Creating list of list based on index and value

Question

I have been given a list of positive integer numbers upto 10^6 and total number of item in the given list will be 10** 4 at most. I want to store these values in a particular order as explained below.

I want to iterate through the list and want to place the index value of item in the inner list corresponding to that item.

Example

Given List : [3,0,1,1]

output : [[1],[2,3],[],[0]] #expected output

basically output list should contains lists with index corresponding to item value in given list which should contain index value

I have comeup with this solution.

sequence = [[]] * 4
given = [3,0,1,1]
index = 0
for num in given:
    sequence[num].append(index)
    index+=1
print(sequence)

but the output I am getting is [[0, 1, 2, 3], [0, 1, 2, 3], [0, 1, 2, 3], [0, 1, 2, 3]] and cannot figure out why.

`sequence = [[]] * 4` will create 4 of the same list so changing one will change the others. try changing it to `sequence = [[] for _ in range(4)]` — Chris Doyle, Nov 07 '19 at 15:37

score 0 · Accepted Answer · answered Nov 07 '19 at 15:48

0

This will do it:

a = [3, 0, 1, 1]
[[j for j, k in enumerate(a) if k == i] for i, _ in enumerate(a)]
#[[1], [2, 3], [], [0]]

answered Nov 07 '19 at 15:48

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Creating list of list based on index and value

Example

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