Question

How to reverse the last N digits of an Integer in C

Answer 1

#include <stdio.h>
#include <math.h>

unsigned reverse_N_digits(unsigned last_N_digits, unsigned digits) {
    unsigned reverse = 0;
    while (digits--) {
        reverse = reverse * 10 + last_N_digits % 10;
        last_N_digits /= 10;
    }
    return reverse;
}

unsigned reverse_last_digits(unsigned x, unsigned digits) {
    unsigned pow10 = (unsigned)ceil(pow(10, digits));
    unsigned last_N_digits = x % pow10;
    unsigned without_last_N_digits = x - last_N_digits;

    return without_last_N_digits + reverse_N_digits(last_N_digits, digits);
}


int main(void) {
    printf("847421 : %u\n", reverse_last_digits(847421, 3));
    printf("981 : %u\n", reverse_last_digits(981, 2));
    printf("530006 : %u\n", reverse_last_digits(530006, 4));
    printf("31 : %u\n", reverse_last_digits(31, 2));
    printf("7 : %u\n", reverse_last_digits(7, 3));

    return 0;
}




/*
run:

847421 : 847124
981 : 918
530006 : 536000
31 : 13
7 : 700

*/

 

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