"Array Subscript is not an integer" c

Question

I am trying to do matrix multiplication using for loops and I am getting an error, "Array Subscript is not an integer" Could I get some help please.

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

int main(void)
{
  float Matrix_1[3][3] = {{3.4, 4.4, 1.2},{5.3, 5.7, 2.2},{6.2, -2.4, 0.9}};
  float Matrix_2[3][3] = {{7.3, 4.9, 3.7},{-2.4, 4.9, -10.2},{7.3, 5.2, 1.7}};
  float i, j, k;
  float result[3][3];  

  for (i = 0; i < 1; i++)
  {
    for (j = 0; j < 3; j++)
    {
        for(k = 0; k < 3; k++)
        {
              result = result + Matrix_1[i][k] * Matrix_2[k][j];    
        }
    }
   }

  printf("The result of multiplying the matrices together\n");
  printf("%.3f\t%.3f\t%.3f\n",result[0][0],result[0][1],result[0][2]);
  printf("%.3f\t%.3f\t%.3f\n",result[1][0],result[1][1],result[1][2]);
  printf("%.3f\t%.3f\t%.3f\n",result[2][0],result[2][1],result[2][2]);

  system("PAUSE");
  return 0;
}

Answer 1

The subscripts, or array indices need to be an int value. Therefore, change this:

float i, j, k; to this: int i, j, k;

Also, you have declared result as a 3X3 matrix. So, when storing data into the matrix, you have to store element - by - element. So, instead of this:

result = result + Matrix_1[i][k] * Matrix_2[k][j]; change it to this: result[i][j] = result[i][j] + Matrix_1[i][k] * Matrix_2[k][j];

One more thing: Initialise the result matrix!! In the above statement, you are using the value of its elements in the matrix, which maybe holding some garbage value, and you may not get the desired result. So, before using the result matrix, initialise it.

A simple way:

for(i=0;i<3;i++)
{
    for(j=0;j<3;j++)
        result[i][j]=0;
}

One last thing: your outermost for loop runs only one time: for (i = 0; i < 1; i++) Probably you wanted this: for (i = 0; i < 3; i++)

Finally, the modified code: http://ideone.com/26GSJa