在列表中查找并列出重复项？ [英] Find and list duplicates in a list?

问题描述

如何在Python列表中找到重复项并创建另一个重复列表？列表只是整数。

How can I find the duplicates in a Python list and create another list of the duplicates? The list is just integers.

推荐答案

要删除重复使用 set（a），打印重复的东西，如

To remove duplicates use set(a), to print duplicates - something like

a = [1,2,3,2,1,5,6,5,5,5]

import collections
print [item for item, count in collections.Counter(a).items() if count > 1]

## [1, 2, 5]

请注意计数器不是特别有效（计时），可能在这里过度， set 将表现更好：

Note that Counter is not particularly efficient (timings) and probably an overkill here, set will perform better:

seen = set()
uniq = []
for x in a:
    if x not in seen:
        uniq.append(x)
        seen.add(x)

或更简洁：

seen = set()
uniq = [x for x in a if x not in seen and not seen.add(x)]    

尽管如此，我不推荐使用后一种风格。

I don't recommend the latter style though.

如果列表元素不可哈希，则不能使用set /例如：

If list elements are not hashable, you cannot use set/dicts and have to resort to a quadratic time solution (compare each which each), for example:

a = [ [1], [2], [3], [1], [5], [3] ]

no_dupes = [x for n, x in enumerate(a) if x not in a[:n]]
print no_dupes # [[1], [2], [3], [5]]

dupes = [x for n, x in enumerate(a) if x in a[:n]]
print dupes # [[1], [3]]

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