For example:
int a[ 15]={ 1,2,3,4,5,6,7,8,9, 10, 1 1, 12, 13, 14, 15 };
int x = 3;
printf("%d ",a[2]);
printf("%d ",a[2 * 3]);
printf("%d ",a[x * x]);
printf("%d ",a[(int)(2.6)];
printf("%d ",a[' \ n ']);