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• Yield only as many are required from a generator

  - by Matt Joiner

  I wish to yield from a generator only as many items are required. In the following code a, b, c = itertools.count() I receive this exception: ValueError: too many values to unpack I've seen several related questions, however I have zero interest in the remaining items from the generator, I only wish to receive as many as I ask for, without providing that quantity in advance. It seems to me that Python determines the number of items you want, but then proceeds to try to read and store more than that number. How can I yield only as many items as I require, without passing in how many items I want?

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• Given a main function and a cleanup function, how (canonically) do I return an exit status in Bash/Linux?

  - by Zac B

  Context: I have a bash script (a wrapper for other scripts, really), that does the following pseudocode: do a main function if the main function returns: $returncode = $? #most recent return code if the main function runs longer than a timeout: kill the main function $returncode = 140 #the semi-canonical "exceeded allowed wall clock time" status run a cleanup function if the cleanup function returns an error: #nonzero return code exit $? #exit the program with the status returned from the cleanup function else #cleanup was successful .... Question: What should happen after the last line? If the cleanup function was successful, but the main function was not, should my program return 0 (for the successful cleanup), or $returncode, which contains the (possibly nonzero and unsuccessful) return code of the main function? For a specific application, the answer would be easy: "it depends on what you need the script for." However, this is more of a general/canonical question (and if this is the wrong place for it, kill it with fire): in Bash (or Linux in general) programming, do you typically want to return the status that "means" something (i.e. $returncode) or do you ignore such subjectivities and simply return the code of the most recent function? This isn't Bash-specific: if I have a standalone executable of any kind, how, canonically should it behave in these cases? Obviously, this is somewhat debatable. Even if there is a system for these things, I'm sure that a lot of people ignore it. All the same, I'd like to know. Cheers!

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• yield – Just yet another sexy c# keyword?

  - by George Mamaladze

  yield (see NSDN c# reference) operator came I guess with .NET 2.0 and I my feeling is that it’s not as wide used as it could (or should) be.   I am not going to talk here about necessarity and advantages of using iterator pattern when accessing custom sequences (just google it).   Let’s look at it from the clean code point of view. Let's see if it really helps us to keep our code understandable, reusable and testable.   Let’s say we want to iterate a tree and do something with it’s nodes, for instance calculate a sum of their values. So the most elegant way would be to build a recursive method performing a classic depth traversal returning the sum.           private int CalculateTreeSum(Node top)         {             int sumOfChildNodes = 0;             foreach (Node childNode in top.ChildNodes)             {                 sumOfChildNodes += CalculateTreeSum(childNode);             }             return top.Value + sumOfChildNodes;         }     “Do One Thing” Nevertheless it violates one of the most important rules “Do One Thing”. Our  method CalculateTreeSum does two things at the same time. It travels inside the tree and performs some computation – in this case calculates sum. Doing two things in one method is definitely a bad thing because of several reasons: ·          Understandability: Readability / refactoring ·          Reuseability: when overriding - no chance to override computation without copying iteration code and vice versa. ·          Testability: you are not able to test computation without constructing the tree and you are not able to test correctness of tree iteration.   I want to spend some more words on this last issue. How do you test the method CalculateTreeSum when it contains two in one: computation & iteration? The only chance is to construct a test tree and assert the result of the method call, in our case the sum against our expectation. And if the test fails you do not know wether was the computation algorithm wrong or was that the iteration? At the end to top it all off I tell you: according to Murphy’s Law the iteration will have a bug as well as the calculation. Both bugs in a combination will cause the sum to be accidentally exactly the same you expect and the test will PASS. J   Ok let’s use yield! That’s why it is generally a very good idea not to mix but isolate “things”. Ok let’s use yield!           private int CalculateTreeSumClean(Node top)         {             IEnumerable<Node> treeNodes = GetTreeNodes(top);             return CalculateSum(treeNodes);         }             private int CalculateSum(IEnumerable<Node> nodes)         {             int sumOfNodes = 0;             foreach (Node node in nodes)             {                 sumOfNodes += node.Value;             }             return sumOfNodes;         }           private IEnumerable<Node> GetTreeNodes(Node top)         {             yield return top;             foreach (Node childNode in top.ChildNodes)             {                 foreach (Node currentNode in GetTreeNodes(childNode))                 {                     yield return currentNode;                 }             }         }   Two methods does not know anything about each other. One contains calculation logic another jut the iteration logic. You can relpace the tree iteration algorithm from depth traversal to breath trevaersal or use stack or visitor pattern instead of recursion. This will not influence your calculation logic. And vice versa you can relace the sum with product or do whatever you want with node values, the calculateion algorithm is not aware of beeng working on some tree or graph.  How about not using yield? Now let’s ask the question – what if we do not have yield operator? The brief look at the generated code gives us an answer. The compiler generates a 150 lines long class to implement the iteration logic.       [CompilerGenerated]     private sealed class <GetTreeNodes>d__0 : IEnumerable<Node>, IEnumerable, IEnumerator<Node>, IEnumerator, IDisposable     {         ...        150 Lines of generated code        ...     }   Often we compromise code readability, cleanness, testability, etc. – to reduce number of classes, code lines, keystrokes and mouse clicks. This is the human nature - we are lazy. Knowing and using such a sexy construct like yield, allows us to be lazy, write very few lines of code and at the same time stay clean and do one thing in a method. That's why I generally welcome using staff like that.   Note: The above used recursive depth traversal algorithm is possibly the compact one but not the best one from the performance and memory utilization point of view. It was taken to emphasize on other primary aspects of this post.

