sum matrix columns in python

Question

I can sum the items in column zero fine. But where do I change the code to sum column 2, or 3, or 4 in the matrix? I'm easily stumped.

def main():
    matrix = []

    for i in range(2):
        s = input("Enter a 4-by-4 matrix row " + str(i) + ": ") 
        items = s.split() # Extracts items from the string
        list = [ eval(x) for x in items ] # Convert items to numbers   
        matrix.append(list)

    print("Sum of the elements in column 0 is", sumColumn(matrix))

def sumColumn(m):
    for column in range(len(m[0])):
        total = 0
        for row in range(len(m)):
            total += m[row][column]
        return total

main()

inspectorG4dget · Accepted Answer · 2014-04-18 00:49:51Z

13

numpy could do this for you quite easily:

def sumColumn(matrix):
    return numpy.sum(matrix, axis=1)  # axis=1 says "get the sum along the columns"

Of course, if you wanted do it by hand, here's how I would fix your code:

def sumColumn(m):
    answer = []
    for column in range(len(m[0])):
        t = 0
        for row in m:
            t += row[column]
        answer.append(t)
    return answer

Still, there is a simpler way, using zip:

def sumColumn(m):
    return [sum(col) for col in zip(*m)]

answered Apr 18, 2014 at 0:49

inspectorG4dget

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3 Comments

Ahmad Over a year ago

I guess axis = 0 gets the sum along the columns

onofricamila Over a year ago

Oh no, he is right. If you specify ´axis = 1´, for every row in the matrix, the columns values will be added up and then stored. The result will be a 1D array containing all the sums.

Coder Over a year ago

Based on the documentation, for columns we use axis = 0 and for row axis = 1.

ChrisW · Accepted Answer · 2015-01-30 17:55:25Z

8

One-liner:

column_sums = [sum([row[i] for row in M]) for i in range(0,len(M[0]))]

also

row_sums = [sum(row) for row in M]

for any rectangular, non-empty matrix (list of lists) M. e.g.

>>> M = [[1,2,3],\
>>>     [4,5,6],\
>>>     [7,8,9]]
>>>
>>> [sum([row[i] for row in M]) for i in range(0,len(M[0]))]
[12, 15, 18] 
>>> [sum(row) for row in M]
[6, 15, 24]

answered Jan 30, 2015 at 17:55

ChrisW

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1 Comment

travc Over a year ago

As much as I like list comprehension magic, @inspectorG4dget's zip based version is ~2X faster for a small (3x3) matrix. zip is 1.86usec vs 3.7usec per loop for nested list comprehension. Old school list(map(sum,zip(*M))) is just as fast (1.87usec).

user3286261 · Accepted Answer · 2014-04-18 02:24:55Z

2

Here is your code changed to return the sum of whatever column you specify:

def sumColumn(m, column):
    total = 0
    for row in range(len(m)):
        total += m[row][column]
    return total

column = 1
print("Sum of the elements in column", column, "is", sumColumn(matrix, column))

edited Apr 18, 2014 at 2:24

answered Apr 18, 2014 at 1:08

user3286261

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Comments

Karol Dowbecki · Accepted Answer · 2018-10-30 11:01:02Z

2

To get the sum of all columns in the matrix you can use the below python numpy code:

matrixname.sum(axis=0)

edited Oct 30, 2018 at 11:01

Karol Dowbecki

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answered Oct 30, 2018 at 10:43

Vaka Chiranjeevi

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1 Comment

onofricamila Over a year ago

Mmm i think it is matrixname.sum(axis=1) instead ... >>> b array([[1, 2], [3, 4]]) >>> b.sum(axis=1) array([3, 7]) >>> b.sum(axis=0) array([4, 6])

Buddy · Accepted Answer · 2018-11-27 22:55:04Z

2

import numpy as np
np.sum(M,axis=1)

where M is the matrix

edited Nov 27, 2018 at 22:55

Buddy

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answered Nov 27, 2018 at 22:47

Fernando

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Comments

michaelmeyer · Accepted Answer · 2014-04-18 00:50:24Z

0

This can be made easier if you represent the matrix as a flat array:

m = [
    1,2,3,4,
    10,11,12,13,
    100,101,102,103,
    1001,1002,1003,1004
]

def sum_column(m, n):
    return sum(m[i] for i in range(n, 4 * 4, 4))

answered Apr 18, 2014 at 0:50

michaelmeyer

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