Why modulus operator does not work on that code?

Question

#include <iostream>
#include <string>

using namespace std;

int main() {
    int n = 12;
    int i, res;
    string digits = to_string(n);


    for(i = 0; i < digits.length(); i++){
        res = 0;
        res = n % digits[i];

    if (res != 0){
        res = false;
        break;
    }

    if (res == 0){
        res = true;
    }
}

The digits[0] is 1, therefore: 12 % 1 = 0 = res. However, when I run the program, res = 12, so the following if conditions do not work correctly. What's wrong?

Thanks

The if conditions are not in the loop, so they're just using res from the last digit. — Barmar
– Barmar, Commented Feb 22, 2020 at 18:17
Because when you use a char as a number, you get the character code. — Barmar
– Barmar, Commented Feb 22, 2020 at 18:21

JohnFilleau · Accepted Answer · 2020-02-22 18:22:12Z

1

digits[0]

Contains the character '1', which has a numeric value of 49. You are evaluating 12 % 49 which is indeed 12.

If you want to convert a numeric character to its value, you can do it like (digits[i] - '0').

Change

res = n % digits[i];

to

res = n % (digits[i] - '0');

But only if you're certain that digits will only contain characters that are digits.

The ASCII character codes can be found by googling for them. This page seems fine.

https://www.asciitable.xyz/

edited Feb 22, 2020 at 18:22

answered Feb 22, 2020 at 18:21

JohnFilleau

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4 Comments

Barmar Over a year ago

Since she created digits by converting a number to a string, the last condition is guaranteed.

JohnFilleau Over a year ago

@Barmar agreed but thought it was worth including for the next iteration of the program where they take user input.

Barmar Over a year ago

Hopefully that input would be done to the int variable, not directly to the string.

JohnFilleau Over a year ago

@Barmar "hopefully"

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