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• ~/.bashrc return can only 'return' from a function or sourced script

  - by Timothy

  I am trying to setup a OpenStack box to have a look at OpenStack Object Storage (Swift). Looking through the web I found this link; http://swift.openstack.org/development_saio.html#loopback-section I followed the instructions line by line but stuck on step 7 in the "Getting the code and setting up test environment" section. When I execute ~/.bashrc I get; line 6: return: can only 'return' from a function or sourced script. Below is the Line 6 extract from ~/.bashrc. My first reaction is to comment this line out, but I dont know what it does. Can anyone help? #If not running interactively, dont't do anything [ -z "$PS1" ] && return I'm running Ubuntu 12.04 as a VM on Hyper-v if knowing that is useful.

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• What to Return? Error String, Bool with Error String Out, or Void with Exception

  - by Ranger Pretzel

  I spend most of my time in C# and am trying to figure out which is the best practice for handling an exception and cleanly return an error message from a called method back to the calling method. For example, here is some ActiveDirectory authentication code. Please imagine this Method as part of a Class (and not just a standalone function.) bool IsUserAuthenticated(string domain, string user, string pass, out errStr) { bool authentic = false; try { // Instantiate Directory Entry object DirectoryEntry entry = new DirectoryEntry("LDAP://" + domain, user, pass); // Force connection over network to authenticate object nativeObject = entry.NativeObject; // No exception thrown? We must be good, then. authentic = true; } catch (Exception e) { errStr = e.Message().ToString(); } return authentic; } The advantages of doing it this way are a clear YES or NO that you can embed right in your If-Then-Else statement. The downside is that it also requires the person using the method to supply a string to get the Error back (if any.) I guess I could overload this method with the same parameters minus the "out errStr", but ignoring the error seems like a bad idea since there can be many reasons for such a failure... Alternatively, I could write a method that returns an Error String (instead of using "out errStr") in which a returned empty string means that the user authenticated fine. string AuthenticateUser(string domain, string user, string pass) { string errStr = ""; try { // Instantiate Directory Entry object DirectoryEntry entry = new DirectoryEntry("LDAP://" + domain, user, pass); // Force connection over network to authenticate object nativeObject = entry.NativeObject; } catch (Exception e) { errStr = e.Message().ToString(); } return errStr; } But this seems like a "weak" way of doing things. Or should I just make my method "void" and just not handle the exception so that it gets passed back to the calling function? void AuthenticateUser(string domain, string user, string pass) { // Instantiate Directory Entry object DirectoryEntry entry = new DirectoryEntry("LDAP://" + domain, user, pass); // Force connection over network to authenticate object nativeObject = entry.NativeObject; } This seems the most sane to me (for some reason). Yet at the same time, the only real advantage of wrapping those 2 lines over just typing those 2 lines everywhere I need to authenticate is that I don't need to include the "LDAP://" string. The downside with this way of doing it is that the user has to put this method in a try-catch block. Thoughts? Is there another way of doing this that I'm not thinking of?

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• The underlying mechanism in 'yield return www' of Unity3D Game Engine

  - by thyandrecardoso

  In the Unity3D game engine, a common code sequence for getting remote data is this: WWW www = new WWW("http://remote.com/data/location/with/texture.png"); yield return www; What is the underlying mechanism here? I know we use the yield mechanism in order to allow the next frame to be processed, while the download is being completed. But what is going on under the hood when we do the yield return www ? What method is being called (if any, on the WWW class)? Is Unity using threads? Is the "upper" Unity layer getting hold of www instance and doing something?

