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  • using lambda instead of let in scheme

    - by Radagaisus
    Hey, In SICP 1.2.1 there is a function that makes a rational number, as follow: (define (make-rat n d) (let ((g (gcd n d))) (cons (/ n g) (/ d g)))) I'm just curious how you can implement the same thing using lambda instead of let, without calling GCD twice. I couldn't figure it out myself.

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  • Lambda functions

    - by Oden
    Hey, I'm really interested in the way of using lambda functions. Does it make sense to use them in a high-level programming language? If yes, why? Is this really just a function embedded in a function (like this) or is there more behind?

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  • Lambda recursive PHP functions.

    - by Kendall Hopkins
    Is it possible to have a PHP function that is both recursive and anonymous (lambda). This is my attempt to get it to work, but it doesn't pass in the function name. $factorial = function( $n ) use ( $factorial ) { if( $n == 1 ) return 1; return $factorial( $n - 1 ) * $n; }; print $factorial( 5 ); I'm also aware that this is a bad way to implement factorial, it's just an example.

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  • Perfect Forwarding to async lambda

    - by Alexander Kondratskiy
    I have a function template, where I want to do perfect forwarding into a lambda that I run on another thread. Here is a minimal test case which you can directly compile: #include <thread> #include <future> #include <utility> #include <iostream> #include <vector> /** * Function template that does perfect forwarding to a lambda inside an * async call (or at least tries to). I want both instantiations of the * function to work (one for lvalue references T&, and rvalue reference T&&). * However, I cannot get the code to compile when calling it with an lvalue. * See main() below. */ template <typename T> std::string accessValueAsync(T&& obj) { std::future<std::string> fut = std::async(std::launch::async, [](T&& vec) mutable { return vec[0]; }, std::forward<T>(obj)); return fut.get(); } int main(int argc, char const *argv[]) { std::vector<std::string> lvalue{"Testing"}; // calling with what I assume is an lvalue reference does NOT compile std::cout << accessValueAsync(lvalue) << std::endl; // calling with rvalue reference compiles std::cout << accessValueAsync(std::move(lvalue)) << std::endl; // I want both to compile. return 0; } For the non-compiling case, here is the last line of the error message which is intelligible: main.cpp|13 col 29| note: no known conversion for argument 1 from ‘std::vector<std::basic_string<char> >’ to ‘std::vector<std::basic_string<char> >&’ I have a feeling it may have something to do with how T&& is deduced, but I can't pinpoint the exact point of failure and fix it. Any suggestions? Thank you! EDIT: I am using gcc 4.7.0 just in case this could be a compiler issue (probably not)

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  • check that int array contains !=0 value using lambda

    - by netmajor
    hey, I have two-dimension array List<List<int>> boardArray How can I enumerate throw this array to check that it contains other value than 0 ? I think about boardArray.Contains and ForEach ,cause it return bool value but I don't have too much experience with lambda expression :/ Please help :)

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  • C# Lambda Expression Speed

    - by Nathan
    I have not used many lambda expressions before and I ran into a case where I thought I could make slick use of one. I have a custom list of ~19,000 records and I need to find out if a record exists or not in the list so instead of writing a bunch of loops or using linq to go through the list I decided to try this: for (int i = MinX; i <= MaxX; ++i) { tempY = MinY; while (tempY <= MaxY) { bool exists = myList.Exists(item => item.XCoord == i && item.YCoord == tempY); ++tempY; } } Only problem is it take ~9 - 11 seconds to execute. Am I doing something wrong is this just a case of where I shouldn't be using an expression like this? Thanks.

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  • LINQ query and lambda expressions

    - by user329269
    I'm trying to write a LINQ query and am having problems. I'm not sure if lambda expressions are the answer or not but I think they may be. I have two combo boxes on my form: "State" and "Color". I want to select Widgets from my database based on the values of these two dropdowns. My widgets can be in one of the following states: Not Started, In Production, In Finishing, In Inventory, Sold. Widgets can have any color in the 'color' table in the database. The 'state' combobox has selections "Not Sold," "In Production/Finishing", "Not Started," "In Production," "In Finishing," "In Inventory," "Sold." (I hope these are self-explanatory.) The 'color' dropdown has "All Colors," and a separate item for each color in the database. How can I create a LINQ query to select the widgets I want from the database based on the dropdowns?

