2D array program shows garbage value in output

Question

I am writing this simple code for dynamically allocated 2D array and displaying it. When I execute this program, it gives garbage value in the first row. How is that? Any errors in my program?

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

int main()
{
    int **ptr, limit, i, j;

    printf("Enter the limit: ");
    scanf("%d", &limit);

    ptr = (int **) malloc(limit * (sizeof(int)));
    for(i=0; i<limit; i++)
    {
        *(ptr+i) = (int *) malloc((i+1) * sizeof(int));

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

    for(i=0; i<limit; i++)
    {
        for(j=0; j<=i; j++)
        {
            printf("%d ", ptr[i][j]);
        }
        printf("\n");
    }

    return 0;
}

Output:

Enter the limit: 4
0 
1 1 
2 2 2 
3 3 3 3 

Enter the limit: 5
9478320 
1 1 
2 2 2 
3 3 3 3 
4 4 4 4 4

If I try to print the 2D array with the limit is less than or equal to 4, it works fine. What am I missing here..

Thanks

Stephan Lechner · Accepted Answer · 2018-06-27 13:33:18Z

6

Maybe there are other issues as well; but one thing is for sure - you are allocating the wrong size for the pointer elements:

ptr = (int **) malloc(limit * (sizeof(int)));

though there should be limit objects of pointers to int, you are allocating ints. So it needs to be...

ptr = malloc(limit * (sizeof(int*)));

BTW: In C, avoid casting the result of malloc.

answered Jun 27, 2018 at 13:33

Stephan Lechner

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