lambda

问题 Here is my List fooList class Foo { private String name; private int code; private int account; private int time; private String others; ... constructor, getters & setters } e.g.(all the value of account has been set to 1) new Foo(First, 200, 1, 400, other1), new Foo(First, 200, 1, 300, other1), new Foo(First, 201, 1, 10, other1), new Foo(Second, 400, 1, 20, other2), new Foo(Second, 400, 1, 40, other2), new Foo(Third, 100, 1, 200, other3), new Foo(Third, 101, 1, 900, other3) I want to

问题 I have a class db_interface. And defined a lambda type: typedef void (*db_interface_lambda)(); When I create lambda in class in such way: [](){ /* do something */ } , it has good type (db_interface_lambda), but when I use [this](){ /* do something */ } , the compiler starts to shout at me. cannot convert ‘db_interface::db_interface(std::ifstream&)::<lambda()>’ to ‘std::map<std::basic_string<char>, void (*)()>::mapped_type {aka void (*)()}’ in assignment How to solve that problem? What is the

问题 List<Y> tmp= new DATA<Y>().findEntities(); List<X> tmp1 = new DATA<X>().findEntities().stream().filter( IN (tmp) ??? ).collect(Collectors.toList()); How to simulate a tipical IN clause (like in mysql or JPA) using a Predicate ? 回答1: I decided to update my comment to an answer. The lambda expression for your requested Predicate<Y> (where Y should be a concrete type) looks as following: element -> tmp.contains(element) Because the collection's contains method has the same signature as the

问题 So let's say that I have the following classes: public class Model { public AnotherModel InnerModel { get; set; } } public class AnotherModel { public String Value{ get; set; } } Now I have the following function: public static void Foo<T, U>(Expression<Func<T, U>> func) { // Get the property info from func } What I would like to do now is the following: Foo<Model, String>(o => o.InnerModel.Value) Here comes the problem: I know that you can fetch the PropertyInfo from the expression func by

问题 So let's say that I have the following classes: public class Model { public AnotherModel InnerModel { get; set; } } public class AnotherModel { public String Value{ get; set; } } Now I have the following function: public static void Foo<T, U>(Expression<Func<T, U>> func) { // Get the property info from func } What I would like to do now is the following: Foo<Model, String>(o => o.InnerModel.Value) Here comes the problem: I know that you can fetch the PropertyInfo from the expression func by

问题 I was intending to write a memorization pattern in C++ and ended up with the following approach std::function<int(int)> Memoize(std::function<int(int)> fn) { std::map<int, int> memo; std::function<int(int)> helper = [=](int pos) { if (memo.count(pos) == 0) { memo[pos] = fn(pos); } return memo[pos]; }; return helper; } Strangely, my compiler VS 2012, refused to compile with the following error 1>Source1.cpp(24): error C2678: binary '[' : no operator found which takes a left-hand operand of

问题 Do simple lambda expressions get inlined? I have a tendency (thanks to F# and other functional forays) to encapsulate repeated code present within a single function into a lambda, and call it instead. I'm curious if I'm incurring a run-time overhead as a result: var foo = a + b; var bar = a + b; vs Func<T1, T2> op = () => a + b; var foo = op(); var bar = op(); Which one costs more to run? 回答1: No. Lambda functions are not inlined but instead are stored as delegates under the hood and incur

问题 I'm relatively new to Java8 and I have a scenario where I need to retrieve all the keys from the Map which matched with the objects. Wanted to know if there is a way to get all keys without iterating them from the list again. Person.java private String firstName; private String lastName; //setters and getters & constructor MAIN Class. String inputCriteriaFirstName = "john"; Map<String, Person> inputMap = new HashMap<>(); Collection<Person> personCollection = inputMap.values(); List<Person>

问题 I'm relatively new to Java8 and I have a scenario where I need to retrieve all the keys from the Map which matched with the objects. Wanted to know if there is a way to get all keys without iterating them from the list again. Person.java private String firstName; private String lastName; //setters and getters & constructor MAIN Class. String inputCriteriaFirstName = "john"; Map<String, Person> inputMap = new HashMap<>(); Collection<Person> personCollection = inputMap.values(); List<Person>

问题 At risk of being off-topic I decided to do a little Q&A as I'm pretty excited about a new function MS is introducing to Excel365; the LAMBDA() function. If the general opinion is such that this is off-topic, please let me know and I can take down the Q&A. The LAMBDA() function is basically your way in Excel itself to create your own function. You then can go ahead and call this function throughout your entire workbook. But the absolute great thing (IMHO) about it is that it is able to call