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  • Problem with nested lambda expressions.

    - by Lehto
    Hey I'm trying to do a nested lambda expression like to following: textLocalizationTable.Where( z => z.SpokenLanguage.Any( x => x.FromCulture == "en-GB") ).ToList(); but i get the error: Member access 'System.String FromCulture' of 'DomainModel.Entities.SpokenLanguage' not legal on type 'System.Data.Linq.EntitySet`1[DomainModel.Entities.SpokenLanguage]. TextLocalization has this relation to spokenlanguage: [Association(OtherKey = "LocalizationID", ThisKey = "LocalizationID", Storage = "_SpokenLanguage")] private EntitySet<SpokenLanguage> _SpokenLanguage = new EntitySet<SpokenLanguage>(); public EntitySet<SpokenLanguage> SpokenLanguage { set { _SpokenLanguage = value; } get { return _SpokenLanguage; } } Any idea what is wrong?

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  • .NET C# setting the value of a field defined by a lambda selector

    - by Frank Michael Kraft
    I have a generic class HierarchicalBusinessObject. In the constructor of the class I pass a lambda expression that defines a selector to a field of TModel. protected HierarchicalBusinessObject (Expression<Func<TModel,string>> parentSelector) A call would look like this, for example: public class WorkitemBusinessObject : HierarchicalBusinessObject<Workitem,WorkitemDataContext> { public WorkitemBusinessObject() : base(w => w.SuperWorkitem, w => w.TopLevel == true) { } } I am able to use the selector for read within the class. For example: sourceList.Select(_parentSelector.Compile()).Where(... Now I am asking myself how I could use the selector to set a value to the field. Something like selector.Body() .... Field...

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  • Lambda expressions and nullable types

    - by Mathew
    I have two samples of code. One works and returns the correct result, one throws a null reference exception. What's the difference? I know there's some magic happening with capturing variables for the lambda expression but I don't understand what's going on behind the scenes here. int? x = null; bool isXNull = !x.HasValue; // this works var result = from p in data.Program where (isXNull) select p; return result.Tolist(); // this doesn't var result2 = from p in data.Program where (!x.HasValue) select p; return result2.ToList();

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  • Using a linq or lambda expression in C# return a collection plus a single value

    - by ahsteele
    I'd like to return a collection plus a single value. Presently I am using a field to create a new list adding a value to the list and then returning the result. Is there a way to do this with a linq or lambda expression? private List<ChargeDetail> _chargeBreakdown = new List<ChargeDetail>(); public ChargeDetail PrimaryChargeDetail { get; set; } public List<ChargeDetail> ChargeBreakdown { get { List<ChargeDetail> result = new List<ChargeDetail>(_chargeBreakdown); result.Add(PrimaryChargeDetail); return result; } }

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  • Using Lambda Statements for Event Handlers

    - by lush
    I currently have a page which is declared as follows: public partial class MyPage : System.Web.UI.Page { protected void Page_Load(object sender, EventArgs e) { //snip MyButton.Click += (o, i) => { //snip } } } I've only recently moved to .NET 3.5 from 1.1, so I'm used to writing event handlers outside of the Page_Load. My question is; are there any performance drawbacks or pitfalls I should watch out for when using the lambda method for this? I prefer it, as it's certainly more concise, but I do not want to sacrifice performance to use it. Thanks.

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  • Wrap Sub as Function for use in Lambda

    - by Luhmann
    I have a problem with VB and Moq. I need to call a verify on a Sub. Like so: logger.Verify(Function(x) x.Log, Times.AtLeastOnce) And my logger looks like this: Public Interface ILogger Sub Log() End Interface But with VB this is not possible, because the Log method is a Sub, and thereby does not produce a value. I don't want to change the method to be a function. Whats the cleanest way of working around this limitation and is there any way to wrap the Sub as a Function like the below? logger.Verify(Function(x) ToFunc(AddressOf x.Log)) I have tried this, but i get: Lambda Parameter not in scope

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  • lambda+for_each+delete on STL containers

    - by rubenvb
    I'm trying to get a simple delete every pointer in my vector/list/... function written with an ultra cool lambda function. Mind you, I don't know c**p about those things :) template <typename T> void delete_clear(T const& cont) { for_each(T.begin(), T.end(), [](???){ ???->delete() } ); T.clear(); } I have no clue what to fill in for the ???'s. Any help is greatly appreciated!

