Search Results

Search found 2455 results on 99 pages for 'lambda expressions'.

Page 2/99 | < Previous Page | 1 2 3 4 5 6 7 8 9 10 11 12  | Next Page >

• lambda expression based reflection vs normal reflection

  - by bitbonk

  What is the difference between normal reflection and the reflection that can be done with lambda expressions such as this (taken form build your own MVVM): public void NotifyOfPropertyChange<TProperty>(Expression<Func<TProperty>> property) { var lambda = (LambdaExpression)property; MemberExpression memberExpression; if (lambda.Body is UnaryExpression) { var unaryExpression = (UnaryExpression)lambda.Body; memberExpression = (MemberExpression)unaryExpression.Operand; } else memberExpression = (MemberExpression)lambda.Body; NotifyOfPropertyChange(memberExpression.Member.Name); } Is the lambda based reflection just using the normal reflection APIs internally? Or is this something significantly different. What is ther perfomance difference?

  Read the article

• Regular Expressions. Remember it, write it, test it.

  - by outcoldman

  I should say that I’m fan of regular expressions. Whenever I see the problem, which I can solve with Regex, I felt a burning desire to do it and going to write new test for new regex. Previously I had installed SharpDevelop Studio just for good regular expression tool in it (Why VS doesn’t have one?). But now I’m a little wiser, and for each Regex I write a separate test. I find it difficult to remember the syntax of regular expressions (I don’t write them very often); I always forget which character is responsible for the beginning of the line, etc. So I use external small and easy articles like this “Regular expressions - An introduction”. Now I want to show you little samples of regular expressions and want to show you how to test these samples. Read more... (redirect to http://outcoldman.ru)

  Read the article

• What's the point of lambda in scheme?

  - by incrediman

  I am learning scheme. I know how to use both lambda and let expressions. However I'm struggling to figure out what the point is of using lambda. Can't you do everything with let that you can with lambda? It would be especially helpful to see an example of a situation where a lambda expression is a better choice than let. One other thing - are there also situations where let is more useful than lambda? If so such an example would be nice as well. Thanks!

  Read the article

• Ambiguous Evaluation of Lambda Expression on Array

  - by Joe

  I would like to use a lambda that adds one to x if x is equal to zero. I have tried the following expressions: t = map(lambda x: x+1 if x==0 else x, numpy.array) t = map(lambda x: x==0 and x+1 or x, numpy.array) t = numpy.apply_along_axis(lambda x: x+1 if x==0 else x, 0, numpy.array) Each of these expressions returns the following error: ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all() My understanding of map() and numpy.apply_along_axis() was that it would take some function and apply it to each value of an array. From the error it seems that the the lambda is being evaluated as x=array, not some value in array. What am I doing wrong? I know that I could write a function to accomplish this but I want to become more familiar with the functional programming aspects of python.

  Read the article

• Lambda expressions - set the value of one property in a collection of objects based on the value of

  - by Michael Rut

  I'm new to lambda expressions and looking to leverage the syntax to set the value of one property in a collection based on another value in a collection Typically I would do a loop: class item{ public string name {get;set;} public string value {get;set;} } class business { item item1 = new item(name="name1"); item item2 = new item(name="name2"); item item3 = new item(name="name3"); Collection<item> items = new Collection() {item1,item2,item3}; //This is what I want to simplify for( int i = 0; i < items.count; i++) { if(items[i].item.name == "name2") { //set the value items[i].item.value = "value2"; } } }

  Read the article

• Extracting pair member in lambda expressions and template typedef idiom

  - by Nazgob

  Hi, I have some complex types here so I decided to use nifty trick to have typedef on templated types. Then I have a class some_container that has a container as a member. Container is a vector of pairs composed of element and vector. I want to write std::find_if algorithm with lambda expression to find element that have certain value. To get the value I have to call first on pair and then get value from element. Below my std::find_if there is normal loop that does the trick. My lambda fails to compile. How to access value inside element which is inside pair? I use g++ 4.4+ and VS 2010 and I want to stick to boost lambda for now. #include <vector> #include <algorithm> #include <boost\lambda\lambda.hpp> #include <boost\lambda\bind.hpp> template<typename T> class element { public: T value; }; template<typename T> class element_vector_pair // idiom to have templated typedef { public: typedef std::pair<element<T>, std::vector<T> > type; }; template<typename T> class vector_containter // idiom to have templated typedef { public: typedef std::vector<typename element_vector_pair<T>::type > type; }; template<typename T> bool operator==(const typename element_vector_pair<T>::type & lhs, const typename element_vector_pair<T>::type & rhs) { return lhs.first.value == rhs.first.value; } template<typename T> class some_container { public: element<T> get_element(const T& value) const { std::find_if(container.begin(), container.end(), bind(&typename vector_containter<T>::type::value_type::first::value, boost::lambda::_1) == value); /*for(size_t i = 0; i < container.size(); ++i) { if(container.at(i).first.value == value) { return container.at(i); } }*/ return element<T>(); //whatever } protected: typename vector_containter<T>::type container; }; int main() { some_container<int> s; s.get_element(5); return 0; }

