有没有内置的方法来获取python中的iterable的长度？ [英] Is there any built-in way to get the length of an iterable in python?

问题描述

例如，Python中的文件是可迭代的 - 它们遍历文件中的行。我想计算行数。

For example, files, in Python, are iterable - they iterate over the lines in the file. I want to count the number of lines.

一种快速方法是：

lines = len(list(open(fname)))

然而，这会将整个文件加载到内存中（同时）。这相当违背了迭代器的目的（只需要将当前行保留在内存中）。

However, this loads the whole file into memory (at once). This rather defeats the purpose of an iterator (which only needs to keep the current line in memory).

这不起作用：

lines = len(line for line in open(fname))

因为生成器没有长度。

除了定义计数函数之外还有什么办法吗？

Is there any way to do this short of defining a count function?

def count(i):
    c = 0
    for el in i: c += 1
    return c

编辑：为了澄清，我知道必须阅读整个文件！我只是不想在内存中一次性=）。

To clarify, I understand that the whole file will have to be read! I just don't want it in memory all at once =).

推荐答案

没有迭代遍历迭代并计算数字迭代，没有。这就是使它成为可迭代而不是列表的原因。这甚至不是特定于python的问题。查看经典的链表数据结构。查找长度是一个O（n）操作，它涉及迭代整个列表以找到元素的数量。

Short of iterating through the iterable and counting the number of iterations, no. That's what makes it an iterable and not a list. This isn't really even a python-specific problem. Look at the classic linked-list data structure. Finding the length is an O(n) operation that involves iterating the whole list to find the number of elements.

正如上面提到的mcrute，你可以将函数减少到：

As mcrute mentioned above, you can probably reduce your function to:

def count_iterable(i):
    return sum(1 for e in i)

当然，如果你要定义自己的可迭代对象，你总是可以实现 __ len __ 你自己并在某处保留元素数。

Of course, if you're defining your own iterable object you can always implement __len__ yourself and keep an element count somewhere.

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