How to input for arrays I don't know the length of?

Question

I'm trying to make a program that accepts the number of students enrolled to an exam, and how many points each of them got. I try to loop the inputs but it gives seemingly random numbers in output

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

int main ()
{
    int studenti;
    scanf("%d", &studenti);
    printf("%d ", studenti);
    int niza[studenti];
    for (int i = 1; i <= studenti; i++){
        scanf("%d", &niza[i]);
        i++;
        printf("%d ",niza[i]);
    }
}

What am I doing wrong? Is there another way to add array elements without knowing how big the array will be beforehand because I don't know how big they are when I pass the checks on my uni website.

Answer 1

The primary issue is that the for loop begins with 1 and continues to i <= studenti. In C, arrays begin with index '0' and the final index in this example is studenti - 1.
Another issue is the for loop increments i and there is an i++; in the body of the loop. i is incremented twice.
Check the return from scanf. It returns the number of items successfully scanned. Here that would be 1, 0 or EOF.

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

int main ()
{
    int studenti = 0; // initialize
    if ( 1 == scanf("%d", &studenti)) { // successful scan
        printf("%d ", studenti);
        int niza[studenti]; // variable length array
        for (int i = 0; i < studenti; i++) { // start from index 0
            if ( 1 == scanf("%d", &niza[i])) {
                printf("%d ",niza[i]);
            }
            else { // scanf returned 0 or EOF
                fprintf ( stderr, "problem scanning array element\n");
                return 2;
            }
        }
    }
    else { // scanf returned 0 or EOF
        fprintf ( stderr, "problem scanning\n");
        return 1;
    }
    printf("\n");
    return 0;
}