Code:

#include <iostream>
#include <string>
using namespace std;
double addition ( double x, double y );
double subtraction ( double x, double y );
double multiplication ( double x, double y );
double division ( double x, double y );
int main ( )
{
string thePassWord = "frozon";
string changePassWord = "fire";
string passWord;
char myChoice;
double x, y;
int count = 0;
do {
cout << "Enter password: \n>";
getline ( cin, passWord, '\n' );
count++;
if ( count == 3 )
{
cout << "Error: You enter the password wrong 3 time. \n";
cout << "Password is being changes. \n";
thePassWord.swap (changePassWord);
}
} while ( passWord != thePassWord );
if ( passWord == thePassWord )
{
while ( true )
{
cout << "Select your choice: \n";
cout << "Select \'a\' for addition: \n";
cout << "Select \'s\' for subtraction: \n";
cout << "Select \'m\' for multiplication: \n";
cout << "Select \'d\' for division: \n>";
cin >> myChoice;
switch ( myChoice )
{
case 'a':
case 'A':
cout << "Addition: \n";
cout << "First number: \n>";
cin >> x;
cout << "Second number: \n>";
cin >> y;
cout << endl;
cout << x << " + " << y << " = " << addition ( x, y ) << "\n\n";
cin.get();
break;
case 's':
case 'S':
cout << "Subraction: \n";
cout << "Frist number: \n>";
cin >> x;
cout << "Second number: \n>";
cin >> y;
cout << endl;
cout << x << " - " << y << " = " << subtraction ( x, y ) << "\n\n";
cin.get();
break;
case 'm':
case 'M':
cout << "multiplication: \n";
cout << "First number: \n>";
cin >> x;
cout << "Second number: \n";
cin >> y;
cout << endl;
cout << x << " * " << y << " = " << multiplication ( x, y ) << "\n\n";
cin.get();
break;
case 'd':
case 'D':
cout << "division: \n";
cout << "First number: \n>";
cin >> x;
cout << "Second number: \n>";
cin >> y;
cout << endl;
if ( y == 0 )
{
cout << "Error: \n\n";
continue;
}
cout << x << " / " << y << " = " << division ( x, y ) << "\n\n";
cin.get();
break;
default:
cout << endl;
cout << "Error: Invalid Choice. \n\n";
cin.get();
break;
}
}
}
cin.get();
return ( 0 );
}
double addition ( double x, double y )
{
return x + y;
}
double subtraction ( double x, double y )
{
return x - y;
}
double multiplication ( double x, double y )
{
return x * y;
}
double division ( double x, double y )
{
return x / y;
}

Just an idea........