Google Dart:.where()函数如何工作? [英] Google Dart : How does .where() function work?

问题描述

var fruits = ['apples', 'oranges', 'bananas'];
fruits[0]; // apples
fruits.add('pears');
fruits.length == 4;
fruits.where((f) => f.startsWith('a')).toList();

文档中的示例显示了上述内容.我也不太了解该方法的文档.

The example in the documentation shows the above. I dont really understand the documentation of the method either.

https://api.dartlang.org/stable/1.21.1/dart-collection/IterableMixin/where.html

我目前看到lambda函数是where内的参数，其中where带有参数f.什么是f?我有点困惑.

I currently see a lambda function as a parameter inside where, with where having the argument f. What is f though? Im a bit confused.

如果我能看到一个可行的例子，那就太好了.就目前而言，我还没有真正理解它.除了它充当某种过滤器之外，我不知道它是如何工作的或者它实际上是做什么的.

It would be great if I could see a working example. As it stands now I dont really get it. I dont know how it works or what it really does apart from that it acts as some sort of filter.

推荐答案

是一个匿名函数， f 是它接受的参数

Is an anonymous function and f is the parameter it accepts

(f) => f.startsWith('a')

where(...)调用为 fruits 中的每个元素传递的函数，并返回一个仅返回函数返回 true <的值的可迭代对象/code>

where(...) calls that passed function for each element in fruits and returns an iterable that only emits the values where the function returned true

where(...)是惰性的，因此，仅当实际访问结果时才进行传递函数的迭代和调用，例如使用 .toList().

where(...) is lazy, therefore the iteration and call of the passed function will only happen when the result is actually accessed, like with .toList().

DartPad示例

更新

匿名"是指该函数没有名称，与诸如的命名函数相反

"anonymous" means the function has no name in contrary to a named function like

myFilter(f) => f.startsWith('a');

main() {
  fruits.where(myFilter).toList();
}

也

myFilter(f) => f.startsWith('a');

只是

myFilter(f) {
  return f.startsWith('a');
}

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