magic_num.cpp

#include <iostream>
using namespace std;

// big -> small
void BubbleSort(int a[]) {
    int i, j, temp;

    for (i = 0; i < 3; i++) {
        for (j = i + 1; j < 4 ; j++) {
            if (a[i] < a[j]) {
                temp = a[i];
                a[i] = a[j];
                a[j] = temp;
            }
        }
    }
}

int Change(int a[4], bool b) {
    if (b == 1) {
        return a[0] * 1000 + a[1] * 100 + a[2] * 10 + a[3];
    }
    else
        return  a[0] + a[1] * 10+ a [2]*100+a[3]*1000;
}

void GetEnchNum(int a[], int n) {
    int fac = 1000, i;

    a[0] = n / fac;

    n= n - a[0] * fac;

    for (i = 1; i < 4; i++) {
        fac = fac / 10;
        a[i] = n / fac;
        n = n - a[i] * fac;
    }
}

int main(void) {
    int n;
    int a[4];

    cout << "Enter a four-digit:";
    cin >> n;

    while (n >= 10000 || n < 999) {
        cout << "Please enter it again:";
        cin >> n;
    }

    while (n != 6174) {
        GetEnchNum (a, n);
        BubbleSort(a);
        n = Change(a, 1) - Change(a,0);
        cout << Change(a,1) << "-" << Change(a,0) << "=" << n <<endl;
    }

    return 0;
}