lambda

问题 I would like to filter an array into an array of every nth item. For examples: fun getNth(array: Array<Any>, n: Int): Array<Any> { val newList = ArrayList<Any>() for (i in 0..array.size) { if (i % n == 0) { newList.add(array[i]) } } return newList.toArray() } Is there an idiomatic way to do this using for example Kotlin's .filter() and without A) provisioning a new ArrayList and B) manually iterating with a for/in loop? 回答1: filterIndexed function is suited exactly for this case: array

问题 I would like to filter an array into an array of every nth item. For examples: fun getNth(array: Array<Any>, n: Int): Array<Any> { val newList = ArrayList<Any>() for (i in 0..array.size) { if (i % n == 0) { newList.add(array[i]) } } return newList.toArray() } Is there an idiomatic way to do this using for example Kotlin's .filter() and without A) provisioning a new ArrayList and B) manually iterating with a for/in loop? 回答1: filterIndexed function is suited exactly for this case: array

问题 I recently struggled with a bug hard to find for me. I tried to pass a lambda to a function taking a std::function object. The lambda was capturing a noncopyable object. I figured out, obviously some copy must happen in between all the passings. I came to this result because I always ended in an error: use of deleted function error. Here is the code which produces this error: void call_func(std::function<void()> func) { func(); } int main() { std::fstream fs{"test.txt", std::fstream::out};

问题 I'm playing with std::variant, lambdas and std::future , and got super weird results when I tried to compose them together. Here are examples: using variant_t = std::variant< std::function<std::future<void>(int)>, std::function<void(int)> >; auto f1 = [](int) { return std::async([] { return 1; }); }; auto f2 = [](int) { return std::async([] { }); }; variant_t v1(std::move(f1)); // !!! why DOES this one compile when it SHOULDN'T? auto idx1 = v1.index(); //equals 1. WHY? variant_t v2(std::move

问题 To be very simple I have class Per{ int a; long b; double c; String d; } Let say I have 3000 Object of Type Per and collected in a List<Per> pers Now I want to achieve:- Skip if object is null or d is null or blank sum of a sum of b aggregated value of operation performed on c Old way is int totalA = 0; long totalB = 0l; long totalC = 0l; for (Per per : pers) { if (per.d != null && !per.d.trim().equals("")) { totalA += per.a; totalB += per.b; totalC += someOperation(per.c); } } someOperation

问题 I've recently got into functional programming and Java 8 lambdas. I have an array of ints and I want to sort it in an ascending order. The way I am trying to do this with lambda is as follows: Arrays.stream(intArray).sorted((x, y) -> Integer.compare(x, y) == -1); The issue with this is that my compiler says: Error:(12, 32) java: method sorted in interface java.util.stream.IntStream cannot be applied to given types; required: no arguments found: (x,y)->Int[...]== -1 reason: actual and formal

问题 I've recently got into functional programming and Java 8 lambdas. I have an array of ints and I want to sort it in an ascending order. The way I am trying to do this with lambda is as follows: Arrays.stream(intArray).sorted((x, y) -> Integer.compare(x, y) == -1); The issue with this is that my compiler says: Error:(12, 32) java: method sorted in interface java.util.stream.IntStream cannot be applied to given types; required: no arguments found: (x,y)->Int[...]== -1 reason: actual and formal

问题 I was playing with recursive lambdas in C# and have found two approaches to do this on the web. One approach uses fixed point combinator and the other does not. In the code below f1 is built using combinator, and f2 is defined directly. My question is, do we need fixed point combinators in C# or the language already provides all we need, so we can leave them alone? class Program { static Func<T, T> F<T>(Func<Func<T,T>,Func<T,T>> f) { return x => f(F(f))(x); } static void Main(string[] args) {

问题 Consider the following code sample: using System; using System.Linq.Expressions; public class Class1<T, Y> { public Class1(Expression<Func<T, Y>> mapExpression) { GetValue = mapExpression.Compile(); } public Func<T, Y> GetValue { get; protected set; } } public class DataClass { public long Data { get; set; } } Now suppose that I make in different places new instances of Class1, e.g. var instance1 = new Class1<DataClass, long>(x => x.Data); var instance2 = new Class1<DataClass, long>(x => x

问题 Consider the following code sample: using System; using System.Linq.Expressions; public class Class1<T, Y> { public Class1(Expression<Func<T, Y>> mapExpression) { GetValue = mapExpression.Compile(); } public Func<T, Y> GetValue { get; protected set; } } public class DataClass { public long Data { get; set; } } Now suppose that I make in different places new instances of Class1, e.g. var instance1 = new Class1<DataClass, long>(x => x.Data); var instance2 = new Class1<DataClass, long>(x => x