使用单个列表中的项目创建带有索引的新python列表

Question

I want to take a list, eg [0, 1, 0, 1, 2, 2, 3] , and make a list of lists of (index + 1) for each of the unique elements. For the above, for example, it would be [[1, 3], [2, 4], [5, 6], [7]] .

Right now my solution is the ultimate in clunkiness:

list_1 = [0, 1, 0, 1, 2, 2, 3]
maximum = max(list_1)
master = []
for i in range(maximum + 1):
    temp_list = []
    for j,k in enumerate(list_1):
        if k == i:
            temp_list.append(j + 1)
    master.append(temp_list)
print master

Any thoughts on a more pythonic way to do this would be much appreciated!

Answer 1

I would do this in two steps:

1. Build a map {value: [list, of, indices], ...} :

    index_map = {} for index, value in enumerate(list_1): index_map.setdefault(value, []).append(index+1) 

2. Extract the value lists from the dictionary into your master list:

    master = [index_map.get(index, []) for index in range(max(index_map)+1)] 

For your example, this would give:

>>> index_map
{0: [1, 3], 1: [2, 4], 2: [5, 6], 3: [7]}
>>> master
[[1, 3], [2, 4], [5, 6], [7]]

This implementation iterates over the whole list only once ( O(n) , where n is len(list_1) ) whereas the others so far iterate over the whole list once for each unique element ( O(n*m) , where m is len(set(list_1)) ). By taking max(d) rather than max(list_1) you only need to iterate over the length of the unique items, which is also more efficient.

The implementation can be slightly simpler if you make d = collections.defaultdict(list) .

Answer 2

master = [[i+1 for i in range(len(list_1)) if list_1[i]==j] for j in range(max(list_1)+1)]

与您当前的代码相同，但是它使用列表理解，这通常是解决此类问题的一种非常好的pythonic方式。

Answer 3

list_1 = [0, 1, 0, 1, 2, 2, 3]
master = []
for i in range(max(list_1)+1):
    master.append([j+1 for j,k in enumerate(list_1) if k==i])