"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;
}

for (i = 0; i < 1; i++) <-- Why do you even need a loop?

Spikatrix
– Spikatrix

2015-05-02 11:32:56 +00:00
Commented May 2, 2015 at 11:32 — Spikatrix
– Spikatrix, Commented May 2, 2015 at 11:32

Ayushi Jha · Accepted Answer · 2015-05-02 11:39:21Z

5

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

edited May 2, 2015 at 11:39

answered May 2, 2015 at 11:31

Ayushi Jha

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2 Comments

Jacob Over a year ago

I'm not sure why the things you changed work but I will ask at uni about it thanks for your help.

Jacob Over a year ago

I had four questions yesterday but I did some more coding this morning and your solutions suddenly made sense. I think I just needed some rest, thank you for your explanations though they helped a lot.

ouah · Accepted Answer · 2015-05-02 11:31:04Z

5

float i, j, k;

have to be:

int i, j, k;

(or any other integer types) as you can only use integer values for array indexes.

result = result + Matrix_1[i][k] * Matrix_2[k][j];
                           ^  ^ have to be integers

answered May 2, 2015 at 11:31

ouah

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1 Comment

Ady96 Over a year ago

What can I do, when I need have size of array e.g. 1 000 000 000 000? How can I iterate throught the array with this size, when integer can't have number as 1 000 000 000 000

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