Question

so this is part of my homework assignment. need some help with it. my task is to swap rows and columns so that the numbers on the main diagonal of a matrix are in descending order. the number of rows and columns is the same. also I need to allocate memory dynamically.

here is an example:
input:

    1 2       1<4 so we swap rows and columns and the final result is 4 3 
    3 4                                                               2 1

THE PROBLEM is that when I insert a 3×3 array or even larger I get some kind of a segmentation fault. here is my code, please help!

    int **t, n, i, j, aux;
    printf("Insert the size of the matrix: ");
    scanf("%d",&n);

    t = malloc(n * sizeof(int *));

    for(i=1; i <= n; i++){
            t[i]=malloc(n * sizeof(int));
    }
    for(i=1; i<=n; i++){
            for(j=1; j<=n; j++){
                    printf("Element [%d][%d] : ", i, j);
                    scanf("%d", &t[i][j]);
            }
    }

    for(i=2; i<=n; i++){
            if(t[i][i] > t[i-1][i-1]){
                    j=i-1;
                    for(i=1; i<=n; i++){
                            aux=t[i][j];
                            t[i][j]=t[i][j+1];
                            t[i][j+1]=aux;
                    }
                    i=j;
                    for(j=1; j<=n; j++){
                            aux=t[i][j];
                            t[i][j]=t[i+1][j];
                            t[i+1][j]=aux;
                    }
                    i=1;
            }
    i++;
    }
    for(i=1; i<= n; i++){
            free(t[i]);
    }
    free(t);

Now the error I get is the following:

    The matrix inserted:
     1  2  3
     4  5  6
     7  8  9
    The matrix after swapping:
     5  4  6
     2  1  3
     8  7  9
    *** glibc detected *** ./6: double free or corruption (out): 0x08e42018 ***
    ======= Backtrace: =========
    /lib/i686/cmov/libc.so.6[0xb7649764]
    /lib/i686/cmov/libc.so.6(cfree+0x96)[0xb764b966]
    ./6[0x80488fb]
    ....

Thank you in advance!!

Answer 1

Array indexes start from 0 to n-1, if it’s size is n. With that said, you have quite a few places to take care of it. Play safe with array indexes. For example –

for(i=1; i <= n; i++){  // Should be i=0; i < n; i++ 
        t[i]=malloc(n * sizeof(int));
}