Why is 1st for loop isn't getting executed in Code blocks?

This is a program to find norm of a matrix. I am using Code Blocks to write codes. The 1st for loop in this code which is used to fill the matrix doesn't get executed in Code Blocks but gets executed in a online c compiler. Why is this happening?

///find the norm of a matrix

#include<stdio.h>
#include<math.h>
void main()
{
    short int r,c;

    register short int i,j;

    //fill matrix
    printf("\nENTER MATRIX DIMENSION");
    printf("\nROWS: ");
    scanf(" %d",&r);
    printf("COLUMNS: ");
    scanf(" %d",&c);

    printf("\ncheck1");

    --r; --c;
    printf("\ncheck2");
    float mat[r][c],sum,norm;

    printf("\ncheck3");

    //1st for loop:fill array                  
    for(i=0 ;i<= r ;i++)
    {
        for(j=0 ;j<= c ;j++)
        {
            printf(" \nELEMENT %d x %d: ",i,j);
            scanf(" %f",&mat[i][j]);
        }
    }

 printf("\ncheck4");
 
    //finding norm
    sum = 0;
     for(i=0;i<=r;i++)
    {
        for(j=0;j<=c;j++)
        {
            sum = sum + pow(mat[i][j],2);
        }
    }

    norm = pow(sum,0.5);
    printf("\n\nNORM: %f",norm);
}

The code is correct but the for loop is not executing because the condition:

for(i=0;i<=r;i++)

i<=r is getting false.

Why? Because garbage value is being stored in your variable r and c.

You are using format specifier %d which is for int. The Format specifier for short is %hd.
Just use %hd and everything will work fine.