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• When should a method of a class return the same instance after modifying itself?

  - by modiX

  I have a class that has three methods A(), B() and C(). Those methods modify the own instance. While the methods have to return an instance when the instance is a separate copy (just as Clone()), I got a free choice to return void or the same instance (return this;) when modifying the same instance in the method and not returning any other value. When deciding for returning the same modified instance, I can do neat method chains like obj.A().B().C();. Would this be the only reason for doing so? Is it even okay to modify the own instance and return it, too? Or should it only return a copy and leave the original object as before? Because when returning the same modified instance the user would maybe admit the returned value is a copy, otherwise it would not be returned? If it's okay, what's the best way to clarify such things on the method?

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• Is there a way to implement Caliburn-like co-routines in VB.NET since there's no yield keyword

  - by Miroslav Popovic

  Note that I'm aware of other yield in vb.net questions here on SO. I'm playing around with Caliburn lately. Bunch of great stuff there, including co-routines implementation. Most of the work I'm doing is C# based, but now I'm also creating an architecture guideline for a VB.NET only shop, based on Rob's small MVVM framework. Everything looks very well except using co-routines from VB. Since VB 10 is used, we can try something like Bill McCarthy's suggestion: Public Function Lines(ByVal rdr as TextReader) As IEnumerable(Of String) Return New GenericIterator(Of String) (Function(ByRef nextItem As String) As Boolean nextItem = rdr.ReadLine Return nextItem IsNot Nothing End Function) End Function I'm just failing to comprehend how a little more complex co-routine method like the one below (taken from Rob's GameLibrary) could be written in VB: public IEnumerable<IResult> ExecuteSearch() { var search = new SearchGames { SearchText = SearchText }.AsResult(); yield return Show.Busy(); yield return search; var resultCount = search.Response.Count(); if (resultCount == 0) SearchResults = _noResults.WithTitle(SearchText); else if (resultCount == 1 && search.Response.First().Title == SearchText) { var getGame = new GetGame { Id = search.Response.First().Id }.AsResult(); yield return getGame; yield return Show.Screen<ExploreGameViewModel>() .Configured(x => x.WithGame(getGame.Response)); } else SearchResults = _results.With(search.Response); yield return Show.NotBusy(); } Any idea how to achieve that, or any thoughts on using Caliburn co-routines in VB?

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• sendmail is using return-path instead of from address

  - by magd1

  I have a customer that is complaining about emails marked as spam. I'm looking at the header. It shows the correct From: [email protected] However, it doesn't like the return-path. Return-Path: <[email protected]> Received-SPF: neutral (google.com: x.x.x.x is neither permitted nor denied by domain of [email protected]) client-ip=x.x.x.x; Authentication-Results: mx.google.com; spf=neutral (google.com: x.x.x.x is neither permitted nor denied by domain of [email protected]) [email protected] How do I configure sendmail to use the From address for the Return-Path?

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• Linux Exim set return-path header automaticly using from header

  - by solomongaby

  Hello, I use Exim on a Centos distribution and have some problems with the mail sending. In order to make all the email pass the spam filters the "Return-path" and "Sender" headers have to be attached to each email. What should I do in order to have "Return-path" and "Sender" headers added by Exim to be exactly the same as the "From" header created by my mail client ? Thanks

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• Which statically typed languages support intersection types for function return values?

  - by stakx

  Initial note: This question got closed after several edits because I lacked the proper terminology to state accurately what I was looking for. Sam Tobin-Hochstadt then posted a comment which made me recognise exactly what that was: programming languages that support intersection types for function return values. Now that the question has been re-opened, I've decided to improve it by rewriting it in a (hopefully) more precise manner. Therefore, some answers and comments below might no longer make sense because they refer to previous edits. (Please see the question's edit history in such cases.) Are there any popular statically & strongly typed programming languages (such as Haskell, generic Java, C#, F#, etc.) that support intersection types for function return values? If so, which, and how? (If I'm honest, I would really love to see someone demonstrate a way how to express intersection types in a mainstream language such as C# or Java.) I'll give a quick example of what intersection types might look like, using some pseudocode similar to C#: interface IX { … } interface IY { … } interface IB { … } class A : IX, IY { … } class B : IX, IY, IB { … } T fn() where T : IX, IY { return … ? new A() : new B(); } That is, the function fn returns an instance of some type T, of which the caller knows only that it implements interfaces IX and IY. (That is, unlike with generics, the caller doesn't get to choose the concrete type of T — the function does. From this I would suppose that T is in fact not a universal type, but an existential type.) P.S.: I'm aware that one could simply define a interface IXY : IX, IY and change the return type of fn to IXY. However, that is not really the same thing, because often you cannot bolt on an additional interface IXY to a previously defined type A which only implements IX and IY separately. Footnote: Some resources about intersection types: Wikipedia article for "Type system" has a subsection about intersection types. Report by Benjamin C. Pierce (1991), "Programming With Intersection Types, Union Types, and Polymorphism" David P. Cunningham (2005), "Intersection types in practice", which contains a case study about the Forsythe language, which is mentioned in the Wikipedia article. A Stack Overflow question, "Union types and intersection types" which got several good answers, among them this one which gives a pseudocode example of intersection types similar to mine above.

