How to add matrices in python

Question

How can I add matrices (list of lists) in python? I know that I can do this:

X = [[12,7,3],
    [4 ,5,6],
    [7 ,8,9]]

Y = [[5,8,1],
    [6,7,3],
    [4,5,9]]

result = [[0,0,0],
         [0,0,0],
         [0,0,0]]

# iterate through rows
for i in range(len(X)):
   # iterate through columns
   for j in range(len(X[0])):
       result[i][j] = X[i][j] + Y[i][j]

for r in result:
   print(r)

But what if the the input of the two lists have different numbers of rows or columns? I need to check first to see if the number of rows and columns are the same before adding and I'm not sure how to do this.

if A is an n × m matrix and B is an m × p matrix, their matrix product AB is an n × p matrix. en.wikipedia.org/wiki/Matrix_multiplication — Karoly Horvath
– Karoly Horvath, Commented Feb 4, 2016 at 21:36
Using numpy the matrix multiplication can be done even using the common multiplication operators, there is absolutely no need for all this mess... — Greg
– Greg, Commented Feb 4, 2016 at 21:38
I'm not trying to multiply the matrices though. I need to ADD them — Michael Minkoff
– Michael Minkoff, Commented Feb 4, 2016 at 21:39
If all you want to do is check that they have the same dimensions, just do (assert len(X) == len(Y)) and (len(X[0]) == len(Y[0])) (assuming that you know all sub-lists will be of the same length) — C_Z_
– C_Z_, Commented Feb 4, 2016 at 21:41

Prune · Accepted Answer · 2016-02-04 21:46:49Z

1

Do the size check and then build the result directly with a doubly-nested list comprehension. Numpy is better, but if you want to avoid an import, this will do.

X = [[12,7,3],
     [4 ,5,6],
     [7 ,8,9]]

Y = [[5,8,1],
     [6,7,3],
     [4,5,9]]

if len(X) == len(Y) and len(X[0]) == len(Y[0]):
    result = [[X[i][j] + Y[i][j] for j in range(len(X[0]))]
              for i in range(len(X))]

print result

Output:

[[17, 15, 4], [10, 12, 9], [11, 13, 18]]

answered Feb 4, 2016 at 21:46

Prune

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2 Comments

Garrett R Over a year ago

the above code will throw an error if the dims don't match, right?

Prune Over a year ago

Only because result isn't defined. To throw a proper exception, add an else clause of your choice.

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