Why is my C matrix multiplication program not working properly?

[James889]

Hello,

I am trying to write a program that multiplies together two 3x3 matrices.

However my program does not really work as intended, and i get some really strange results.

Any ideas?

#include <stdio.h>
#include <stdlib.h>

int main() {

int result_matrix[3][3];

int matrixA[3][3] = { {2,2,2},
                             {2,2,2},
                             {2,2,2} };

int matrixB[3][3] = { {4,4,4},
                            {4,4,4},
                            {4,4,4} };
        for(int rows=0; rows<3; rows++) {

                for(int columns=0; columns<3; columns++){

                for(int r=0,c=0; r<3,c<3; r++,c++){
         printf("Element at result_matrix[%d][%d] is MatrixA[%d][%d] * MatrixB[%d][%d]\n",rows,columns,r,c,c,r);
                result_matrix[rows][columns] += matrixA[r][c] * matrixB[c][r];

                }
        }
}
printf("%d\n",result_matrix[0][0]);
printf("%d\n",result_matrix[0][1]);
printf("%d\n",result_matrix[0][2]);
printf("%d\n",result_matrix[1][0]);
printf("%d\n",result_matrix[1][1]);
printf("%d\n",result_matrix[1][2]);
printf("%d\n",result_matrix[2][0]);
printf("%d\n",result_matrix[2][1]);
printf("%d\n",result_matrix[2][2]);
return 0;
}

 

[MisterX]

There's no initialization of the elements of result_matrix.

 

[I like Serena]

Hi James889!

And you inner loop is not right.
For starters, it should not cycle over r and c, but only over c.

 

[James889]

Element at result_matrix[0][0] is: 25
Element at result_matrix[0][1] is: 24
Element at result_matrix[0][2] is: 24
Element at result_matrix[1][0] is: 25
Element at result_matrix[1][1] is: 24
Element at result_matrix[1][2] is: -1077941132
Element at result_matrix[2][0] is: -1077941124
Element at result_matrix[2][1] is: 672789176
Element at result_matrix[2][2] is: -1077941296

This is after i changed the inner loop to only loop through c

 

[I like Serena]

This is after i changed the inner loop to only loop through c

From the wikipedia page about http://en.wikipedia.org/wiki/Matrix_multiplication" :

Do you see the difference with your code?

Furthermore, MisterX already remarked that you need to initialize your result matrix to zero before trying to multiply the matrices.

Cheers!

 

[James889]

result_matrix[3][3] = { {0} };

./run

Element at result_matrix[0][0] is: 24
Element at result_matrix[0][1] is: 24
Element at result_matrix[0][2] is: 24
Element at result_matrix[1][0] is: 24
Element at result_matrix[1][1] is: 24
Element at result_matrix[1][2] is: 24
Element at result_matrix[2][0] is: 24
Element at result_matrix[2][1] is: 24
Element at result_matrix[2][2] is: 24

 

[I like Serena]

And yet another satisfied customer!

 

[MisterX]

result_matrix[3][3] = { {0, 0, 0}, {0, 0, 0}, {0, 0, 0} };