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Comment by cgore | Hacker News Reader

Actually, array indices do make sense as negative. For example, you might want an array going from a[-500] to a[700], without having to adjust the index all the time. It is much nicer if the array does it for you. Fortran lets you change the starting index if I remember correctly.

Most commonly you'll actually want to go positive, and have an array going from a[1970] to a[2013] or something similar.

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Perhaps, though from a language design standpoint I think that adds unnecessary complexity. Either way, we're talking about C here, and in that context signed integers don't make sense as array indexes.

The fact that we're talking about C doesn't really make a difference, if you genuinely desire negative indices:

    int array[FOO];
    int *shifted_array = array + FOO/2;

That's actually a nice trick. I just hope the compiler doesn't complain about it. I'll have to try that sometime.

EDIT: just tried it. The following works:

    #include <stdio.h>
    int main(void)
    {
        int array[100];
        int *shifted_array = array - 1980;
        for (int i = 0; i < 100; i++)
            array[i] = i;
        for (int j = 1980; j < 2014; j++)
            printf("shifted_array[%d] = %d\n", j, shifted_array[j]);
        return 0;
    }

Yeah, there's a couple cases where this kind if thing cleans things up; use it with caution though.

More amusing, but less useful outside the IOCCC:

The C subscripting operator performs an basically syntactic transformation, turning a[i] into (*(a + i)). This means that, counter-intuitively (if you're thinking of [] as a lookup) the construct i[a] works too because plus is commutative.

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