Question

Possible Duplicate:
Negative array indexes in C?

Can I use negative indices in arrays?

#include <stdio.h>

int main(void)
{
    char a[] = "pascual";
    char *p = a;

    p += 3;
    printf("%c\n", p[-1]); /* -1 is valid here? */
    return 0;
}

Answer 1

Yes, -1 is valid in this context, because it points to a valid location in memory allocated to your char a[] array. p[-1] is equivalent to *(p-1). Following the chain of assignments in your example, it is the same as a+3-1, or a+2, which is valid.

EDIT : The general rule is that an addition / subtraction of an integer and a pointer (and by extension, the equivalent indexing operations on pointers) need to produce a result that points to the same array or one element beyond the end of the array in order to be valid. Thanks, Eric Postpischil for a great note.