Thread: Encrypt a number

Just to show you what I had in mind:

Code:

#include <stdio.h>

int main(void) {
    int number = 0;
    do {
        fputs("Enter a four digit number: ", stdout);
        scanf("%d", &number);
        while (getchar() != '\n'); // discard all characters not convertible into an int
    } while (number < 1000 || number > 9999);


    // ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


    /*
    // I
    int digits[4] = { 0 };
    for (int divisor = 1000, idx = 0; idx < 4; divisor /= 10, ++idx) {
        digits[idx] = ((number / divisor + 7) % 10);
        number %= divisor;
    }
    const int newNumber = digits[0] * 10 +
                          digits[1] +
                          digits[2] * 1000 +
                          digits[3] * 100;
    */


    /*
    // II
    int newNumber = 0;
    for (int divisor = 1000, factor = 10; divisor; divisor /= 10) {
        if (divisor == 100) {
            factor = 1;
        } else if (divisor == 10) {
            factor = 1000;
        } else if (divisor == 1) {
            factor = 100;
        } // else, both divisor and factor still have their initial values


        newNumber += ((number / divisor + 7) % 10) * factor;
        number %= divisor;
    }
    */


    // III
    static const int factors[] = { 100, 1000, 1, 10 };
    int newNumber = 0;
    for (const int *iter = factors, *const end = iter + 4; iter < end; ++iter) {
        newNumber += *iter * ((number + 7) % 10);
        number /= 10;
    }


    // ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


    printf("New formed number: %04d\n", newNumber);
    return 0;
}

I and II is using your math with the divisor variable. In I you'll find my original proposal to separate the digits first. In II branching is used to choose the right factor. This will add some unnecessary complexity though. III is what I would have come up after all. No additional branching, and no divisor variable with its additional division operations.

Thanks aGerman, it is useful.