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:

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#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

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Code:

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

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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:

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#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;
}

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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:

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}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

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Code:

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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){

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