用C++编写程序代码，求大神解答

#include <iostream>
using namespace std;

int fun(int num)
{
    int mod, sum = 0, x = 10;

    while (num)
    {
        mod = num % x;
        sum += mod;
        num /= x;
    }
    return sum;
}

int main()
{
    int num;

    while (true)
    {
        cout << "输入一个4位数：\n";
        cin >> num;
        int test = num / 1000;
        if (test >= 1 && test < 10)
            break;
        cout << "输入错误，请重新输入：\n";
    }
    int sum = fun(num);
    cout << sum;

    return 0;
}

#include <iostream>
using namespace std;
int fun(int num)
{
    int mod, sum = 0, x = 10;
    while (num)
    {
        mod = num % x;
        sum += mod;
        num /= x;
    }
    return sum;
}
int main()
{
    int num;
    while (true)
    {
        cout << "输入一个4位数：\n";
        cin >> num;
        int test = num / 1000;
        if (test >= 1 && test < 10)
            break;
        cout << "输入错误，请重新输入：\n";
    }
    int sum = fun(num);
    cout << sum;
    return 0;
}