Swift3 中的数组过滤器 [英] Array filter in Swift3

问题描述

我有一段代码.我没有得到这段代码里面的内容.谁能解释一下?

I have a piece of code. I am not getting whats going inside in this code. Can anybody explain it?

 let wordFreqs = [("k", 5), ("a", 7), ("b", 3)]

        let res = wordFreqs.filter(
        {
            (e) -> Bool in

            if e.1 > 3 {
                return true
            } else {
                return false
            }

        }).map { $0.0 }

        print(res)

给出输出:

["k","a"]

推荐答案

如果我们一个接一个地处理这段代码:

If we take the parts of this code one after the other:

let wordFreqs = [("k", 5), ("a", 7), ("b", 3)]

您从一组元组开始.

来自 Swift 文档:

From the Swift documentation:

元组类型是以逗号分隔的类型列表，括在括号中.

A tuple type is a comma-separated list of types, enclosed in parentheses.

和:

元组将多个值分组为一个复合值.元组中的值可以是任何类型.

Tuples group multiple values into a single compound value. The values within a tuple can be of any type.

在这种情况下，元组是 2 个值的一对"，一个是 String 类型，另一个是 Int 类型.

In this case, the tuples are "couples" of 2 values, one of type String and 1 of type Int.

        let res = wordFreqs.filter(
        {
            (e) -> Bool in

这部分在数组上应用过滤器.您可以在此处看到过滤器的闭包采用元素 e(因此，在我们的示例中为一个元组)，并返回一个 Bool.使用 'filter' 函数，返回 true 表示保留该值，而返回 false 表示将其过滤掉.

This part applies a filter on the array. You can see here that the closure of the filter takes an element e (so, in our case, one tuple), and returns a Bool. With the 'filter' function, returning true means keeping the value, while returning false means filtering it out.

            if e.1 > 3 {
                return true
            } else {
                return false
            }

e.1 语法返回索引 1 处元组的值.因此，如果索引 1(第二个)处的元组值超过 3，则过滤器返回 true(因此元组将被保留)；如果不是，过滤器返回 false(因此从结果中排除元组).此时，过滤器的结果将是 [("k", 5), ("a", 7)]

The e.1 syntax returns the value of the tuple at index 1. So, if the tuple value at index 1 (the second one) is over 3, the filter returns true (so the tuple will be kept) ; if not, the filter returns false (and therefore excludes the tuple from the result). At that point, the result of the filter will be [("k", 5), ("a", 7)]

        }).map { $0.0 }

map 函数基于元组数组创建一个新数组:对于输入数组 ($0) 的每个元素，它返回索引 0 处的元组值.所以新数组是 ["k", "一个"]

The map function creates a new array based on the tuple array: for each element of the input array ($0), it returns the tuple value at index 0. So the new array is ["k", "a"]

        print(res)

这会将结果打印到控制台.

This prints out the result to the console.

这些函数(filter、map、reduce 等)在函数式编程中非常常见.它们通常使用点语法链接，例如 [1, 2, 3].filter({ }).map({ }).reduce({ })

These functions (filter, map, reduce, etc.) are very common in functional programming. They are often chained using the dot syntax, for example, [1, 2, 3].filter({ }).map({ }).reduce({ })

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