计算数组数组中的项目？

If I have an object that is declared as

如果我有一个声明为的对象

let compoundArray = [Array<String>]

is there a property that would give me the number of strings in all the arrays contained in compoundArray?

有没有一个属性可以给我在compoundArray中包含的所有数组中的字符串数？

I can do it by adding up all the items in each array within:

我可以通过在每个数组中添加所有项目来实现：

var totalCount = 0
for array in compoundArray {
   totalCount += array.count }
//totalCount = total items in all arrays within compoundArray

But that seems clunky and it seems that swift would have a property/method of Array to do this, no?

但这似乎很笨拙，似乎swift会有一个Array的属性/方法来做到这一点，不是吗？

Thanks!

谢谢！

3 个解决方案

#1

You can add the nested array counts with

您可以使用添加嵌套数组计数

let count = compoundArray.reduce(0) { $0 + $1.count }

Performance comparison for large arrays (compiled and run on a MacBook Pro in Release configuration):

大型阵列的性能比较（在发布配置中在MacBook Pro上编译和运行）：

let N = 20_000
let compoundArray = Array(repeating: Array(repeating: "String", count: N), count: N)

do {
    let start = Date()
    let count = compoundArray.joined().count
    let end = Date()
    print(end.timeIntervalSince(start))
    // 0.729196012020111
}

do {
    let start = Date()
    let count = compoundArray.flatMap({$0}).count
    let end = Date()
    print(end.timeIntervalSince(start))
    // 29.841913998127
}

do {
    let start = Date()
    let count = compoundArray.reduce(0) { $0 + $1.count }
    let end = Date()
    print(end.timeIntervalSince(start))
    // 0.000432014465332031
}

#2

You can use joined or flatMap for that.

您可以使用join或flatMap。

Using joined

使用加入

let count = compoundArray.joined().count

Using flatMap

使用flatMap

let count = compoundArray.flatMap({$0}).count

#3

Since you are asking for a property, I thought I'd point out how to create one (for all collections, while we're at it):

既然你要求一个属性，我想我会指出如何创建一个属性（对于所有的集合，我们在它的时候）：

extension Collection where Iterator.Element: Collection {
    var flatCount: Int {
        self.reduce(0) { $0 + $1.count } // as per Martin R
    }
}

Making this recursive seems to be an interesting exercise.

使这种递归似乎是一个有趣的练习。

#1