  Read the article

• How to properly diagram lambda expressions or traversals through them in Architecture Explorer?

  - by MainMa

  I'm exploring a piece of code in Architecture Explorer in Visual Studio 2010 to study the relations between methods. I noticed a strange behavior. Take the following source code. It generates a hello message based on a template and a template engine, the template engine being a method (a sort of strategy pattern simplified at a maximum for demo purposes). public string GenerateHelloMessage(string personName) { return this.ApplyTemplate( this.DefaultTemplateEngine, this.GenerateLocalizedHelloTemplate(), personName); } private string GenerateLocalizedHelloTemplate() { return "Hello {0}!"; } public string ApplyTemplate( Func<string, string, string> templateEngine, string template, string personName) { return templateEngine(template, personName); } public string DefaultTemplateEngine(string template, string personName) { return string.Format(template, personName); } The graph generated from this code is this one: Change the first method from this: public string GenerateHelloMessage(string personName) { return this.ApplyTemplate( this.DefaultTemplateEngine, this.GenerateLocalizedHelloTemplate(), personName); } to this: public string GenerateHelloMessage(string personName) { return this.ApplyTemplate( (a, b) => this.DefaultTemplateEngine(a, b), this.GenerateLocalizedHelloTemplate(), personName); } and the graph becomes: While semantically identical, those two versions of code produce different dependency graphs, and Architecture Explorer shows no trace of the lambda expression (while Visual Studio's code coverage, for example, shows them, as well as Code analysis seems to be able to understand that the link exists). How would it be possible, without changing the source code, to: Either force Architecture Explorer to display everything, including lambda expressions, Or make it traverse lambda expressions while drawing a dependency through them (so in this case, drawing the dependency from GenerateHelloMessage to DefaultTemplateEngine in the second example)?

  Read the article

• What triggered the popularity of lambda functions in modern programming languages?

  - by Giorgio

  In the last few years anonymous functions (AKA lambda functions) have become a very popular language construct and almost every major / mainstream programming language has introduced them or is planned to introduce them in an upcoming revision of the standard. Yet, anonymous functions are a very old and very well-known concept in Mathematics and Computer Science (invented by the mathematician Alonzo Church around 1936, and used by the Lisp programming language since 1958, see e.g. here). So why didn't today's mainstream programming languages (many of which originated 15 to 20 years ago) support lambda functions from the very beginning and only introduced them later? And what triggered the massive adoption of anonymous functions in the last few years? Is there some specific event, new requirement or programming technique that started this phenomenon?

  Read the article

• Ruby: convert proc to lambda?

  - by Alan O'Donnell

  Is it possible to convert a proc-flavored Proc into a lambda-flavored Proc? Bit surprised that this doesn't work, at least in 1.9.2: my_proc = proc {|x| x} my_lambda = lambda &p my_lambda.lambda? # = false!

  Read the article

• custom C++ boost::lambda expression help

  - by aaa

  hello. A little bit of background: I have some strange multiple nested loops which I converted to flat work queue (basically collapse single index loops to single multi-index loop). right now each loop is hand coded. I am trying to generalized approach to work with any bounds using lambda expressions: For example: // RANGE(i,I,N) is basically a macro to generate `int i = I; i < N; ++i ` // for (RANGE(lb, N)) { // for (RANGE(jb, N)) { // for (RANGE(kb, max(lb, jb), N)) { // for (RANGE(ib, jb, kb+1)) { // is equivalent to something like (overload , to produce range) flat<1, 3, 2, 4>((_2, _3+1), (max(_4,_3), N), N, N) the internals of flat are something like: template<size_t I1, size_t I2, ..., class L1_, class L2, ..._> boost::array<int,4> flat(L1_ L1, L2_ L2, ...){ //boost::array<int,4> current; class variable bool advance; L2_ l2 = L2.bind(current); // bind current value to lambda { L1_ l1 = L1.bind(current); //bind current value to innermost lambda l1.next(); advance = !(l1 < l1.upper()); // some internal logic if (advance) { l2.next(); current[0] = l1.lower(); } } //..., } my question is, can you give me some ideas how to write lambda (derived from boost) which can be bound to index array reference to return upper, lower bounds according to lambda expression? thank you much

  Read the article

• What are some resources for learning Lambda Calculus?

