問題描述

例如，Python 中的文件是可迭代的——它們迭代文件中的行.我想計算行數.

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

一種快速的方法是這樣做:

One quick way is to do this:

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

但是，這會將整個文件加載到內存中(一次).這反而違背了迭代器的目的(只需要將當前行保留在內存中).

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).

這不起作用:

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

因為生成器沒有長度.

除了定義一個計數函數之外，還有什么方法可以做到這一點嗎?

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

為了澄清，我知道必須閱讀整個文件！我只是不想一下子把它放在內存中

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

推薦答案

沒有遍歷iterable并計算迭代次數，沒有.這就是使它成為可迭代而不是列表的原因.這甚至不是一個特定于 python 的問題.看看經典的鏈表數據結構.查找長度是一個 O(n) 操作，涉及迭代整個列表以查找元素的數量.

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，您可能可以將您的功能簡化為:

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

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

當然，如果您要定義自己的可迭代對象，您始終可以自己實現 __len__ 并在某處保留元素計數.

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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