7.3

#include <iostream>
#include <cmath>

void f(int x, int& primality);

int main(void) {
    using namespace std;
    int x, primality = 0;

    cout << "Введите число: ";
    cin >> x;
    f(x, primality);

    if(primality == 1)
        cout << "Число простое" << endl;
    else
        cout << "Число составное" << endl;

    return 0;
}

void f(int x, int& primality) {
    primality = 1;
    for(int i = 2; (i <= sqrt(x)) && (primality == 1); ++i) {
        if(x % i == 0)
            primality = 0;
    }
}