# Matrix Addition in Python - list

I'm trying to write Matrix Addition function using Python.

Here is the code I've been trying, but it gives me a list index error and I cannot figure out why.

def matrixADD(A,B): Z = [] #TODO for i in range(0,len(A)): for column in range(0, len(A)): result = A[i][column] + B[i][column] Z[i][column] = (result) return Z

using the following lists:

A = [[2,4], [7,0], [6,3]] B = [[3,1], [-1,8], [-3, 3]]

So in theory, A[0][0] + B[0][0] would equal 5, and I would want to add that value to position Z[0][0].

However I keep receiving the error: IndexError: list index out of range

## Answers

>>> A = [[2,4], [7,0], [6,3]] >>> B = [[3,1], [-1,8], [-3, 3]] >>> Z = [map(sum, zip(*t)) for t in zip(A, B)] >>> Z [[5, 5], [6, 8], [3, 6]]

As for how you could fix your current code:

Z = [] for i in range(len(A)): row = [] for j in range(len(A[i])): row.append(A[i][j] + B[i][j]) Z.append(row)

The important parts here being that you cannot just assign to Z[i][j] unless that row/column already exists, so you need to construct each inner list separately and append them to Z. Also the inner loop needs to end at the length of a row, so I changed range(len(A)) to range(len(A[i])).

len(A) = 3 but you matrix have dimension 3x2 so when you try to access A[2][2] (because column is between 0 and len(A)) you are out of bounds.

For column you are using range 0 to len(A) (which is 3). A[i][2] will be out of range, because A[i]'s length is only 2.

Try using column range to end a len(A[i]) instead of len(A):

def matrixADD(A,B): Z = [] #TODO for i in range(0,len(A)): for column in range(0, len(A[i])): result = A[i][column] + B[i][column] Z[i][j] = (result) return Z