Error checking user input

I am coding this currency conversion app (into to prog class) and need to error check the user inputs. Currently if a user inputs an ASCII alpha character it throws the app into an infinante loop.


My logic is to simply check the input to see if its an integer:

{
(currency !== int) // ????????
}

However the HOW to do this escapes me. I simply do not want the user to enter in an Alpha character, but I dont want to explicitly limit the range either.

Any suggestions?

Below is my code for your review.

Thanks in advance.

Code:

---

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

using namespace std;

    //................................
    // defines constants
    //................................
 
#define EUR 1.07892
#define BRI 1.57719
#define JAP 0.00835838
#define AUS 0.601142
#define IRA 3.21647

int main ()

{

    //................................
    //head
    //................................
   
    cout<<"Currency Conversion Calculator\n\n";

    //.................................
    //Declarations of variables
    //.................................
   
    int input = 0;
    double currency = 0;
    double EURC = 0;
    double BRIC = 0;
    double JAPC = 0;
    double AUSC = 0;
    double IRAC = 0;
 
   
    //.................................   
    //request user input
    //................................
   
    while (input == 0 || !(input>= 1 && input <= 5))
       
    {

    cout<<"Select from the following 5 options:\n\n";

    cout<<"To convert the Euro to US $, please press 1\n\n";

    cout<<"To convert the British Pound to US $, please press 2\n\n";

    cout<<"To convert the Yen to US $, please press 3\n\n";

    cout<<"To convert the Australian $ to US $, please press 4\n\n";

    cout<<"To convert the Iraqi Dinar to US $, please press 5\n\n";
   
    cout<<"....:";
   
    cin>> input;
   
    cout<<"\n";
   
    }

 
    //.................................   
    // request user's amount to be converted
    //................................
    {
       
    cout<<"Input the Currency Ammount to be Converted: ";
    cin>> currency;
   
   
    //................................
    // Conversion table
    //................................
   
    EURC = currency * EUR;
    BRIC = currency * BRI;
    JAPC = currency * JAP;
    AUSC = currency * AUS;
    IRAC = currency * IRA;
     
    } 
   
/*  switch(currency)

    {
   
    case1: 
    { 
   
    (currency !=int)
   
    }
   
    default:
    {
        printf("Invalid input, please enter a valid currency. \n\n");
        break;
    }
     
     

    }
*/
    //................................
    // Calculate conversions and print
    //................................
   
    switch(input)
    {
   
    case 1:
    {
          printf ("%0.2lf Euros is equal to %0.2lf US dollars\n\n", currency, EURC);
          break;
    }
    case 2: 
   
    {
          printf ("%0.2lf British pounds is equal to %0.2lf US dollars\n\n", currency, BRIC);
          break;
    }
   
    case 3: 
   
    {
          printf ("%0.2lf Yen is equal to %0.2lf US dollars\n\n", currency, JAPC);
          break;
    }
   
    case 4: 
    {
          printf ("%0.2lf Australian dollars is equal to %0.2lf US dollars\n\n", currency, AUSC);
          break;
         
    }   
   
    case 5:
    {    printf ("%0.2lf Iraqi Dinar is equal to %0.2lf US dollars\n\n", currency, IRAC);
          break;
         
    }   
   
   
    default:
    {
        printf("Invalid input, please enter a valid currency. \n\n");
        break;
    }
     
    }
    //................................
    // stablize shell
    //................................
   
  system("PAUSE");       
 
    //................................
    // statisfy interger expectation of int main
    //................................
   
  return 0;
}

---

There are several solutions. One solution is to read the input one line at a time. Another solution is to read each character at a time. In both cases, use isdigit() to validate an integer.

Kuphryn

isdigit() is in ctype.h

This:

cin>> currency;


is *already* checking to see if it's a number. If it's not a valid double, cin will enter a fail state, so cin will evaluate to false. When the error does happen, you're going to need to find the error and reset the stream completely.

Here's a little example for you:

Code:

---

#include <iostream>


int main(){
        double d;

        std::cout << "Enter a number > ";
        std::cin >> d;

        while (!std::cin){
                std::cout << "Not a number! Try again. > ";
                std::cin.clear(); // reset the stream to a good state
                std::cin.ignore(0x7FFFL,'\n'); // Ignore the whole buffer
                std::cin >> d;
        }

        std::cout << "You entered " << d << std::endl;
}

---

Here's one sample output:

Code:

---


Enter a number > a
Not a number! Try again. > 37.gh
You entered 37

---

Note that there is still information in the stream (namely, gh) so you would have to detect and purge it. Note also that "37.gh" is legal to scan for a double because 37. is a legal number. You'll need more sophistication than this to deal with that input.

Also, cin>>d cannot HELP but leave you with a number in d. Doubles cannot hold anything besides numbers, ever.

One possibility is to use the string class (#include <string>) and then use std::getline(std::cin, myString); to read an entire line into a string; then you can process the string to get the character.

Any special reason why you use 0x7fff (32767) as max number of char's to be discarded ?

Code:

---

std::cin.ignore(0x7FFFL,'\n'); // Ignore the whole buffer

---

It was big enough to fit the bill (who is going to type that much text, anyway) and it's not possible to be interpreted by the compiler as a negative number.