Error checking

I am working on this currency program, so far it works however whenever a user is prompted to enter an amount and enters a bunch of letters the program goes crazy. For example a user enters "afbajkdbak" - program goes in a infinite loop and I have to close the program. When a user enters "11a" the program does the error checking and the program still works. What am I doing wrong, I tried changing my code around, but still it doesn't work quite right.

Code:

#include <iostream.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <iomanip.h>

void main(void)
{
   // Declaration of Variables
	const double Jap_Rate = 1.177;  // Japanese Rate
	const double Can_Rate = 1.021;  // Canadian Rate
   const double Bpd_Rate = 1.559;  // British Pounds Rate
	const double Aus_Rate = 0.652;  // Australian Rate
	const double Eur_Rate = 1.833;  // Euro Rate
	bool bCont = true;
 	char choice;              // Hold selection of which currency is chosen
	char curName[25];        // Hold name of currency
	double curRate;          // Hold foreign rates
	float usAmount;          // Hold US dollar amount
	double convertAmount;    // Hold converted amount

	do{
	  cout << "\tCurrency Conversion Program"<< endl;
	  cout << "\t---------------------------"<< endl << endl;
    // User will be prompted for input
	  cout << "What currency would like to converted?" << endl;
	  cout << "\t1 for Japanese Yen" << endl;
	  cout << "\t2 for Canadian" << endl;
	  cout << "\t3 for British Pounds" << endl;
	  cout << "\t4 for Australian Dollar" << endl;
	  cout << "\t5 for Euro" << endl;
     cout << "\t6 Exit" << endl;
     cin >> choice;

   // Switch for the currency type and rate
	  switch(choice){
		 case '1':
         strcpy(curName,"Yen");
			curRate = Jap_Rate;
			break;
		 case '2':
			strcpy(curName,"Canadian");
			curRate = Can_Rate;
			break;
		 case '3':
			 strcpy(curName,"British Pounds");
			 curRate = Bpd_Rate;
			 break;
		 case '4':
			 strcpy(curName,"Australian");
			 curRate = Aus_Rate;
			 break;
		 case '5':
			 strcpy(curName,"Euro");
			 curRate = Eur_Rate;
			 break;
       case '6':
          exit(1);
       default:
			 cout << "\tTry Again" << endl << endl;
          break;
       }

	  if(choice < '6'){
	     // Get amount to be converted
	     cout << "Enter amount to convert: ";
	     cin >> usAmount;
        // Error checking for anything not numerical
        if(usAmount >= 0 && usAmount <= 10000){
             // Perform conversion
		     convertAmount = usAmount * curRate;
           // Conversion output
           // The following line formats the output to show a dollar format
           cout << setiosflags(ios::showpoint | ios::fixed) << setprecision(2);
		     cout << "Rate as of today: " << usAmount << " of " << curName << " is ";
           cout << convertAmount << " US Dollar(s)." << endl << endl;
          }else{
            cout << "\tError: Invalid Choice." << endl << endl;
            bCont = false;  // The loop will end upon invalid input
            exit(1);
            }
          }else{
         cout << "\tError: Invalid Choice." << endl << endl;
         }
       } while(bCont);
} // End Main

Code:

float usAmount;   
...
...
cout << "Enter amount to convert: ";
cin >> usAmount;

If a user enters letters, then your program attempts to read letters into a float variable, and cin goes haywire in response. In order to prevent your cin stream from going into error mode, you can read everything in as a string. Then, you can use what's called a stringstream to try and obtain a number from the input. An input stringstream is like a string object but you can use the operator>> on it to try and read data into variables of any type. An input stringstream works like this:

Code:

#include<iostream>
#include<string>
#include<sstream>

using namespace std;


int main()
{
	string input = "12.3abc";
	istringstream inStr(input);
		
	float usAmount = 0;
		
	inStr>>usAmount;
	if(usAmount != 0)
		cout<<usAmount<<endl;
	else
		cout<<"please enter a number"<<endl;
	
	

	string input2 = "abc2.4";
	istringstream inStr2(input2);
	
	usAmount = 0;

	inStr2>>usAmount;
	if(usAmount != 0)
		cout<<usAmount<<endl;
	else
		cout<<"please enter a number"<<endl;

	return 0;
}

By adding this cin.clear( ), we can clear the instream buffer if the attempt to (cin >> useramount) fails.. we can get rid of the garbage before starting another iteration of the do/while loop:

Code:

 }else{
         cout << "\tError: Invalid Choice." << endl << endl;
		 
         }		 
	 cin.clear(); //clear the instream buffer of a previous failed attempt to (cin >> float)
       } while(bCont);
} // End Main

Code:

if(choice < '6'){
	     // Get amount to be converted
	     usAmount = 0; //IF the user enters bogus data, at least
                            //you won't be offering a currency exchange 
                            //display leftover from a previous loop iteration.
	     cout << "Enter amount to convert: ";
	     cin >> usAmount;
        // Error checking for anything not numerical
        if(usAmount >= 0 && usAmount <= 10000){

[edit] Using stringstream though is a more robust approach to handling user input [/edit]