Unable to enter additional input after validation

This is a discussion on Unable to enter additional input after validation within the C Programming forums, part of the General Programming Boards category; Hi, I am trying to write a program that asks the user to enter a number between 0 and 9. ...

  1. #1
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    Unable to enter additional input after validation

    Hi,

    I am trying to write a program that asks the user to enter a number between 0 and 9. I also have to validate the input. That part works just fine. The problem comes in when they enter something other than 0-9. The error message displays and the prompt appears again which asks for them to enter a number between 0-9. So, the validation works like a charm. For whatever reason, however, when I tested the program I couldn't type anything at this point to input something else. Any ideas what is causing that?

    Here is the code:

    Code:
    #include <stdafx.h>
    
    int main ()
    {
    
    	 int user_input;
    
    printf("Please enter a number between 0 and 9: ");
    scanf("%d", &user_input);
    
    if (user_input < 0 || user_input > 9)
    
    {
            printf("Error: That is not a valid entry!\n");
    		printf("Please enter a number between 0 and 9: ");
    		scanf("%d", &user_input);
    }
    
    else printf("Thank you! You selected %d\n", user_input);
      
      return 0;
    }

  2. #2
    Registered User slingerland3g's Avatar
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    Have you thought of using a while loop to check for the proper condition?

  3. #3
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    Yeah, I tried a while loop, but when I entered incorrect information to test validation the text "Error: That is not a valid entry! Please enter a number between 0 and 9:" just kept scrolling across my screen.

  4. #4
    and the hat of wrongness Salem's Avatar
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    scanf doesn't clean the input if you make a mistake.

    Which is why you should use fgets() to read a line, then sscanf to parse it (perhaps).
    If you reject the line at the parsing stage, you just go round and read another line.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
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  5. #5
    Banned master5001's Avatar
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    Flush your input.

  6. #6
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    How do I do that? I'm completely new to this, so please forgive uninformed questions.

  7. #7
    Registered User slingerland3g's Avatar
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    oh and why the use of the pre compiled header include statement? Why not use stdio.h?

  8. #8
    Registered User slingerland3g's Avatar
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    For what ever reason I can not duplicate your scenario. What compiler are you using. With the simple change from your if to the a while statement and removing the else statement should do the trick for you:

    Code:
    #include <stdio.h>           /* !note this change!*/
    
    int main ()
    {
    
             int user_input;
    
    printf("Please enter a number between 0 and 9: ");
    scanf("%d", &user_input);
    
    while (user_input < 0 || user_input > 9)
    
    {
            printf("Error: That is not a valid entry!\n");
                    printf("Please enter a number between 0 and 9: ");
                    scanf("%d", &user_input);
    }
    
    printf("Thank you! You selected %d\n", user_input);
    
      return 0;
    }

  9. #9
    Registered User slingerland3g's Avatar
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    If the user does not input a valid number then there will be an issue. Thus flushing may overcome that problem.

  10. #10
    CSharpener vart's Avatar
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    Quote Originally Posted by RenFromPenn View Post
    How do I do that? I'm completely new to this, so please forgive uninformed questions.
    There is FAQ on this site

    FAQ > How do I... (Level 2) > Flush the input buffer
    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

  11. #11
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    I am using Microsoft Visual C++ to write my C program. When I try to use stdio.h it generates an error message and won't let me proceed.

    I tried entering the code just as you provided it and the same error appeared,: fatal error C1010: unexpected end of file while looking for precompiled header. Did you forget to add '#include "stdafx.h"' to your source:

    When I replaced it with stdafx.h I experienced the same loop that I mentioned previously.

  12. #12
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    Okay, I read the FAQs page and I changed the program to this:

    Code:
    #include <stdafx.h>
    
    int main ()
    {
    
    	 int user_input;
    
    	 puts;
    
    printf("Please enter a number between 0 and 9: ");
    scanf("%d", &user_input);
    
    if (user_input < 0 || user_input > 9)
    
    {
            printf("Error: That is not a valid entry!\n");
    		printf("Please enter a number between 0 and 9: ");
    		scanf("%d", &user_input);
    }
    
    else printf("Thank you! You selected %d\n", user_input);
      
      return 0;
    }
    That still didn't allow me to enter another number if I initially entered incorect input.

  13. #13
    CSharpener vart's Avatar
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    1. you should use loop
    2. you should check the return value of scanf
    3. you should clear the input stream

    if you want to discard the stdafx.h - disable the precompiled headers in the project settings
    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

  14. #14
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    Quote Originally Posted by vart View Post
    1. you should use loop
    2. you should check the return value of scanf
    3. you should clear the input stream

    if you want to discard the stdafx.h - disable the precompiled headers in the project settings
    Well, okay, maybe I should do all of that, but I don't know how. The loop doesn't wrok as it results in an infinate loop. I thought my program did check the value of scanf. I tried clearing the input stream, but puts didn't do anything but create an error.

    **Update**

    I disabled the precompiled headers as suggested. I'm going to run the while loop now to see if it works.
    Last edited by deleted_user; 11-20-2008 at 03:31 PM.

  15. #15
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    I just tried it and the text just keeps printing and printing.

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