Code

#include <iostream>
using namespace std;

int biggestDigit(int x);

int main() {
  int x = 0;
  while (x < 1000) {
    cout << "Please enter an integer greater than 1000: ";
    cin >> x;
  }
  cout << "The largest digit in " << x << " is: " << biggestDigit(x) << endl;
  return 0;
}

int biggestDigit(int x) {
  int max = 0;
  int current;
  while (x != 0) {
    current = x % 10;
    if (current > max) {
  max = current;
    }
    x /= 10;
  }
  return max;
}