Thread: Checking input in a while loop...

[edit]
nonpuz:
It's ok, we all have those days - That's why they put an eraser on the back of pencils.

I try to avoid
* Posting a fact that I can't find a source for .
* Posting code that has not been compiled & tested.

So when I have those days, I can catch myself out before posting it.
[/edit]

Here is a sample code showing what the problem is.

After a failed attempt to read, it will show you what was left on stdin

Code:

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


int main(void)
{
    unsigned long darts;


    printf(">>");
    fflush(stdout);


    while((scanf("%lu", &darts)) != 1)
    {
        int c;


        while ((c = getchar()) != '\n' && c != EOF)
        {
            putchar(c);
            //continue;
        }


        printf("!\n>>");
        fflush(stdout);
    }


     return EXIT_SUCCESS;
}

Enjoy.

Originally Posted by nonpuz

Code:

#include <ctype.h>
...
   while((scanf("%lu", &darts)) != 1)
   {
      printf("\n\nThis is not an acceptable entry. please try again.\n");
      printf("%s", prompt);
      int c;
      while (!isdigit(c = getchar()) && c != EOF)
         ;
      if (isdigit(c)) ungetc(c, stdin);
   }

The problem with this approach is that it will correctly reject non-numeric input like "Hello world", but it will accept invalid input such as

Code:

Hello world123

In that case I would prefer to treat that input as an error and ask for new input. Using the approach from nonpuz will instead treat the above as a valid integer 123.