  - by Simucal

  So the Wikipedia entry on Lambda Calculus was interesting but I've finished it. I wish to dive a little deeper and get a better understanding of Lambda Calculus. Can anyone recommend what they consider to be the best book or primer to Lambda Calculus?

  Read the article

• scheme recursive lambda

  - by Mike

  Is there a way to have a recursive lambda expression in scheme without relying an external identifier? I know you can have (define fact (lambda (n) (if (= n 0) 1 (fact (- n 1)))) but it would be nice if fact wasn't hard coded in the lambda expression, it seems improper.

  Read the article

• boost::lambda bind expressions can't get bind to string's empty() to work

  - by navigator

  Hi, I am trying to get the below code snippet to compile. But it fails with: error C2665: 'boost::lambda::function_adaptor::apply' : none of the 8 overloads could convert all the argument types. Sepcifying the return type when calling bind does not help. Any idea what I am doing wrong? Thanks. #include <boost/lambda/lambda.hpp> #include <boost/lambda/bind.hpp> #include <string> #include <map> int main() { namespace bl = boost::lambda; typedef std::map<int, std::string> types; types keys_and_values; keys_and_values[ 0 ] = "zero"; keys_and_values[ 1 ] = "one"; keys_and_values[ 2 ] = "Two"; std::for_each( keys_and_values.begin(), keys_and_values.end(), std::cout << bl::constant("Value empty?: ") << std::boolalpha << bl::bind(&std::string::empty, bl::bind(&types::value_type::second, _1)) << "\n"); return 0; }

  Read the article

• varargs in lambda functions in Python

  - by brain_damage

  Is it possible a lambda function to have variable number of arguments? For example, I want to write a metaclass, which creates a method for every method of some other class and this newly created method returns the opposite value of the original method and has the same number of arguments. And I want to do this with lambda function. How to pass the arguments? Is it possible? class Negate(type): def __new__(mcs, name, bases, _dict): extended_dict = _dict.copy() for (k, v) in _dict.items(): if hasattr(v, '__call__'): extended_dict["not_" + k] = lambda s, *args, **kw: not v(s, *args, **kw) return type.__new__(mcs, name, bases, extended_dict) class P(metaclass=Negate): def __init__(self, a): self.a = a def yes(self): return True def maybe(self, you_can_chose): return you_can_chose But the result is totally wrong: >>>p = P(0) >>>p.yes() True >>>p.not_yes() # should be False Traceback (most recent call last): File "<pyshell#150>", line 1, in <module> p.not_yes() File "C:\Users\Nona\Desktop\p10.py", line 51, in <lambda> extended_dict["not_" + k] = lambda s, *args, **kw: not v(s, *args, **kw) TypeError: __init__() takes exactly 2 positional arguments (1 given) >>>p.maybe(True) True >>>p.not_maybe(True) #should be False True

  Read the article

• What triggered the popularity of lambda functions in modern mainstream programming languages?

  - by Giorgio

  In the last few years anonymous functions (AKA lambda functions) have become a very popular language construct and almost every major / mainstream programming language has introduced them or is planned to introduce them in an upcoming revision of the standard. Yet, anonymous functions are a very old and very well-known concept in Mathematics and Computer Science (invented by the mathematician Alonzo Church around 1936, and used by the Lisp programming language since 1958, see e.g. here). So why didn't today's mainstream programming languages (many of which originated 15 to 20 years ago) support lambda functions from the very beginning and only introduced them later? And what triggered the massive adoption of anonymous functions in the last few years? Is there some specific event, new requirement or programming technique that started this phenomenon? IMPORTANT NOTE The focus of this question is the introduction of anonymous functions in modern, main-stream (and therefore, maybe with a few exceptions, non functional) languages. Also, note that anonymous functions (blocks) are present in Smalltalk, which is not a functional language, and that normal named functions have been present even in procedural languages like C and Pascal for a long time. Please do not overgeneralize your answers by speaking about "the adoption of the functional paradigm and its benefits", because this is not the topic of the question.

