CODEWITHSUNDEEP
python
mike991
mike991

Reputation: 11

How can I rewrite this list comprehension as a for loop?

How can I rewrite this using nested for loops instead of a list comprehension? 

final= [[]]  
    for i in array_list:
        final.extend([sublist + [i] for sublist in final])


    return final

Upvotes: 0

Views: 80

Answers (3)

Fluous
Fluous

Reputation: 2475

This code also seems to give the same results as your code. 

final = [[]]
for i in array_list:
    for sublist in list(final):
        final.extend([sublist + [i]])
return final

It seems like your code takes the elements of the last iteration and combines them with the element of the current iteration (See 'Example' below). In order to do this with a traditional for loop, you need to prevent the list that is being looped over from being updated while looping. I do this by breaking the link with the 'final' list variable. This can be done with the list() function or by slicing it (then you would need to replace with as proposed by Morgan).

Example

For the array_list of [1,2,3] you get the following. Each line (except for the lines) is a new element of the 'final' list variable.

[]
--------------------------------
1           > array_list[0]
--------------------------------
2           > array_list[1]
1, 2        > array_list[0] + array_list[1]
--------------------------------
3           > array_list[2]
1, 3        > array_list[0] + array_list[2]
2, 3        > array_list[1] + array_list[2]
1, 2, 3     > array_list[0] + array_list[1] + array_list[2]

Upvotes: 0

Morgan Thrapp
Morgan Thrapp

Reputation: 9986

If you try to iterate over final as you extend it, it creates an infinite loop. Because every time you go to the next element, you add another element, so you never reach the end of the list.

If you want to do the inner loop as a for loop instead of a list comprehension, you need to iterate over a copy of final. 

final = [[]] 
for i in [1, 2, 3]:
    for sublist in final[:]:
        final.extend([sublist + [i]])

Upvotes: 2

Patrick Haugh
Patrick Haugh

Reputation: 60954

Your solution looks to be a very good for loop one. A one-liner using itertools is possible, but ugly

list(itertools.chain(list(itertools.combinations(arr, i)) for i in range(len(arr) + 1)))

EDIT:

Prettier:

list(itertools.chain(*[itertools.combinations(arr, i) for i in range(len(arr)+1)]))

Upvotes: 0

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