Visual C++ :: Roman Numeral Converter To Decimal Using Array Of Strings

This program compiles, but has a bunch of logical errors. I know my problem is somewhere in the while loop that I have, but I can't figure out where. Here are some of the issues I am experiencing:

1. At the beginning of the program it asks you to enter a number, and when you do it does nothing while proceeding to the while loop where I have it asking the same question

Code:
"Please enter a number between 1 and 20 (Enter 0 to stop)
";
cin >> num;
cout << endl;

I want to be able to eliminate that first statement but if I only run this in the loop without the above statement the program will display nothing on the screen and proceeds to stop.

2. This code runs fine, except that if you make a mistake, it will prompt you to enter a valid number, however; it ignores your first response if the number you enter is valid and asks you to enter a valid number anyway. Once you enter it a second time, it will accept it and the program will continue on.

Code:
while(num != SENTINEL) {
cout << "Please enter a number between 1 and 20 (Enter 0 to stop) ";
cin >> num;
cout << endl;

Also if you type in 0 on your first response, it will prompt you that it is not a valid number and ask you to try again, instead of stopping the program like it is supposed to do. On your second response the program will accept your 0 and stop the program correctly.

//Write a program that displays the roman numeral equivalent of any decimal number between 1 and 20 that the user enters. The roman numerals should be stored in an array of strings and the decimal number that the user enters should be used to locate the array element holding the roman numeral equivalent. The program should have a loop that allows the user to continue entering numbers until an end sentinel of 0 is entered.

Input validation: Do not accept scores less than 0 or greater than 20

#include <iostream>
#include <string>
using namespace std;
int main() {
// Declare constants and variables
const int romanNum = 21; // Size of the elements in the array

[Code] ....