Populating a dynamically allocated 2D array

Question

I am trying to populate a dynamically allocated 2D array.

Specifically, I am trying to input random numbers into an array.

Code below:

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

int main(void)
{

int i, j, num_students = 10, num_grades = 4;
int **array;

srand(time(NULL));


array = malloc(sizeof(int *) * num_students);

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

for (i = 0; i < num_students; ++i)
{
    printf("Student %d has grades: ", i);
    for (j = 0; j < num_grades; ++j)
        array[i][j] = rand() % 101;
        printf("%d ", array[i][j]);

    printf("\n");
    free(array[i]);
}

free(array);

return 0;

}

The output:

Student 0 has grades: 0 
Student 1 has grades: 0 
Student 2 has grades: 0 
Student 3 has grades: 0 
Student 4 has grades: 0 
Student 5 has grades: 0 
Student 6 has grades: 0 
Student 7 has grades: 0 
Student 8 has grades: 0 
Student 9 has grades: 0

I do not know why it is printing 0 instead of the random numbers.

Note: Had code not defined `j` until the loop like `for (int j = 0; j < num_grades; ++j)`, then the compiler would have error-ed on `printf("%d ", array[i][j]);` — chux - Reinstate Monica, Jan 18 '18 at 21:16

score 3 · Accepted Answer · answered Jan 18 '18 at 21:11

3

You missed the { ... }...

for (j = 0; j < num_grades; ++j)
        array[i][j] = rand() % 101;
        printf("%d ", array[i][j]);

Is the same as

for (j = 0; j < num_grades; ++j) {
        array[i][j] = rand() % 101;
}
printf("%d ", array[i][j]);

,but it should be

for (j = 0; j < num_grades; ++j) {
        array[i][j] = rand() % 101;
        printf("%d ", array[i][j]);
}

answered Jan 18 '18 at 21:11

Stephan Lechner

34,891
4
35
58

Thank you kind one of the glorious hills. – elMentat Jan 18 '18 at 21:21

score 1 · Answer 2 · answered Jan 18 '18 at 21:10

1

You're missing brackets around the inner for loop:

for (j = 0; j < num_grades; ++j)
{
    array[i][j] = rand() % 101;
    printf("%d ", array[i][j]);
}

answered Jan 18 '18 at 21:10

Mike Holt

4,452
1
17
24

score 1 · Answer 3 · answered Jan 18 '18 at 21:10

Indentation doesn't control your scope (as it does in Python, for example). Your for-loop only iterates over the first line:

for (j = 0; j < num_grades; ++j)
    array[i][j] = rand() % 101; // <- only this is iterated over
    printf("%d ", array[i][j]); // <- this only runs once, after the for-loop

Ensure that you've encapsulated both lines in braces:

for (j = 0; j < num_grades; ++j) {
    array[i][j] = rand() % 101;
    printf("%d ", array[i][j]);
}

Many thanks for picking up on the simple mistake. – elMentat Jan 18 '18 at 21:22 — elMentat, Jan 18 '18 at 21:22

Populating a dynamically allocated 2D array

3 Answers3