runtime error in sandbox for array examples from week 2 lecture

Question

This is confusing me a little and I want to fully understand it before moving through the rest of the lecture. I usually code alongside David's examples on screen and was trying to understand why this makes sense

#include <cs50.h>
#include <stdio.h>

int main(void)
{
    int n = get_int("Number of scores: ");
    int scores[n];
    for(int i = 0; i < n; i++)
    {
        scores[i] = get_int("Score %i: ", i + 1);
    }

    printf("Average: TO DO\n");
}

and yet this doesn't (and also produces a run-time error in sandbox):

#include <cs50.h>
#include <stdio.h>

int main(void)
{
    int n = get_int("Number of scores: ");
    int scores[n];
    for(int i = 0; i < n; i++)
    {
        scores[n] = get_int("Score %i: ", i + 1);
    }

    printf("Average: TO DO\n");
}

The error I got was

runtime error: index 3 out of bounds for type 'int [n]'

I'm confused as to why the one in the loop can't be n? I can't even begin to describe my logic on this...

Answer 1

When the int array scores is declared here int scores[n]; it will have n elements, indexed to 0. When 3 is entered, the elements of scores are scores[0], scores[1] and scores[2]. There is no scores[3]. (Notice the loop executes until i < n to ensure it will be within bounds). This line scores[n] = get_int("Score %i: ", i + 1); will try to populate scores[3], which produces the "out of bounds" error.