How do I sum the column values in a nested list without using any imports or the sum function?

Question

I have been asked to find the sum of the first value in each column in a nested list. How do I do this without using imports or the sum function?

 def column_sums(square):
    """Returns the sum of the columns"""

    result = []
    i = 0
    for element in square:
        n = 0
        for item in element:
            if item == element[n]:
                i = i + item
        n = n + 1
    result.append(i)
    i = 0
    return result

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

This is what I have come up with but it only returns the value of the first column. How can I get the sum of all the columns set out like this?:

[28, 32, 36, 40]

Can you be more specific about what the issue is? Have you written any pseudocode/design? — AMC
– AMC, Commented May 1, 2020 at 9:46

user2390182 · Accepted Answer · 2020-05-01 09:44:12Z

2

For a purely loop based approach without any built-ins:

def column_sums(square):
    result = [0] * len(square[0])
    for row in square:
        for i in range(len(result)):
            result[i] += row[i]
    return result

The following uses the zip(*...) transpositional pattern:

def column_sums(square):
    result = []
    for col in zip(*square):
        total = 0
        for num in col:
            total += num
        result.append(total)
    return result

square = [
    [1, 2, 3, 4],
    [5, 6, 7, 8],
    [9, 10, 11, 12],
    [13, 14, 15, 16]
]
print(column_sums(square))
# [28, 32, 36, 40]

Note that you get the columns by zipping the rows. If you could use sum and all other built-ins, the following would be the shortest:

print(list(map(sum, zip(*square))))

edited May 1, 2020 at 9:44

answered May 1, 2020 at 9:37

user2390182

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1 Comment

Heidi Felix Over a year ago

Thank you, that really helped. I didn't know about zip(* )!

U13-Forward · Accepted Answer · 2020-05-01 09:44:52Z

1

Use this:

def column_sums(square):
    result = dict.fromkeys(len(square), 0)
    for i, v in enumerate(zip(*square)):
        for x in v:
            result[i] += x
    return result

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

Or with sum you can actually just use:

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

edited May 1, 2020 at 9:44

answered May 1, 2020 at 9:38

U13-Forward

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3 Comments

user2390182 Over a year ago

"How do I do this without [...] the sum function?"

user2390182 Over a year ago

Nope, wasn't me

CKM Over a year ago

sum is disallowed as per OP.

CKM · Accepted Answer · 2020-05-01 09:48:17Z

1

Here you go with the simplest sol that came to my mind without using any fancy stuff.

def column_sums(square):
    """Returns the sum of the columns"""

    result = [0,0,0,0]
    for element in square:

        for i,item in enumerate(element):
                result[i] = result[i] + item   
    return result

edited May 1, 2020 at 9:48

answered May 1, 2020 at 9:41

CKM

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