How to sum up individual nested lists in python

Question

    data = [[1, 1, 0, 1, 1, 0, 0], [0, 0, 1, 0, 0, 1, 0], [0, 0, 1, 0, 0, 1, 0], [0, 0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0, 0], [0, 0, 0, 1, 0, 2, 3], [1, 1, 2, 0, 1, 1, 0], [1, 0, 0, 2, 1, 1, 2]]

Hello, I have this list with nested lists called data. I would like to sum up the indivual nested lists only so the output is:

    [[4],[2],[2],[0],[0],[0],[0],[0],[6],[6],[7]]

I tried to do this:

    for numbers in data:
            new_list = []
            y = sum(numbers)
            new_list.append(y) 
     print (new_list)

However, this only gives the sum of the last nested list, because for every for-loop the new_list seems to "reset".

Thanks

because you keep creating new_list = [], then appending to it, then on the next iteration, doing new_list = [], which discards the previous list. — juanpa.arrivillaga
– juanpa.arrivillaga, Commented Mar 5, 2021 at 21:13
As an aside, having a list with single-element lists seems pretty pointless, why not just list(map(sum, data))? You coudl get what you posted originally, though, with something like [[sum(numbers)] for numbers in data] which you coudl get in your for-loop by just doing new_list.append([y]) — juanpa.arrivillaga
– juanpa.arrivillaga, Commented Mar 5, 2021 at 21:13

Alain T. · Accepted Answer · 2021-03-05 21:15:44Z

3

Not sure why you would want each sum to be a single element list but you can do it in a list comprehension:

[ [sum(sl)] for sl in data ]
                         
[[4], [2], [2], [0], [0], [0], [0], [0], [6], [6], [7]]

answered Mar 5, 2021 at 21:15

Alain T.

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Daniyar Aubekerov · Accepted Answer · 2021-03-05 21:12:44Z

1

The reason is that your new_list must be outside of the loop. Right now, after each iteration, you are overwriting your new_list. Also, if you want each element to be a list you will need to change to [y] Try to:

new_list = []
for numbers in data:
            y = sum(numbers)
            new_list.append([y]) 
 print (new_list)

Also, if you want to use python's list comprehension feature you can do:

new_list = [[sum(numbers)] for numbers in data]

answered Mar 5, 2021 at 21:12

Daniyar Aubekerov

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