C - reverse a number

Question

I am coding in C on linux, and I need to reverse a number. (EG: 12345 would turn into 54321), I was going to just convert it into a string using itoa and then reverse that,

Answer 1

iota() is not a standard C function, but snprintf() serves the purpose just as well.

/* assume decimal conversion */
const char * my_itoa (int input, char *buffer, size_t buffersz) {
    if (snprintf(buffer, sz, "%d", input) < sz) return buffer;
    return 0;
}

Since the input cannot be negative, you can use an unsigned type:

unsigned long long irev (unsigned input) {
    unsigned long long output = 0;
    while (input) {
        output = 10 * output + input % 10;
        input /= 10;
    }
    return output;
}

Reversing the input may result in a value that no longer fits the input type, so the return result attempts to use a wider type. This may still fail if unsigned and unsigned long long have the same width. For such cases, it is probably easiest to use a string to represent the reversed value. Or, if the only goal is to print the number, you can just use a loop to print the digits in reverse order.

void print_irev (unsigned input) {
    if (input) {
        do {
            putchar('0' + input % 10);
            input /= 10;
        } while (input);
    } else {
        putchar('0');
    }
    putchar('\n');
}