The sum of indexes within nested lists. Adding elements from separate lists

Question

Just doing an exercise question as follows: Take a ' number square' as a parameter and returns a list of the column sums.

e.g

square = [
[1, 2, 3, 4],
[5, 6, 7, 8],
[9, 10, 11, 12],
[13, 14, 15, 16]
]

Result should be =

[28, 32, 36, 40]

What I have so far:

def column_sums(square):
    col_list = []
    for i in range(len(square)): 
        col_sum = 0
        for j in range(len(square[i])): 
            col_sum += square[j]
        col_list.append(col_sum)
    return col_list

So I think I have all the parameters laid out, with all the indexing correct, but I am stuck because I get an error

builtins.TypeError: unsupported operand type(s) for +=: 'int' and 'list'

Which shouldn't happen if I am referencing the element within the list I thought.

Also it's probably easier to use the SUM command here, but couldn't figure out a way to use it cleanly.

Adding elements from separate lists via indexing.

Possible duplicate of Sum of nested list without using SUM function (exercise) — Julien
– Julien, Commented Aug 9, 2016 at 6:40
Change square[j] to square[i][j] that is the specific location of the square. Currently, you are typecasting a list to an integer — Vaibhav Bajaj
– Vaibhav Bajaj, Commented Aug 9, 2016 at 6:41

Moses Koledoye · Accepted Answer · 2016-08-09 07:38:49Z

2

A more simpler solution would be to transpose the list of lists with zip and take the sum across each new sublist:

def column_sums(square):
    return [sum(i) for i in zip(*square)]

zip(*square) unpacks the list of lists and returns all items from each column (zipped as tuples) in successions.

>>> column_sums(square)
[28, 32, 36, 40]

You could also use numpy.sum to do this by setting the axis parameter to 0, meaning sum along rows (i.e. sum on each column):

>>> import numpy as np
>>> square = np.array(square)
>>> square.sum(axis=0)
array([28, 32, 36, 40])

edited Aug 9, 2016 at 7:38

answered Aug 9, 2016 at 7:08

Moses Koledoye

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Comments

eol · Accepted Answer · 2016-08-09 06:55:45Z

1

It should be col_sum += square[j][i] since you want to access the element at position j of the list at position i.

edited Aug 9, 2016 at 6:55

answered Aug 9, 2016 at 6:40

eol

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3 Comments

Robert Hemingway Over a year ago

Hmm but with that addition it just gives me back the sum of each list e.g [10, 26, 42, 58] to the above statement

okorkut Over a year ago

That gives you the row sums not the column sums

eol Over a year ago

Just swap the indices to get the col sum and not the row sum

Israel Unterman · Accepted Answer · 2016-08-09 07:03:13Z

1

You should change the line of summing to:

col_sum += square[j][i]

because square[j] is the j'th row (list), but you need the current column, which is the i'th element in that row.

But here is my solution using sum and list comprehensions:

def column_sums2(square):
    def col(i):
        return [row[i] for row in square]
    return [sum(col(i)) for i in range(len(square))]

edited Aug 9, 2016 at 7:03

answered Aug 9, 2016 at 6:54

Israel Unterman

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