如何获取符合`Sequence`的类型的计数？ [英] How to get the count of a type conforming to `Sequence`?

问题描述

假设我有以下代码：

func work<S: Sequence>(sequence: S) {
    // do stuff
}

我怎么知道其中有多少个元素序列？

我想要的明显版本效率很低：

How could I figure out how many elements there are in sequence?
The obvious version I'd go for is pretty inefficient:

var count = 0
for element in sequence {
    count += 1
}

肯定有更好的方法吧？

推荐答案

我不认为对于符合
SequenceType 的任意类型，有更好的方法。关于序列的唯一已知信息是
具有 generate（）方法，该方法返回 GeneratorType ，而
具有 next（）方法。 next（）方法前进到序列的下一个
元素并返回它，或者返回 nil 如果没有
没有下一个元素。

I do not think that there is a better method for an arbitrary type conforming to SequenceType. The only thing that is known about a sequence is that is has a generate() method returning a GeneratorType, which in turn has a next() method. The next() method advances to the next element of the sequence and returns it, or returns nil if there is no next element.

请注意， next（）最终返回
nil ：一个序列可能包含无限元素。

Note that it is not required at all that next() eventually returns nil: a sequence may have "infinite" elements.

因此，请枚举该序列是计算其
元素的唯一方法。但这不必终止。因此答案
也可能是：带有序列参数的函数不必知道
元素的总数。

Therefore enumerating the sequence is the only method to count its elements. But this need not terminate. Therefore the answer could also be: A function taking a sequence argument should not need to know the total number of elements.

对于符合 CollectionType 的类型，可以使用
countElements（）函数（重命名为在Swift 1.2中是 count（）。

For types conforming to CollectionType you can use the countElements() function (renamed to count() in Swift 1.2).

还有 underestimateCount（）：

/// Return an underestimate of the number of elements in the given
/// sequence, without consuming the sequence.  For Sequences that are
/// actually Collections, this will return countElements(x)
func underestimateCount<T : SequenceType>(x: T) -> Int

但这不一定返回确切的元素数。

but that does not necessarily return the exact number of elements.

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