Adding two matrices in python

Question

def addM(a, b):
    res = []
    for i in range(len(a)):
        row = []
        for j in range(len(a[0])):
            row.append(a[i][j]+b[i][j])
        res.append(row)
    return res

I found this code here which was made by @Petar Ivanov, this code adds two matrices, i really don't understand the 3rd line, why does he use len(a) and the 5th line, why does he use len(a[0]). In the 6th line, also why is it a[i][j] +b[i][j]?

sashkello · Accepted Answer · 2013-11-11 00:55:59Z

2

The matrix here is a list of lists, for example a 2x2 matrix will look like: a=[[0,0],[0,0]]. Then it is easy to see:

len(a) - number of rows.
len(a[0]) - number of columns (since this is a matrix, the length of a[0] is the same as length of any a[i]).
This way, i is the number of row, j is the number of column and a[i][j]+b[i][j] is simply adding up the elements of two matrices which are placed in the same locations in the matrices.

For all this to work, a and b should be of the same shapes (so, numbers of rows and columns would match).

edited Nov 11, 2013 at 0:55

answered Nov 11, 2013 at 0:50

sashkello

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2 Comments

user2971015 Over a year ago

if they are not the same (matrices) will i get an error or do i have to modify the code in order to return none? if len(a) != len(b): return None

sashkello Over a year ago

@user2971015 It would certainly be a good idea to handle len(a)!=len(b) or len(a[0])!=len(b[0]) to be sure the input is right. You also need to check if all the row lengths for each of a and b are the same, otherwise they won't be considered matrices.

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