Loop question.

[robski]

Hi all.

Just a quick question for anyone who may be able to ehelp.

If a user is to enter a choice and i use scanf to read it in.
When that choice is a character that is entered, if the user enters either a character that is not that which is allowed or they enter a number the loop will work fine and it will refresh and get the user to enter it again.
However, i have set up some do while loops when the user is to enter a number and i can set up a do while that will set a range that the number has to be in and if a number outside this is entered the loop works fine but if a character is entered it starts to flicker and freezes up.

Does anyone know how i can overcome this. It is not a huge issue as i have finished my assignments but it would be nice to sort out this last bug! I guess it is some form of discrimination? Any help would be very appreciated.

Thanks everyone :)

[Salem]

Yes, use fgets() to read the whole line, then sscanf() to parse it.

If the line is rubbish, then you've already cleaned up the input stream to have another go.

[Narue]

scanf fails if it can't convert the incoming stream into a valid number, and it doesn't extract the bad characters. So unless you extract them yourself you'll get an infinite loop as scanf continues to fail on the same stuff.

// Unsafe; risks an infinite loop
while (scanf("%d", &x) != 1) {
  printf("Invalid number. Please try again: ");
  fflush(stdout);
}

// Safe; clearing the stream each time
while (scanf("%d", &x) != 1) {
  int ch;

  printf("Invalid number. Please try again: ");
  fflush(stdout);

  // Clear the stream state
  clearerr(stdin);

  // "Flush" the stream
  while ((ch = getc(stdin)) != '\n' && ch != EOF)
    ;
}

[robski]

Hi thanks for the info. Just not too sure what "fflush" and "clearerr" are?

Would you mind just explaining what they are doing in that piece of code. These are a couple of commands i haven't used before.
Thanks :)

[Narue]

>Just not too sure what "fflush" and "clearerr" are?
That's what reference manuals are for.