Partial Array Initializer Fills Zeros

1#include <stdio.h>
2 
3int main() {
4    int a[5] = {1, 2};
5    int sum = 0;
6    for (int i = 0; i < 5; i++)
7        sum += a[i];
8    printf("%d\n", sum);
9    return 0;
10}