保持冻结集中元素的顺序 [英] Maintaining the order of the elements in a frozen set

问题描述

我有一个元组列表，每个元组包含一个字符串和两个整数.列表如下所示:

I have a list of tuples, each tuple of which contains one string and two integers. The list looks like this:

x = [('a',1,2), ('b',3,4), ('x',5,6), ('a',2,1)]

该列表包含成千上万个这样的元组.现在，如果我想获得唯一的组合，则可以在列表上执行frozenset，如下所示:

The list contains thousands of such tuples. Now if I want to get unique combinations, I can do the frozenset on my list as follows:

y = set(map(frozenset, x))

这给了我以下结果:

{frozenset({'a', 2, 1}), frozenset({'x', 5, 6}), frozenset({3, 'b', 4})}

我知道set是一个无序数据结构，这是正常情况，但是我想在这里保留元素的顺序，以便以后可以将元素插入pandas数据帧中.数据框将如下所示:

I know that set is an unordered data structure and this is normal case but I want to preserve the order of the elements here so that I can thereafter insert the elements in a pandas dataframe. The dataframe will look like this:

 Name  Marks1  Marks2
0    a       1       2
1    b       3       4
2    x       5       6

推荐答案

除了直接在frozenset的set上进行操作外，您还可以将其仅用作辅助数据结构-如 unique_everseen itertools部分中的食谱(逐字复制):

Instead of operating on the set of frozensets directly you could use that only as a helper data-structure - like in the unique_everseen recipe in the itertools section (copied verbatim):

from itertools import filterfalse

def unique_everseen(iterable, key=None):
    "List unique elements, preserving order. Remember all elements ever seen."
    # unique_everseen('AAAABBBCCDAABBB') --> A B C D
    # unique_everseen('ABBCcAD', str.lower) --> A B C D
    seen = set()
    seen_add = seen.add
    if key is None:
        for element in filterfalse(seen.__contains__, iterable):
            seen_add(element)
            yield element
    else:
        for element in iterable:
            k = key(element)
            if k not in seen:
                seen_add(k)
                yield element

基本上，这将在您使用key=frozenset时解决此问题:

Basically this would solve the issue when you use key=frozenset:

>>> x = [('a',1,2), ('b',3,4), ('x',5,6), ('a',2,1)]

>>> list(unique_everseen(x, key=frozenset))
[('a', 1, 2), ('b', 3, 4), ('x', 5, 6)]

这将按原样返回元素 ，并且还保持元素之间的相对顺序.

This returns the elements as-is and it also maintains the relative order between the elements.

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