Fast transposing matrix in C

[equals]

Hey everyone!
I'm kind of a beginner so please bear with me.
I was given pseudo-code and using that I needed to build my own function. That said I'm finding a problem during the execution even though i get no particular error after compiling

My code is as follows:

void transposeSparseMatrix(Triple * a, Triple * b){    
    int i;
    int * cur_pos = 0;
    int num_col = a[0].column;
    int num_val = a[0].value;
    b[0].row = num_col;
    b[0].column = a[0].row;
    b[0].value = num_val;


    int * terminiRiga = malloc(sizeof(Triple *)*NUMR);
    int * posIniziale = malloc(sizeof(Triple *)*NUMC);


    if (num_val > 0){
        for (i = 0; i<=num_col-1; i++){
            terminiRiga[i] = 0;
        }
        for (i = 1; i<=num_val; i++){
            terminiRiga[a[i].column] = terminiRiga[a[i].column]+1;
        }
        posIniziale[0] = 1;
        for (i = 1; i <= num_col-1; i++){
            posIniziale[i] = posIniziale[i-1] + terminiRiga[i-1];
        }
        for (i = 1; i<=num_val; i++){
            *cur_pos = posIniziale[a[i].column];  //this line is what my debugger points as reason for the function not working
            printf("\ncur_pos: %d\n", *cur_pos);  //printf best debugger, it gives me weird values
            posIniziale[a[i].column] = posIniziale[a[i].column]+1;
            b[*cur_pos].row = a[i].column;
            b[*cur_pos].column = a[i].row;
            b[*cur_pos].value = a[i].value;
        }
    }


}

Would anyone mind explaining what I'm doing wrong?

Thanks in advance

[laserlight]

This declares a pointer and initialises it to be a null pointer:

int * cur_pos = 0;

Consequently, cur_pos needs to actually point to an int object, otherwise you cannot do this:

*cur_pos = posIniziale[a[i].column];

since you cannot dereference a null pointer. Perhaps you should just make cur_pos an int instead.

Also, this looks suspect:

int * terminiRiga = malloc(sizeof(Triple *)*NUMR);
int * posIniziale = malloc(sizeof(Triple *)*NUMC);

terminiRiga is a pointer to an int, so we would expect that it either points to a single standalone int object, or it points to some int object (typically the first) in an array of int objects, or perhaps one past the end of such an array of int objects. However, according to the malloc call, you allocate space for NUMR objects of type pointer to Triple. If this malloc call is correct, then we would expect terminiRiga to be a pointer to a pointer to Triple:

Triple **terminiRiga = malloc(sizeof(Triple*) * NUMR);

and in fact we could write:

Triple **terminiRiga = malloc(sizeof(terminiRiga[0]) * NUMR);

However, it looks like you use terminiRiga as if it were an array of int, in which case you should have written:

int *terminiRiga = malloc(sizeof(terminiRiga[0]) * NUMR);

Likewise for posIniziale.

That said, this might make no real difference because it could be the case that sizeof(Triple *) == sizeof(int), i.e., it would be good for correctness, but not necessarily change anything in the resulting compiled code.

[equals]

I managed to make it work, thanks! I did also use your correction regarding this

1	int *terminiRiga = malloc(sizeof(terminiRiga[0]) * NUMR);

so I'm guessing I was having troubles with memory allocation