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() 6 Answers
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)] 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] 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)) To get the sum of all columns in the matrix you can use the below python numpy code:
matrixname.sum(axis=0) import numpy as np np.sum(M,axis=1) where M is the matrix
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)) 