  Read the article

• How to implement lambda as a function called "lambda" in Clojure?

  - by dirtyvagabond

  I'd like to be able to define lambdas using common Lisp syntax, in Clojure. For example: (lambda (myarg) (some-functions-that-refer-to myarg)) This needs to result in the same as: #(some-functions-that-refer-to %) In my case, I know I'll always have exactly one arg, so perhaps that simplifies things. (But it can be called anything -- "myarg" or whatever.) I suspect a workable solution is to "(defmacro lambda ...". If so, I'm not sure of the best way to proceed. How to cleanly translate the arg name to %? And how to end up with the correct function? Or, is there a simpler solution than writing my own macro that actually re-implements Clojure's... lambda?

  Read the article

• Anonymous Methods / Lambda's (Coding Standards)

  - by Mystagogue

  In Jeffrey Richter's "CLR via C#" (the .net 2.0 edtion page, 353) he says that as a self-discipline, he never makes anonymous functions longer than 3 lines of code in length. He cites mostly readability / understandability as his reasons. This suites me fine, because I already had a self-discipline of using no more than 5 lines for an anonymous method. But how does that "coding standard" advice stack against lambda's? At face value, I'd treat them the same - keeping a lambda equally as short. But how do others feel about this? In particular, when lambda's are being used where (arguably) they shine brightest - when used in LINQ statements - is there genuine cause to abandon that self-discipline / coding standard?

  Read the article

• Python lambda returning None instead of empty string

  - by yoshi

  I have the following lambda function: f = lambda x: x == None and '' or x It should return an empty string if it receives None as the argument, or the argument if it's not None. For example: >>> f(4) 4 >>> f(None) >>> If I call f(None) instead of getting an empty string I get None. I printed the type of what the function returned and I got NoneType. I was expecting string. type('') returns string, so I'd like to know why the lambda doesn't return an empty string when I pass None as an argument. I'm fairly new to lambdas so I might have misunderstood some things about how they work.

  Read the article

• C# Lambda Problem

  - by Chris Klepeis

  Probably something simple, but as I'm new to lambda expressions, the problem evades me: m => m.contactID == contactID && m.primaryAddress == true && (m.addressTypeID == 2 || m.addressTypeID == 3) I tried to use that lambda expression but I receive an invalid operator. Is there a way to simplify this so that it would work? Edit: The equivolent sql query would be: SELECT * FROM Contact WHERE contactID = 3 AND primaryAddress = 1 AND (addressTypeID = 2 OR addressTypeID = 3) I have a repository function defined like so: public E Single(Expression<Func<E, bool>> where) { return objectSet.Single<E>(where); } I'm passing the lambda expression above into this function. Invalid operator error.

  Read the article

• Examining a lambda expression at runtime in C#

  - by Ben Aston

  I have a method Get on a type MyType1 accepting a Func<MyType2, bool> as a parameter. An example of its use: mytype1Instance.Get(x => x.Guid == guid)); I would like create a stub implementation of the method Get that examines the incoming lambda expression and determines what the value of guid is. Clearly the lambda could be "anything", but I'm happy for the stub to make an assumption about the lambda, that it is trying to match on the Guid property. How can I do this? I suspect it involves the use of the built-in Expression type?

  Read the article

• List filtering: list comprehension vs. lambda + filter

  - by Agos

  I happened to find myself having a basic filtering need: I have a list and I have to filter it by an attribute of the items. My code looked like this: list = [i for i in list if i.attribute == value] But then i thought, wouldn't it be better to write it like this? filter(lambda x: x.attribute == value, list) It's more readable, and if needed for performance the lambda could be taken out to gain something. Question is: are there any caveats in using the second way? Any performance difference? Am I missing the Pythonic Way™ entirely and should do it in yet another way (such as using itemgetter instead of the lambda)? Thanks in advance

