How to flatten a list in python?

Question

I have a program which outputs solutions, which are lists of numbers as such:

    [[1, [[1, 5], [2, 4], [3, 3], [4, 2], [5, 1]]], [2, [[1, 4], [2, 3], [3, 2], [4, 1]]], [3, [[1, 3], [2, 2], [3, 1]]], [4, [[1, 2], [2, 1]]], [5, [[1, 1]]]]

What I want this to be turned into is:

    [[1,1,5],[1,2,4],[1,3,3],[1,4,2],[1,5,1],[2,1,4],[2,2,3],[2,3,2],[2,4,1],[3,1,3],[3,2,2],[3,3,1],[4,1,2],[4,2,1],[5,1,1]]

Basically, each term in the whole list is headed by a number, which forms the first item of all possible solutions with that number, and then the following lists indicate what is to be added.

Looks like an interesting question, but it isn't very well explained. "1 is part of all solutions" "6 starts a new solution", how is a 'solution' calculated? For example you have the following object in your output array "[6,7,8]", by what logic have you included 8 and skipped 9? — Parsa, Jan 02 '16 at 15:31
So its a program to calculate the lists of numbers of a particular length that add up to a certain number. That example was a bit rubbish, I'm sorry. Changing it to a trial output from the program. — Mrahta, Jan 02 '16 at 15:32

score 0 · Accepted Answer · answered Jan 02 '16 at 15:46

0

This should work. It works with the example that you have atleast.

result = [[[i[0]] + j for j in i[1:][0] ] for i in arr][0]

answered Jan 02 '16 at 15:46

mattsap

3,790
1
15
36

score 0 · Answer 2 · edited May 23 '17 at 12:15

Introduce two functions: merge and flatten.

The second function is taken from here:

def flatten(lst):
    return [item for sublist in lst for item in sublist]

The first function is defined as:

def merge(lst):
    return [[lst[0]] + x for x in flatten(lst[1:])]

Then call them:

 s = [[1, [[1, 5], [2, 4], [3, 3], [4, 2], [5, 1]]], [2, [[1, 4], [2, 3], [3, 2], [4, 1]]], [3, [[1, 3], [2, 2], [3, 1]]], [4, [[1, 2], [2, 1]]], [5, [[1, 1]]]]
 print flatten([merge(x) for x in s])

Output is:

[[1, 1, 5], [1, 2, 4], [1, 3, 3], [1, 4, 2], [1, 5, 1], [2, 1, 4], [2, 2, 3], [2, 3, 2], [2, 4, 1], [3, 1, 3], [3, 2, 2], [3, 3, 1], [4, 1, 2], [4, 2, 1], [5, 1, 1]]

score 0 · Answer 3 · answered Jan 02 '16 at 15:46

0

items = [[1, [[1, 5], [2, 4], [3, 3], [4, 2], [5, 1]]], [2, [[1, 4], [2, 3], [3, 2], [4, 1]]], [3, [[1, 3], [2, 2], [3, 1]]], [4, [[1, 2], [2, 1]]], [5, [[1, 1]]]]
flat_items = []
for item in items:
    leading = item[0]
    for i in item[1]:
        flat_items.append([leading]+i)
print(flat_items)

answered Jan 02 '16 at 15:46

kia

828
5
12

This works here, but if the lists go deeper, so instead of that [1,5] there was something like [1,[2,3]], I would want it to output [[1,1,2],[1,1,3],... as in the code would have to do what yours does for the [1,[2,3]], but then using that do it again with the original 1, and this could go on indefinitely. – Mrahta Jan 02 '16 at 15:53

score 0 · Answer 4 · answered Jan 02 '16 at 15:54

0

Yet another solution that gives you an output as you needed:

def flatten(lst):
    result = []
    for i in lst:
        for j in i[1]:
            pair = j[:]
            pair.insert(0, i[0])
            result.append(pair)
    return result

answered Jan 02 '16 at 15:54

vrs

1,922
16
23

How to flatten a list in python?

4 Answers4