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  • Using linq to combine objects

    - by DotnetDude
    I have 2 instances of a class that implements the IEnumerable interface. I would like to create a new object and combine both of them into one. I understand I can use the for..each to do this. Is there a linq/lambda expression way of doing this?

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  • Entity Framework - Condition on one to many join (Lambda)

    - by nirpi
    Hi, I have 2 entities: Customer & Account, where a customer can have multiple accounts. On the account, I have a "PlatformTypeId" field, which I need to condition on (multiple values), among other criterions. I'm using Lambda expressions, to build the query. Here's a snippet: var customerQuery = (from c in context.CustomerSet.Include("Accounts") select c); if (criterions.UserTypes != null && criterions.UserTypes.Count() > 0) { List<short> searchCriterionsUserTypes = criterions.UserTypes.Select(i => (short)i).ToList(); customerQuery = customerQuery.Where(CommonDataObjects.LinqTools.BuildContainsExpression<Customer, short>(c => c.UserTypeId, searchCriterionsUserTypes)); } // Other criterions, including the problematic platforms condition (below) var customers = customerQuery.ToList(); I can't figure out how to build the accounts' platforms condition: if (criterions.Platforms != null && criterions.Platforms.Count() > 0) { List<short> searchCriterionsPlatforms = criterions.Platforms.Select(i => (short)i).ToList(); customerQuery = customerQuery.Where(c => c.Accounts.Where(LinqTools.BuildContainsExpression<Account, short>(a => a.PlatformTypeId, searchCriterionsPlatforms))); } (The BuildContainsExpression is a method we use to build the expression for the multi-select) I'm getting a compilation error: The type arguments for method 'System.Linq.Enumerable.Where(System.Collections.Generic.IEnumerable, System.Func)' cannot be inferred from the usage. Try specifying the type arguments explicitly. Any idea how to fix this? Thanks, Nir.

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  • Convert Lambda from C# to VB.NET

    - by Iosu
    How would I translate this C# lambda expression into VB.NET ? query.ExecuteAsync(op => op.Results.ForEach(Employees.Add)); using System; using System.Net; using System.Windows; using System.Windows.Controls; using System.Windows.Documents; using System.Windows.Ink; using System.Windows.Input; using System.Windows.Media; using System.Windows.Media.Animation; using System.Windows.Shapes; using System.Collections.ObjectModel; using IdeaBlade.Core; using IdeaBlade.EntityModel; namespace SimpleSteps { public class MainPageViewModel { public MainPageViewModel() { Employees = new ObservableCollection(); var mgr = new NorthwindIBEntities(); var query = mgr.Employees; query.ExecuteAsync(op = op.Results.ForEach(Employees.Add)); } public ObservableCollection<Employee> Employees { get; private set; } } }

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  • Help me clean up this crazy lambda with the out keyword

    - by Sarah Vessels
    My code looks ugly, and I know there's got to be a better way of doing what I'm doing: private delegate string doStuff( PasswordEncrypter encrypter, RSAPublicKey publicKey, string privateKey, out string salt ); private bool tryEncryptPassword( doStuff encryptPassword, out string errorMessage ) { ...get some variables... string encryptedPassword = encryptPassword(encrypter, publicKey, privateKey, out salt); ... } This stuff so far doesn't bother me. It's how I'm calling tryEncryptPassword that looks so ugly, and has duplication because I call it from two methods: public bool method1(out string errorMessage) { string rawPassword = "foo"; return tryEncryptPassword( (PasswordEncrypter encrypter, RSAPublicKey publicKey, string privateKey, out string salt) => encrypter.EncryptPasswordAndDoStuff( // Overload 1 rawPassword, publicKey, privateKey, out salt ), out errorMessage ); } public bool method2(SecureString unencryptedPassword, out string errorMessage) { return tryEncryptPassword( (PasswordEncrypter encrypter, RSAPublicKey publicKey, string privateKey, out string salt) => encrypter.EncryptPasswordAndDoStuff( // Overload 2 unencryptedPassword, publicKey, privateKey, out salt ), out errorMessage ); } Two parts to the ugliness: I have to explicitly list all the parameter types in the lambda expression because of the single out parameter. The two overloads of EncryptPasswordAndDoStuff take all the same parameters except for the first parameter, which can either be a string or a SecureString. So method1 and method2 are pretty much identical, they just call different overloads of EncryptPasswordAndDoStuff. Any suggestions? Edit: if I apply Jeff's suggestions, I do the following call in method1: return tryEncryptPassword( (encrypter, publicKey, privateKey) => { var result = new EncryptionResult(); string salt; result.EncryptedValue = encrypter.EncryptPasswordAndDoStuff( rawPassword, publicKey, privateKey, out salt ); result.Salt = salt; return result; }, out errorMessage ); Much the same call is made in method2, just with a different first value to EncryptPasswordAndDoStuff. This is an improvement, but it still seems like a lot of duplicated code.

