Thread: Error Checking

FAQ > How do I... (Level 1) > How do I get a number from the user (C)

Consider using scanf() to scan in a float, assign the float to an integer and compare the two. If they subtract to 0 (or divide to 1), then the user typed an integer, if they don't, then it had a decimal in it.

Code:

		while (!scanf("%d", &menuChoice));
			{	
		        printf ("Enter a choice 1, 2, 3, 4,  5, or 6.\n");
		        scanf ("%d", &menuChoice);
			}

You have an extra semicolon there. This isn't a do-while loop.

This is the program you're looking for from the FAQ (http://faq.cprogramming.com/cgi-bin/...043284385#opt2):

Code:

#include <stdio.h> 

int main(void)
{
  int temp;
  
  printf ("Input your number: ");

  while (scanf("%d", &temp) != 1)
  {
    while (getchar() != '\n');
    printf ("Try again: ");
  }

  printf ("You entered %d\n", temp);
  
  return(0);
}

You might also want to add a clearerr into the loop:

Code:

#include <stdio.h> 

int main(void)
{
  int temp;
  
  printf ("Input your number: ");

  while (scanf("%d", &temp) != 1)
  {
    clearerr(stdin);
    while (getchar() != '\n');
    printf ("Try again: ");
  }

  printf ("You entered %d\n", temp);
  
  return(0);
}

BTW, you can greatly reduce the calls to printf() by taking advantage of the fact that the preprocessor concatenates strings together:

Code:

		printf ("Please enter a number from the menu.\n");
		printf ("1. Inches to centimeters\n");
		printf ("2. Feet to meters\n");
		printf ("3. Miles to kilometers\n");
		printf ("4. Pounds to kilograms\n");
		printf ("5. Gallons to liters\n");
		printf ("6. Quit the program\n");
		printf ("What Would You Like To Do?:");

->

Code:

		printf ("Please enter a number from the menu.\n"
		    "1. Inches to centimeters\n"
		    "2. Feet to meters\n"
		    "3. Miles to kilometers\n"
		    "4. Pounds to kilograms\n"
		    "5. Gallons to liters\n"
		    "6. Quit the program\n"
		    "What Would You Like To Do?:");

GCC complains if you make a string longer than 509 chars, though.

try this one, it has the error checking i think you want (but it requires arrays and checking the array).

Code:

#include <stdio.h>
#include <ctype.h>

int main(void) {
    char ec[100];
    int choice, i, error;
    
    while(1) {
        error = 0;
        
        printf ("Please enter a number from the menu.\n"
        "1. Inches to centimeters\n"
        "2. Feet to meters\n"
        "3. Miles to kilometers\n"
        "4. Pounds to kilograms\n"
        "5. Gallons to liters\n"
        "6. Quit the program\n"
        "What Would You Like To Do?:");
        
        scanf("%s", ec);
        getchar();
        
        // check for error in input
        for(i=0;ec[i] !='\0';++i) {
            if(!isdigit(ec[i])) {
                error = 1;
                break;
            }
        }
        
        choice = atoi(ec);
        
        if((choice > 0 && choice < 7) && !error) {
            puts("input is correct");
            break;
        }
        else puts("Error in input");

    }
    
    printf("You wanted choice %d\n", choice);
    while(getchar()!=10);
    return 0;
}

Code:

    int menuChoice;
      
    do
	{
		printf ("Please enter a number from the menu.\n");
		printf ("1. Inches to centimeters\n");
		printf ("2. Feet to meters\n");
		printf ("3. Miles to kilometers\n");
		printf ("4. Pounds to kilograms\n");
		printf ("5. Gallons to liters\n");
		printf ("6. Quit the program\n");
		printf ("What Would You Like To Do?:");
	
		scanf ("%d", &menuChoice);
        if (menuChoice < 1 || menuChoice > 6)
        {
            printf("Please enter one of the given choices.\n");
        }
	} while ((menuChoice < 1) || (menuChoice > 6));

this works...

Try entering a letter and you'll see why it doesn't.

The best way to do it is to use the FAQ's program, as already mentioned:

Code:

#include <stdio.h> 

int main(void)
{
  int temp;
  
  printf ("Input your number: ");

  while (scanf("%d", &temp) != 1)
  {
    while (getchar() != '\n');
    printf ("Try again: ");
  }

  printf ("You entered %d\n", temp);
  
  return(0);
}