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• Is it bad to have multiple return statements?

  - by scot

  Hi, I have a code somethg like below: int method(string a ,int b , int c){ if(cond1) return -1; if(cond2 || cond3) return 3; if(cond1 && cond2) return 0; else return -999; } Does it perform badly when compared to having multiple if else and have single return?

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• Java return to position N in call stack.

  - by Finbarr

  Using the return keyword in Java code will return execution to the last piece of calling code in the call stack. If object foo calls baz.bar(), the return keyword in the bar method will continue code execution in foo. Let's say I have object foo that calls foofoo that calls foofoofoo in the above scenario, and foofoofoo calls baz.bar(). Is there anyway in Java to use the return keyword, or something else, so that the code in the bar method can return all the way back up to foo? (WITHOUT THROWING AN EXCEPTION)

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• unable to return 'true' value in C function

  - by Byron

  If Im trying to check an input 5 byte array (p) against a 5 byte array stored in flash (data), using the following function (e2CheckPINoverride), to simply return either a true or false value. But it seems, no matter what I try, it only returns as 'false'. I call the function here: if (e2CheckPINoverride(pinEntry) == 1){ PTDD_PTDD1 = 1; } else{ PTDD_PTDD1 = 0; } Here is the function: BYTE e2CheckPINoverride(BYTE *p) { BYTE i; BYTE data[5]; if(e2Read(E2_ENABLECODE, data, 5)) { if(data[0] != p[0]) return FALSE; if(data[1] != p[1]) return FALSE; if(data[2] != p[2]) return FALSE; if(data[3] != p[3]) return FALSE; if(data[4] != p[4]) return FALSE; } return TRUE; } I have already assigned true and false in the defines.h file: #ifndef TRUE #define TRUE ((UCHAR)1) #endif #ifndef FALSE #define FALSE ((UCHAR)0) #endif and where typedef unsigned char UCHAR; when i step through the code, it performs all the checks correctly, it passes in the correct value, compares it correctly and then breaks at the correct point, but is unable to process the return value of true? please help?

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• Omit return type in C++0x

  - by Clinton

  I've recently found myself using the following macro with gcc 4.5 in C++0x mode: #define RETURN(x) -> decltype(x) { return x; } And writing functions like this: template <class T> auto f(T&& x) RETURN (( g(h(std::forward<T>(x))) )) I've been doing this to avoid the inconvenience having to effectively write the function body twice, and having keep changes in the body and the return type in sync (which in my opinion is a disaster waiting to happen). The problem is that this technique only works on one line functions. So when I have something like this (convoluted example): template <class T> auto f(T&& x) -> ... { auto y1 = f(x); auto y2 = h(y1, g1(x)); auto y3 = h(y1, g2(x)); if (y1) { ++y3; } return h2(y2, y3); } Then I have to put something horrible in the return type. Furthermore, whenever I update the function, I'll need to change the return type, and if I don't change it correctly, I'll get a compile error if I'm lucky, or a runtime bug in the worse case. Having to copy and paste changes to two locations and keep them in sync I feel is not good practice. And I can't think of a situation where I'd want an implicit cast on return instead of an explicit cast. Surely there is a way to ask the compiler to deduce this information. What is the point of the compiler keeping it a secret? I thought C++0x was designed so such duplication would not be required.

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• How to Pythonically yield all values from a list?

  - by bodacydo

  Suppose I have a list that I wish not to return but to yield values from. What is the most Pythonic way to do that? Here is what I mean. Thanks to some non-lazy computation I have computed the list ['a', 'b', 'c', 'd'], but my code through the project uses lazy computation, so I'd like to yield values from my function instead of returning the whole list. I currently wrote it as following: List = ['a', 'b', 'c', 'd'] for item in List: yield item But this doesn't feel Pythonic to me. Looking forward to some suggestions, thanks. Boda Cydo.