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  • Using lambda expressions and linq

    - by Andy
    So I've just started working with linq as well as using lambda expressions. I've run into a small hiccup while trying to get some data that I want. This method should return a list of all projects that are open or in progress from Jira Here's the code public static List<string> getOpenIssuesListByProject(string _projectName) { JiraSoapServiceService jiraSoapService = new JiraSoapServiceService(); string token = jiraSoapService.login(DEFAULT_UN, DEFAULT_PW); string[] keys = { getProjectKey(_projectName) }; RemoteStatus[] statuses = jiraSoapService.getStatuses(token); var desiredStatuses = statuses.Where(x => x.name == "Open" || x.name == "In Progress") .Select(x=>x.id); RemoteIssue[] AllIssues = jiraSoapService.getIssuesFromTextSearchWithProject(token, keys, "", 99); IEnumerable<RemoteIssue> openIssues = AllIssues.Where(x=> { foreach (var v in desiredStatuses) { if (x.status == v) return true; else return false; } return false; }); return openIssues.Select(x => x.key).ToList(); } Right now this only select issues that are "Open", and seems to skip those that are "In Progress". My question: First, why am I only getting the "Open" Issues, and second is there a better way to do this? The reason I get all the statuses first is that the issue only stores that statuses ID, so I get all the statuses, get the ID's that match "Open" and "In Progress", and then match those ID numbers to the issues status field.

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  • Create lambda action from function expression

    - by Martin Robins
    It is relatively easy to create a lambda function that will return the value of a property from an object, even including deep properties... Func<Category, string> getCategoryName = new Func<Category, string>(c => c.Name); and this can be called as follows... string categoryName = getCategoryName(this.category); But, given only the resulting function above (or the expression originally used to create the function), can anybody provide an easy way to create the opposing action... Action<Category, string> setCategoryName = new Action<Category, string>((c, s) => c.Name = s); ...that will enable the same property value to be set as follows? setCategoryName(this.category, ""); Note that I am looking for a way to create the action programatically from the function or expression - I hope that I have shown that I already know how to create it manually. I am open to answers that work in both .net 3.5 and 4.0. Thanks.

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  • for loop vs std::for_each with lambda

    - by Andrey
    Let's consider a template function written in C++11 which iterates over a container. Please exclude from consideration the range loop syntax because it is not yet supported by the compiler I'm working with. template <typename Container> void DoSomething(const Container& i_container) { // Option #1 for (auto it = std::begin(i_container); it != std::end(i_container); ++it) { // do something with *it } // Option #2 std::for_each(std::begin(i_container), std::end(i_container), [] (typename Container::const_reference element) { // do something with element }); } What are pros/cons of for loop vs std::for_each in terms of: a) performance? (I don't expect any difference) b) readability and maintainability? Here I see many disadvantages of for_each. It wouldn't accept a c-style array while the loop would. The declaration of the lambda formal parameter is so verbose, not possible to use auto there. It is not possible to break out of for_each. In pre- C++11 days arguments against for were a need of specifying the type for the iterator (doesn't hold any more) and an easy possibility of mistyping the loop condition (I've never done such mistake in 10 years). As a conclusion, my thoughts about for_each contradict the common opinion. What am I missing here?

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