Why is my modulo operation giving me bogus values?

Question

When I run this code, I get outputs that don't really make any sense. I'm most likely just missing something, but I've been working on trying to find the problem with my code by working out the problems by hand, but I'm getting the values I should be getting when I do it by hand. A simplified version of this calculation is (c%10^n - c%10^(n-1)) / 10^(n-1).

The goal of this calculation is to assign the digits of a number to an array of ints. I'm not really looking for alternate solutions.

int cNumberV[nLength];
for(int n = nLength; n > 0; n--) {
    cNumberV[nLength - n] = (cNumber % (long long) pow(10, n) - cNumber % (long long) pow(10, n - 1)) / (long long) pow(10, n - 1);
    printf("%i\n", cNumberV[n]);
}

This is my output when cNumber = 5105105105105100 and nLength = 16:

-1981492631
232830
-1530494976
1188624
-397102900
134514540
-1081801416
1188624
0
1
5
0
1
5
0
1

Answer 1

The problem is that your loop sets cNumberV[nLength - n], but then prints out cNumberV[n].

So the first half of the loop prints uninitialized array entries, and the second half of the loop prints the result of the first half's calculation in reverse order (but due to an off-by-one error as pointed out by rowan.G, it never prints the first digit).

Answer 2

pow() is an expensive and inaccurate floating function. You only need simple integer divide by ten to get digits. If you really want to get them left-to-right as you do above, make a lookup table with the 19 powers of 10 as integers.

#include <stdio.h>

#define nLength 20

long long cNumber = 5105105105105100;
int cNumberV[nLength];
int negative = 0;

int main(int argc, char *argv[]) {
    if (cNumber < 0) {
        negative = 1;
        cNumber = -cNumber;
    }

    int n;
    for (n = nLength - 1; n >= 0; n -= 1) {
        cNumberV[n] = cNumber % 10;
        cNumber /= 10;

        if (0 == cNumber) break;
    }

    if (negative) printf("-");
    for (int i = n; i < nLength; i += 1) {
        printf("%1d", cNumberV[i]);
    }
    printf("\n");
}