c#中的谓词是什么? [英] What is a predicate in c#?

问题描述

我对使用谓词很陌生，刚刚学会了如何编写:

I am very new to using predicates and just learned how to write:

Predicate<int> pre = delegate(int a){ a %2 == 0 };

谓词将返回什么，它在编程时有何用处?

What will the predicate return, and how is it useful when programming?

推荐答案

Predicate 是一个函数式结构，提供了一种基本测试给定 T 是否为真的便捷方法 对象.

Predicate<T> is a functional construct providing a convenient way of basically testing if something is true of a given T object.

例如假设我有一堂课:

class Person {
    public string Name { get; set; }
    public int Age { get; set; }
}

现在假设我有一个 List人们，我想知道列表中是否有人叫Oscar.

Now let's say I have a List<Person> people and I want to know if there's anyone named Oscar in the list.

不使用Predicate(或Linq，或任何其他花哨的东西)，我总是可以通过执行以下操作来完成此操作:

Without using a Predicate<Person> (or Linq, or any of that fancy stuff), I could always accomplish this by doing the following:

Person oscar = null;
foreach (Person person in people) {
    if (person.Name == "Oscar") {
        oscar = person;
        break;
    }
}

if (oscar != null) {
    // Oscar exists!
}

这很好，但是假设我想检查是否有一个叫Ruth"的人?还是一个 17 岁的人?

This is fine, but then let's say I want to check if there's a person named "Ruth"? Or a person whose age is 17?

使用 Predicate，我可以使用更少的代码找到这些东西:

Using a Predicate<Person>, I can find these things using a LOT less code:

Predicate<Person> oscarFinder = (Person p) => { return p.Name == "Oscar"; };
Predicate<Person> ruthFinder = (Person p) => { return p.Name == "Ruth"; };
Predicate<Person> seventeenYearOldFinder = (Person p) => { return p.Age == 17; };

Person oscar = people.Find(oscarFinder);
Person ruth = people.Find(ruthFinder);
Person seventeenYearOld = people.Find(seventeenYearOldFinder);

注意我说的是更少的代码，而不是更快.开发人员的一个常见误解是，如果某样东西只需要一行，它的性能肯定比需要 10 行的东西更好.但在幕后，采用 Predicate 的 Find 方法毕竟只是枚举.Linq 的许多功能也是如此.

Notice I said a lot less code, not a lot faster. A common misconception developers have is that if something takes one line, it must perform better than something that takes ten lines. But behind the scenes, the Find method, which takes a Predicate<T>, is just enumerating after all. The same is true for a lot of Linq's functionality.

那么我们来看看你问题中的具体代码:

So let's take a look at the specific code in your question:

Predicate<int> pre = delegate(int a){ return a % 2 == 0; };

这里我们有一个 Predicatepre 接受一个 int a 并返回 a % 2 == 0.这本质上是测试偶数.这意味着:

Here we have a Predicate<int> pre that takes an int a and returns a % 2 == 0. This is essentially testing for an even number. What that means is:

pre(1) == false;
pre(2) == true;

等等.这也意味着，如果你有一个 List;ints 并且你想找到第一个偶数，你可以这样做:

And so on. This also means, if you have a List<int> ints and you want to find the first even number, you can just do this:

int firstEven = ints.Find(pre);

当然，与您可以在代码中使用的任何其他类型一样，为您的变量提供描述性名称是个好主意；所以我建议将上面的 pre 更改为 evenFinder 或 isEven 之类的东西 - 沿着这些路线的东西.那么上面的代码就清晰了很多:

Of course, as with any other type that you can use in code, it's a good idea to give your variables descriptive names; so I would advise changing the above pre to something like evenFinder or isEven -- something along those lines. Then the above code is a lot clearer:

int firstEven = ints.Find(evenFinder);

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