Question

I have two two-dimensional arrays, and i don’t know why, or how, the addresses of two elements one from each array, coincide..
Here’s the source code:

#include <stdio.h>

int main()
{
    int i,j,m,n,o,p,*ptr;
    printf("Enter dimension of 1st matrix: ");
    scanf("%d * %d",&m,&n);
    printf("Enter dimension of 2nd matrix: ");
    scanf("%d * %d",&o,&p);
    int *a[m][n];
    int *b[o][p];
    if (n!=o) return 0;

    printf("\nEnter 1st matrix:\n");
    for (i=0;i<m;i++)
        for (j=0;j<n;j++)
        {   printf("%d   ",(a+i*(n-1)+i+j)); scanf("%d",(a+i*(n-1)+i+j));   }

    printf("\nEnter 2nd matrix:\n");
    for (i=0;i<o;i++)
        for (j=0;j<p;j++)
        {   printf("%d   ",(b+i*(p-1)+i+j)); scanf("%d",(b+i*(p-1)+i+j));   }

    /*Printing the matrices*/
    puts("");puts("");
    for (i=0;i<m;i++)
        {for (j=0;j<n;j++)
            {   ptr = (a+i*(n-1)+i+j);
                printf(" %d ",*ptr);    }   puts("");}puts("");
    for (i=0;i<o;i++)
        {for (j=0;j<p;j++)
            {   ptr = (b+i*(p-1)+i+j);
                printf(" %d ",*ptr);    }   puts("");}
}

And here’s a print screen;

Due to this, i have been getting errors in a simple program to calculate the product of two matrices. The question is, is this usual? Shouldn’t the compiler or the OS have taken care of this?

Also, why do i have to do ptr = (a+i*(n-1)+i+j); printf(" %d ",*ptr);?

Why won’t printf(" %d ",*(a+i*(n-1)+i+j)); work?

Answer 1

First of all, a and b are arrays of pointers, and the pointers are never initialized.

int *a[m][n];
int *b[o][p];

My guess is that it was meant to read:

int a[m][n];
int b[o][p];

(The rest of the code would need to be changed accordingly.)

Secondly, you’re treating pointers as ints (e.g. in %d). Bear in mind that a pointer can be wider than an int. For example, on my platform pointers are 64-bit and ints are 32-bit.