL designers add these overloads?

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• scheme struct question

  - by qzar

  ;; definition of the structure "book" ;; author: string - the author of the book ;; title: string - the title of the book ;; genre: symbol - the genre (define-struct book (author title genre)) (define lotr1 (make-book "John R. R. Tolkien" "The Fellowship of the Ring" 'Fantasy)) (define glory (make-book "David Brin" "Glory Season" 'ScienceFiction)) (define firstFamily (make-book "David Baldacci" "First Family" 'Thriller)) (define some-books (list lotr1 glory firstFamily)) ;; count-books-for-genre: symbol (list of books) -> number ;; the procedure takes a symbol and a list of books and produces the number ;; of books from the given symbol and genre ;; example: (count-books-for-genre 'Fantasy some-books) should produce 1 (define (count-books-for-genre genre lob) (if (empty? lob) 0 (if (symbol=? (book-genre (first lob)) genre) (+ 1 (count-books-for-genre (rest lob) genre)) (count-books-for-genre (rest lob) genre) ) ) ) (count-books-for-genre 'Fantasy some-books) It produce following exception first: expected argument of type non-empty list; given 'Fantasy, I don't understand whats the problem. Can somebody give me some explanation ? Thank you very much !

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• Linq-to-SQL produces bad SQL?

  - by strager

  I am trying to run a Linq-to-SQL query, but when the query is evaluated, I get the following exception: System.Data.OleDb.OleDbException was unhandled Message=The SELECT statement includes a reserved word or an argument name that is misspelled or missing, or the punctuation is incorrect. Source=Microsoft JET Database Engine ErrorCode=-2147217900 StackTrace: at System.Data.OleDb.OleDbCommand.ExecuteCommandTextErrorHandling(OleDbHResult hr) at System.Data.OleDb.OleDbCommand.ExecuteCommandTextForSingleResult(tagDBPARAMS dbParams, Object& executeResult) at System.Data.OleDb.OleDbCommand.ExecuteCommandText(Object& executeResult) at System.Data.OleDb.OleDbCommand.ExecuteCommand(CommandBehavior behavior, Object& executeResult) at System.Data.OleDb.OleDbCommand.ExecuteReaderInternal(CommandBehavior behavior, String method) at System.Data.OleDb.OleDbCommand.ExecuteReader(CommandBehavior behavior) at System.Data.OleDb.OleDbCommand.ExecuteDbDataReader(CommandBehavior behavior) at System.Data.Common.DbCommand.ExecuteReader() at System.Data.Linq.SqlClient.SqlProvider.Execute(Expression query, QueryInfo queryInfo, IObjectReaderFactory factory, Object[] parentArgs, Object[] userArgs, ICompiledSubQuery[] subQueries, Object lastResult) at System.Data.Linq.SqlClient.SqlProvider.ExecuteAll(Expression query, QueryInfo[] queryInfos, IObjectReaderFactory factory, Object[] userArguments, ICompiledSubQuery[] subQueries) at System.Data.Linq.SqlClient.SqlProvider.System.Data.Linq.Provider.IProvider.Execute(Expression query) at System.Data.Linq.DataQuery`1.System.Linq.IQueryProvider.Execute[S](Expression expression) at System.Linq.Queryable.FirstOrDefault[TSource](IQueryable`1 source) at xxx.InventoryPopulator`2.Clear(String barcode) in F:\Projects\C#\xxx\xxx\InventoryPopulator.cs:line 38 [..etc..] InnerException: The debugger shows my query is: SELECT [t0].[SupplierID] AS [Id], [t0].[SupplierSKU] AS [Sku], [t0].[LocalSKU] AS [LocalSku], [t0].[ManufacturersBarcode] AS [Barcode], [t0].[QuantityAvailable] FROM [inventorySupplier] AS [t0] WHERE [t0].[ManufacturersBarcode] = @p0 And the Linq query which generates the above is: var items = from item in this.supplierItems where item.Barcode == barcode select item; How do I fix my query?

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• Natural language grammar and user-entered names

  - by Owen Blacker

  Some languages, particularly Slavic languages, change the endings of people's names according to the grammatical context. (For those of you who know grammar or studied languages that do this to words, such as German or Russian, and to help with search keywords, I'm talking about noun declension.) This is probably easiest with a set of examples (in Polish, to save the whole different-alphabet problem): Dorothy saw the cat — Dorota zobaczyla kota The cat saw Dorothy — Kot zobaczyl Dorote It is Dorothy’s cat — To jest kot Doroty I gave the cat to Dorothy — Dalam kota Dorotie I went for a walk with Dorothy — Poszlam na spacer z Dorota “Hello, Dorothy!” — “Witam, Doroto!” Now, if, in these examples, the name here were to be user-entered, that introduces a world of grammar nightmares. Importantly, if I went for Katie (Kasia), the examples are not directly comparable — 3 and 4 are both Kasi, rather than *Kasy and *Kasie — and male names will be wholly different again. I'm guessing someone has dealt with this situation before, but my Google-fu appears to be weak today. I can find a lot of links about natural-language processing, but I don'think that's quite what I want. To be clear: I'm only ever gonna have one user-entered name per user and I'm gonna need to decline them into known configurations — I'll have a localised text that will have placeholders something like {name nominative} and {name dative}, for the sake of argument. I really don't want to have to do lexical analysis of text to work stuff out, I'll only ever need to decline that one user-entered name. Anyone have any recommendations on how to do this, or do I need to start calling round localisation agencies ;o) Further reading (all on Wikipedia) for the interested: Declension Grammatical case Declension in Polish Declension in Russian Declension in Czech nouns and pronouns Disclaimer: I know this happens in many other languages; highlighting Slavic languages is merely because I have a project that is going to be localised into some Slavic languages.

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• Why an auto_ptr can "seal" a container

  - by icephere

  auto_ptr on wikipedia said that "an auto_ptr containing an STL container may be used to prevent further modification of the container.". It used the following example: auto_ptr<vector<ContainedType> > open_vec(new vector<ContainedType>); open_vec->push_back(5); open_vec->push_back(3); // Transfers control, but now the vector cannot be changed: auto_ptr<const vector<ContainedType> > closed_vec(open_vec); // closed_vec->push_back(8); // Can no longer modify If I uncomment the last line, g++ will report an error as t05.cpp:24: error: passing ‘const std::vector<int, std::allocator<int> >’ as ‘this’ argument of ‘void std::vector<_Tp, _Alloc>::push_back(const _Tp&) [with _Tp = int, _Alloc = std::allocator<int>]’ discards qualifiers I am curious why after transferring the ownership of this vector, it can no longer be modified? Thanks a lot!

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