Telephone Digits The following program reads the letter codes A to Z and prints the corresponding telephone digit. This program uses a sentinel-controlled while loop. To stop the program, the user is prompted for the sentinel, which is #. This is also an example of a nested control structure, in which if. . .else, switch, and the while loop are nested.

//**********************************************************
// Program: Telephone Digits
// This is an example of a sentinel-controlled loop. This
// program converts uppercase letters to their corresponding
// telephone digits.
//**********************************************************

#include <iostream>
using namespace std;
int main()
{
char letter; //Line 1
cout << "Program to convert uppercase "
<< "letters to their corresponding "
<< "telephone digits." << endl; //Line 2
cout << "To stop the program enter #."
<< endl; //Line 3
cout << "Enter a letter: "; //Line 4
cin >> letter; //Line 5
cout << endl; //Line 6
while (letter != '#') //Line 7
{
cout << "The letter you entered is: "
<< letter << endl; //Line 8
cout << "The corresponding telephone "
<< "digit is: "; //Line 9
if (letter >= 'A' && letter <= 'Z') //Line 10
switch (letter) //Line 11
{
case 'A':
case 'B':
case 'C':
cout << 2 <<endl; //Line 12
break; //Line 13
case 'D':
case 'E':
case 'F':
cout << 3 << endl; //Line 14
break; //Line 15
case 'G':
case 'H':
case 'I':
cout << 4 << endl; //Line 16
break; //Line 17
case 'J':
case 'K':
case 'L':
cout << 5 << endl; //Line 18
break; //Line 19
case 'M':
case 'N':
case 'O':
cout << 6 << endl; //Line 20
break; //Line 21
case 'P':
case 'Q':
case 'R':
case 'S':
cout << 7 << endl; //Line 22
break; //Line 23
case 'T':
case 'U':
case 'V':
cout << 8 << endl; //Line 24
break; //Line 25
case 'W':
case 'X':
case 'Y':
case 'Z':
cout << 9 << endl; //Line 26
}
else //Line 27
cout << "Invalid input." << endl; //Line 28
cout << "\nEnter another uppercase "
<< "letter to find its "
<< "corresponding telephone digit."
<< endl; //Line 29
cout << "To stop the program enter #."
<< endl; //Line 30
cout << "Enter a letter: "; //Line 31
cin >> letter; //Line 32
cout << endl; //Line 33
}//end while
return 0;
}