Thread: How do I check data entered and generate an error

I'm writing a class assignment and we're supposed to check the validity of the data entered. Keeping it simple, I can write the number of francs, and do the loop if it's a negative number, but what if the user enters one or more alpha characters or just presses ENTER. How do I generate the error condition then? I've tried to figure out ISALPHA and ISDIGIT but it didn't work, or at least I'm not good enought to figure it out.

Code:

#include <stdio.h>
#include <system.h>

main()
{
/* 	
	Author: Ken Buckner
	Created May 11, 2004
	Version 2.0
*/
	/* Declare variables */
	int error;
	float conversion_rate, francs;

	/* Initialize variables */
	error = 0;
	conversion_rate = 5.47396;
	francs = 1.0;
	/* Conversion rates downloaded on April 30, 2004 */	
	
		/* Output prompt */
		printf("Currency Conversion.\n\n");
		printf("Please enter the amount of French Francs you'd like to convert.\n\n");
		
		/* Start DO WHILE loop */	
		do   
		{	/* Reads user input */
			scanf("%lf", &francs);
			/* Checks user input, if not valid, sets ERROR to 1 (ON) */
			if(error = (francs <= 0.0))
			{
				/* Clears the screen */
				clrscr();
				/* If Error prompt */
				printf("Please enter a valid amount.\n\n");
			}	
		}
		/* End DO WHILE loop, if not ERROR */
		while (error);
		
		/* Clears the screen */
		clrscr();
		/* Output information, with calculation */
		printf("%.3f French Francs = $%.2f U.S. Dollars.\n\n", francs, (francs / conversion_rate));
		printf("Conversion rates accurate as of April 30, 2004.\n");
		
	return(0);
}