Question

here is the other question from my c++ class. Please help me solve it. Thank you so much !!

9. A personal identification number may be set to be any 4 digits.
(a) How many possible PINs are there?
(b) How many possible PINs are there that do not have a repeated digit?
(c) How many possible PINs are there that do have a repeated digit?

Answer 1

#include <iostream>
#include <string>
using namespace std;

bool has_repeated_digits(string s) {
    return s[0] == s[1]  || s[0] == s[2] || s[0] == s[3] || s[1] == s[2] || s[1] == s[3] || s[2] == s[3];
}

int main() {
    int c1 = 0, c2 = 0, c3 = 0;
    for (int i = '0'; i <= '9'; ++i) {
        for (int j = '0'; j <= '9'; ++j) {
            for (int k = '0'; k <= '9'; ++k) {
                for (int l = '0'; l <= '9'; ++l) {
                    string s;
                    s += i;
                    s += j;
                    s += k;
                    s += l;
                    c1++;
                    if (has_repeated_digits(s)) {
                        c2++;
                    } else {
                        c3++;
                    }
                }
            }
        }
    }
    cout << "Total number of possible PINs = " << c1 << endl;
    cout << "Total number of possible PINs that do not have a repeated digit = " << c3 << endl;
    cout << "Total number of possible PINs that do have a repeated digit = " << c2 << endl;
    return 0;
}