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;
}

---

Have you thought of using a while loop to check for the proper condition?

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.

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.

Flush your input.

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

oh and why the use of the pre compiled header include statement? Why not use stdio.h?

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;
}

---

If the user does not input a valid number then there will be an issue. Thus flushing may overcome that problem.

Quote:

---

Originally Posted by RenFromPenn

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

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.

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.

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

Quote:

---

Originally Posted by vart

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.

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

Quote:

---

Originally Posted by RenFromPenn

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

---

you should break out of the loop after the user has done what you asked for

I don't even get a chance to enter more input. When i enter the incorrect information and press enter the text just starts scrolling across the screen. I have no opportunity to try entering the correct number a second time around.

Again you have to flush the buffer, especially when using Visual C++. This is somewhat liken to having to call and extra getchar() after any user input.

I'm not using Visual C++. I am writing in C. I am just using this complier to write the program in. Could you please just tell me how to flush it? I tried and it didn't work. I have googled it and I can't find the information.

Okay, I got it to work. I broke the while loop by adding flushall();

Thank you all for your help!

Quote:

---

Originally Posted by RenFromPenn

Okay, I got it to work. I broke the while loop by adding flushall();

Thank you all for your help!

---

That is the wrong solution - flushall is a function that isn't guaranteed to do anything useful or meaningfull for input channels. _DO_ look up the FAQ link that was posted a few posts back. It contains a loop that does getchar(), which is safe and correct.

--
Mats

I did look at the FAQs, but I didn't understand how to implement the flush. Would someone please just paste it into my code and help me with this?

Code:

---

  int  ch;

  while ((ch = getchar()) != '\n' && ch != EOF);

---

put int ch in with your variables, and the line starting with "while" where you want to clean up your input buffer. Calling this after any scanf should be fine (assuming you don't expect someone to enter a bunch of stuff on one line, but you read it using several scanf stataments, e.g a loop to read 10 integers, and someone expects to be able to enter the 10 numbers in a line).

You could also put all of the above code into a function, and then call that function when you feel a need to clean up the input.

The above function is ALSO very useful if you are mixing scanf() and fgets() or scanf with "%s" or "%c", since scanf leaves newlines in the input buffer, and when you then call fgets, it sees the newline from your the input to scanf, and takes the input to be blank.

--
Mats