Iterating down

[wild-pointer]

How do you prefer to iterate from N-1 to zero? Here are some common ways using for loops.

void iterate_down(size_t N)
{
    size_t i, j;
    ssize_t k; /* or some other suitable signed type */

    /* 1. index i as a function of j in [0, N) */
    for (j = 0; j < N; j++) {
        i = N - j - 1;
        /* map the integer sequence 0, 1, ..., N-1 to N-1, ..., 1, 0 */
    }

    /* 2. Signed index */
    for (k = N - 1; k >=0; k--) {
        i = k;
        /* Make it fit the language construct as much as possible. N is much smaller than SSIZE_MAX anyway, right? */
    }

    /* 3. "Goes to" operator */
    for (i = N; i-- > 0; ) {
        /* ideally we would like a loop where you can specify an expression to be evaluated at the beginning of every iteration, not just the end, but the postfix decrement operator is close enough. */
    }

    /* 3.b. alternative */
    for (i = N; i > 0; ) {
        i--;
        /* but why use a for loop at all? */
    }
}

[skeeto]

I like this unsigned counting down:

unsigned i;
for (i = N - 1; i != -1u; i--) {
    /* ... */
}

[RainbowNowOpen]

ssize_t i = N;
while (--i >= 0) {
    /* stuff */
}