  Read the article

• What’s New in The Second Edition of Regular Expressions Cookbook

  - by Jan Goyvaerts

  %COOKBOOKFRAME% The second edition of Regular Expressions Cookbook is a completely revised edition, not just a minor update. All of the content from the first edition has been updated for the latest versions of the regular expression flavors and programming languages we discuss. We’ve corrected all errors that we could find and rewritten many sections that were either unclear or lacking in detail. And lack of detail was not something the first edition was accused of. Expect the second edition to really dot all i’s and cross all t’s. A few sections were removed. In particular, we removed much talk about browser inconsistencies as modern browsers are much more compatible with the official JavaScript standard. There is plenty of new content. The second edition has 101 more pages, bringing the total to 612. It’s almost 20% bigger than the first edition. We’ve added XRegExp as an additional regex flavor to all recipes throughout the book where XRegExp provides a better solution than standard JavaScript. We did keep the standard JavaScript solutions, so you can decide which is better for your needs. The new edition adds 21 recipes, bringing the total to 146. 14 of the new recipes are in the new Source Code and Log Files chapter. These recipes demonstrate techniques that are very useful for manipulating source code in a text editor and for dealing with log files using a grep tool. Chapter 3 which has recipes for programming with regular expressions gets only one new recipe, but it’s a doozy. If anyone has ever flamed you for using a regular expression instead of a parser, you’ll now be able to tell them how you can create your own parser by mixing regular expressions with procedural code. Combined with the recipes from the new Source Code and Log Files chapter, you can create parsers for whatever custom language or file format you like. If you have any interest in regular expressions at all, whether you’re a beginner or already consider yourself an expert, you definitely need a copy of the second edition of Regular Expressions Cookbook if you didn’t already buy the first. If you did buy the first edition, and you often find yourself referring back to it, then the second edition is a very worthwhile upgrade. You can buy the second edition of Regular Expressions Cookbook from Amazon or wherever technical books are sold. Ask for ISBN 1449319432.

  Read the article

• Lambda Expression to be used in Select() query

  - by jameschinnock

  Hi, I am trying to build a lambda expression, containing two assignments (as shown further down), that I can then pass to a Queryable.Select() method. I am trying to pass a string variable into a method and then use that variable to build up the lambda expression so that I can use it in a LINQ Select query. My reasoning behind it is that I have a SQL Server datasource with many column names, I am creating a charting application that will allow the user to select, say by typing in the column name, the actual column of data they want to view in the y-axis of my chart, with the x-axis always being the DateTime. Therefore, they can essentially choose what data they chart against the DateTime value (it’s a data warehouse type app). I have, for example, a class to store the retrieved data in, and hence use as the chart source of: public class AnalysisChartSource { public DateTime Invoicedate { get; set; } public Decimal yValue { get; set; } } I have (purely experimentaly) built an expression tree for the Where clause using the String value and that works fine: public void GetData(String yAxis) { using (DataClasses1DataContext db = new DataClasses1DataContext()) { var data = this.FunctionOne().AsQueryable<AnalysisChartSource>(); //just to get some temp data in.... ParameterExpression pe = Expression.Parameter(typeof(AnalysisChartSource), "p"); Expression left = Expression.MakeMemberAccess(pe, typeof(AnalysisChartSource).GetProperty(yAxis)); Expression right = Expression.Constant((Decimal)16); Expression e2 = Expression.LessThan(left, right); Expression expNew = Expression.New(typeof(AnalysisChartSource)); LambdaExpression le = Expression.Lambda(left, pe); MethodCallExpression whereCall = Expression.Call( typeof(Queryable), "Where", new Type[] { data.ElementType }, data.Expression, Expression.Lambda<Func<AnalysisChartSource, bool>>(e2, new ParameterExpression[] { pe })); } } However……I have tried a similar approach for the Select statement, but just can’t get it to work as I need the Select() to populate both X and Y values of the AnalysisChartSource class, like this: .Select(c => new AnalysisChartSource { Invoicedate = c.Invoicedate, yValue = c.yValue}).AsEnumerable(); How on earth can I build such an expression tree….or….possibly more to the point…..is there an easier way that I have missed entirely?

  Read the article

• Lambda Expressions

  - by Asad Khan

  Can somebody explain me lambda expressions & what they can be used for. I have googled for it & have a rough idea. most of the examples give c# code. How about lambda expressions in plain old C...?

  Read the article

• Can you reverse order a string in one line with LINQ or a LAMBDA expression

  - by Student for Life

  Not that I would want to use this practically (for many reasons) but out of strict curiousity I would like to know if there is a way to reverse order a string using LINQ and/or LAMBDA expressions in one line of code, without utilising any framework "Reverse" methods. e.g. string value = "reverse me"; string reversedValue = (....); and reversedValue will result in "em esrever" EDIT Clearly an impractical problem/solution I know this, so don't worry it's strictly a curiosity question around the LINQ/LAMBDA construct.

  Read the article

< Previous Page | 1 2 3 4 5 6 7 8 9 10 11 12  | Next Page >