From Astrachan, pp 143-144.
---------------------------
#include <iostream.h>
#include "CPstring.h"

// converts two digit numbers to English equivalent
// Owen Astrachan, 6/8/95

string DigitToString(int num)
// precondition: 0 <= num < 10
// postcondition: returns english equivalent, e.g., 1->one,...9->nine     
{
    if (0 == num)       return "zero";
    else if (1 == num)  return "one";
    else if (2 == num)  return "two";
    else if (3 == num)  return "three";
    else if (4 == num)  return "four";
    else if (5 == num)  return "five";
    else if (6 == num)  return "six";
    else if (7 == num)  return "seven";
    else if (8 == num)  return "eight";
    else if (9 == num)  return "nine";
    else return "?";
}

string TensPrefix(int num)
// precondition: 10 <= num <= 99 and num % 10 == 0
// postcondition: returns ten, twenty, thirty, forty, etc.
//                corresponding to num, e.g., 50->fifty     
{
    if (10 == num) return "ten";
    else if (20 == num) return "twenty";
    else if (30 == num) return "thirty";
    else if (40 == num) return "forty";
    else if (50 == num) return "fifty";
    else if (60 == num) return "sixty";
    else if (70 == num) return "seventy";
    else if (80 == num) return "eighty";
    else if (90 == num) return "ninety";
    else return "?";
}

string TeensToString(int num)
// precondition: 11 <= num <= 19
// postcondition: returns eleven, twelve, thirteen, fourteen, etc.
//                corresponding to num, e.g., 15 -> fifteen
{
    if (11 == num) return "eleven";
    else if (12 == num) return "twelve";
    else if (13 == num) return "thirteen";
    else if (14 == num) return "fourteen";
    else if (15 == num) return "fifteen";
    else if (16 == num) return "sixteen";
    else if (17 == num) return "seventeen";
    else if (18 == num) return "eighteen";
    else if (19 == num) return "nineteen";
    else return "?";
}

string NumToString(int num)
// precondition: 0 <= num <= 99
// postcondition: returns english equivalent, e.g., 1->one, 13->thirteen
{
    if (0 <= num && num < 10)
    {
        return DigitToString(num);
    }
    else if (10 < num && num < 20)
    {
        return TeensToString(num);
    }
    else if (num % 10 == 0)
    {
        return TensPrefix(num);
    }
    else
    {
	// concatenate ten's digit with one's digit
        return TensPrefix(10 * (num/10)) + "-" + DigitToString(num % 10);
    }
}

int main()
{
    int number;
    cout << "enter number between 0 and 99: ";
    cin >> number;
    cout << number  << " = " << NumToString(number) << endl;
    return 0;
}