How to loop but reject non-numerical inputs? - c++

Question

How do I make a function that will terminate the program if the user says e, and loop if the user presses l at any time?
How do I make the program reask the user for number input if the user inputs letters instead of numbers? Currently, the program terminates when I input blah, for instance. My obstacle is the bool die definition: I'm not sure how to use bool die to loop instead of die (my teacher required bool die usage.)
#include <iostream>
#include <string>
#include <stdlib.h>
using namespace std;
bool die(const string & msg);
int main() {
// declaring variables:
unsigned QUARTERS;
unsigned DIMES;
unsigned NICKELS;
unsigned PENNIES;
double total;
cout << "********************************************************" << endl;
cout << " Welcome to Crazy Coin Counter! " << endl;
cout << "********************************************************" << endl << endl;
// user input:
//QUARTERS
do {
cout << " # QUARTERS: ";
cin >> QUARTERS;
if (cin){
if (QUARTERS < 1000)
cout << " --> Input Successful!" << endl;
}
else die(" --> :( Input Unsuccessful!");
cout << " You must put in less than 1000 quarters! Please try again." << endl << endl << endl;
} while (QUARTERS >= 1000 );
//DIMES
do{
cout << endl << " # DIMES: ";
cin >> DIMES;
if (cin){
if (DIMES < 1000)
cout << " --> Input Successful!" << endl;
}
else die(" --> :( Input Unsuccessful!");
if (DIMES >= 1000)
cout << " You must put in less than 1000 dimes! Please try again." << endl << endl << endl;
} while (DIMES >= 1000);
//NICKELS
do {
cout << endl << " # NICKLES: ";
cin >> NICKELS;
if (cin){
if (NICKELS < 1000)
cout << " --> Input Successful!" << endl;
}
else die(" --> :( Input Unsuccessful!");
if (NICKELS >= 1000)
cout << " You must put in less than 1000 nickels! Please try again." << endl << endl << endl;
} while (NICKELS >= 1000);
//PENNIES
do {
cout << endl << " # PENNIES: ";
cin >> PENNIES;
if (cin){
if (PENNIES < 1000)
cout << " --> Input Successful!" << endl;
}
else die(" --> :( Input Unsuccessful!");
if (PENNIES >= 1000)
cout << " You must put in less than 1000 pennies! Please try again." << endl;
} while (PENNIES >= 1000);
// calculations:
total = (QUARTERS * 0.25) + (DIMES * 0.1) + (NICKELS * 0.05) + (PENNIES * 0.01);
// output:
cout << endl <<endl<< "Congrats! You have $" << total << " worth of coins! " << endl << endl << endl;
}
// function definition
bool die(const string & msg){
cout << " " << msg << endl;
exit(EXIT_FAILURE);
}

Answer 1

Try something like this:
while ( true ) // Loop forever
{
cout << "Enter 'e' to exit:";
std::string answer;
getline(cin, answer);
if (answer == "e")
{
break; // Break out of the loop
}
else
{
cout << "\nWrong answer.\n";
continue; // The continue would start at the top of the loop.
}
}
There are many other techniques that you can find by searching StackOverflow for "c++ terminate loop".
Edit 1: Checking numerical input
The simplest method for checking numerical input is to test the result of inputting the number:
unsigned int quarters; // Using unsigned because quantities can't be negative.
cout << "Enter number of quarters: ";
if (cin >> quarters) // Input and test in same statement.
{
cout << "Your total is " << (quarters * 0.25) << "\n";
}
else
{
// Handle incorrect input
cout << "Invalid input, try again.\n";
}