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• Method return values and exceptions

  - by dnagirl

  I have an interface called iIncident which defines a single method when(). when() should return a DateTime object. I'm trying to decide what to do if $object->when() has no DateTime to return as might be the case just after an object is instantiated and before all its properties are set. My choices are: return false throw some kind of Exception return some default DateTime like '9999-01-01' My inclination is to go with an Exception since $object really can't act as an incident until it knows when it occurred. I don't want to return a default DateTime because it complicates comparisons and it's not true. And I don't really want to return false because then I have to check for it every time I call the method- but if that is the preferred method, I guess I will. Is throwing an exception the best way? And is there a predefined exception type I should use (none of the SPL ones struck me as particularly appropriate- but that might just indicate my lack of experience)?

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• Return from parent sub in Perl

  - by JS Bangs

  I want to write a subroutine which causes the caller to return under certain conditions. This is meant to be used as a shortcut for validating input to a function. What I have so far is: sub needs($$) { my ($condition, $message) = @_; if (not $condition) { print "$message\n"; # would like to return from the *parent* here } return $condition; } sub run_find { my $arg = shift @_; needs $arg, "arg required" or return; needs exists $lang{$arg}, "No such language: $arg" or return; # etc. } The advantage of returning from the caller in needs would then be to avoid having to write the repetitive or return inside run_find and similar functions.

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• C++ return a "NULL" object if search result not found

  - by aduric

  I'm pretty new to C++ so I tend to design with a lot of Java-isms while I'm learning. Anyway, in Java, if I had class with a 'search' method that would return an object T from a Collection< T that matched a specific parameter, I would return that object and if the object was not found in the collection, I would return a NULL. Then in my calling function I would just check if(T != NULL) { ... } In C++, I'm finding out that I can't return a NULL if the object doesn't exist. I just want to return an 'indicator' of type T that notifies the calling function that no object has been found. I don't want to throw an exception because it's not really an exceptional circumstance. class Node { .... Attr& getAttribute(const string& attribute_name) const { //search collection //if found at i return attributes[i]; //if not found return NULL; } private: vector<Attr> attributes; }

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• what is the point of return in ruby?

  - by Sam

  What is the difference between return and just putting a variable such as the following: return def write_code(number_of_errors) if number_of_errors > 1 mood = "WTF" else mood = "No Problem" end mood end no return def write_code(number_of_errors) if number_of_errors > 1 mood = "WTF" else mood = puts "No Problem" end return mood end

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• Is return an operator or a function?

  - by eSKay

  This is too basic I think, but how do both of these work? return true; // 1 and return (true); // 2 Similar: sizeof, exit My guess: If return was a function, 1 would be erroneous. So, return should be a unary operator that can also take in brackets... pretty much like unary minus: -5 and -(5), both are okay. Is that what it is - a unary operator?

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• Return an empty C-String

  - by Evorlor

  Simple Question: How do you return an empty C-String with as little code as possible? I have code that needs to return an empty char*. I am looking for something along the lines of return "";. I know there are several ways to do this, but I am looking for the most efficient way possible. Using return ""; gives warning: conversion from string literal to 'char *' is deprecated [-Wdeprecated-writable-strings] Thanks!

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• Is thread-safe to use the 'yield' operator inside a static method'?

  - by SDReyes

  Is a static method using yield thread-safe? For example: public static T Select<T>( this IDataReader reader, Func<IDataReader, T> convertString ) { { while( reader.Read( ) ) yield return convertString( reader ); } }

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• Is thread-safe to use the 'yield' operator inside an extension method'?

  - by SDReyes

  Would be thread-safe to use the yield operator inside an extension method? For example: public static CartItem GetItems( this Cart cart ) { { while( cart.hasNext() ) yield return cart.GetNextItem( ); } }

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• const return value and template instantiation

  - by Rimo

  From Herb Sutter's GotW #6 Return-by-value should normally be const for non-builtin return types. .... Note: Lakos (pg. 618) argues against returning const value, and notes that it is redundant for builtins anyway (for example, returning "const int"), which he notes may interfere with template instantiation. .... While Sutter seems to disagree on whether to return a const value or non-const value when returning an object of a non-built type by value with Lakos, he generally agrees that returning a const value of a built-in type (e.g const int) is not a good idea. While I understand why that is useless because the return value cannot be modified as it is an rvalue, I cannot find an example of how that might interfere with template instantiation. Please give me an example of how having a const qualifier for a return type might interfere with template instantiation.

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