Dynamically allocated 2d array

Question

I'm trying to create a 2d array, specifically an adjacency matrix for directed graphs. I've never tried this with dynamic memory allocation before and I've hit a snag. Here's the code:

int n, i;
printf("Number of nodes is: ");
scanf("%d", &n);

int ** array = malloc(n * sizeof(int*));
for(i = 0; i < n; i++)
    array[i] = malloc(n * sizeof(int));

printf("Number of edges is: ");
scanf("%d", &m);

int x, y;
for(i=0;i<m;i++)
{
    scanf("%d %d", &x, &y);
    array[x][y]=1;
}

As soon as I finish entering all the edges, the program stops working and throws the usual "exe has stopped working".

Where's the problem?

EDIT: houssam spotted my error. The first "for" should have gone from 1 to n. When I entered 1 6 as an edge, the program crashed because there were only nodes 0-5. Thanks for spotting that. Such a simple mistake.....

Where exactly does it crash? Could you mark the point at the code? Also, did you validate your own input? — Zach P
– Zach P, Commented May 12, 2015 at 17:08
Could you give the whole program? Nothing here (other than out of bounds inputs) would cause a crash.. — mtijanic
– mtijanic, Commented May 12, 2015 at 17:12
You probably also want to explicitly initialize each array[i][j] to 0, either with another loop or by using calloc. — Jeff Ames
– Jeff Ames, Commented May 12, 2015 at 17:17

Community · Accepted Answer · 2017-05-23 12:14:41Z

you may entered wrong values for x or y , their values should be less than n and greater than or equal to 0:

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

int main()
{
    int m ,n, i;
    printf("Number of nodes is: ");
    scanf("%d", &n);

    int ** array = malloc(n * sizeof(int*));
    for(i = 0; i < n; i++)
        array[i] = malloc(n * sizeof(int));

    printf("Number of edges is: ");
    scanf("%d", &m);

    int x, y;
    for(i=0;i<m;i++)
    {
        scanf("%d %d", &x, &y);
        if (x >=0 && y >= 0 && x < n && y < n) // here
            array[x][y]=1;
        else
            printf("error in your values x=%d y=%d\n" , x , y);
    }
    return 0;
}

EDIT #1:
based on comment from user : M Oehm to check the return value of scanf , see:
scanf fails why? , I can refactor the code to be like:

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

int main()
{
    int m ,n, i;
    printf("Number of nodes is: ");
    scanf("%d", &n);

    int ** array = malloc(n * sizeof(int*));
    for(i = 0; i < n; i++)
        array[i] = malloc(n * sizeof(int));

    printf("Number of edges is: ");
    scanf("%d", &m);

    int x, y;
    for(i=0;i<m;i++)
    {
        //int num_read = scanf("%d %d", &x, &y);
        if(scanf("%d %d", &x, &y) < 2)
        {
            printf("please enter valid two integers..\n");          
            while (getchar() != '\n'); // read all characters in the input stream.
        }
        else if (x >=0 && y >= 0 && x < n && y < n) // here
        {
            array[x][y]=1;
            printf("array[%d][%d]=1;\n" , x, y);

        }
        else
            printf("error in your values x=%d y=%d\n" , x , y);
    }
    return 0;
}

While you're at it: Why not check the return value of scanf?

Collectives™ on Stack Overflow

Dynamically allocated 2d array

1 Answer 1

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related