C - Infinite loop after read a large decimal input from integer var

Question

I am new to C. I am writing this do while loop but when I accidentally enter a decimal/double to the input, it will enter a infinite loop. Would you please correct me? Thank you!

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>

int main(void)
{
    int items;
   do {
        printf("Input item quantity: ");
        scanf("%d", &items);
        if (items < 1 || items > 10)
            printf("ERROR: It must between 1 - 10.\n");
    } while (items < 1 || items > 10);
    return 0;
    
}

If I try to input 300.5, the output will become a loop

```
Input item quantity: ERROR: It must between 1 - 10.
Input item quantity: ERROR: It must between 1 - 10.
Input item quantity: ERROR: It must between 1 - 10.
```

In the future, always include a minimal reproducible example with debugging questions. — Eric Postpischil
– Eric Postpischil, Commented Jun 15, 2022 at 23:42
@EricPostpischil dear sir, added #included <studio.h> and the int main (void) just now. Thank you — Alpaca Ma
– Alpaca Ma, Commented Jun 15, 2022 at 23:58
The simple answer is: don't bother. scanf is not designed for robust user input with error recovery. There are ways to flush the offending input if the user makes a mistake, but if this is just a toy program for learning, you can cheat and tell the user (you) to not make mistakes. If you want to do robust input, it's far easier to use something other than scanf. — Steve Summit
– Steve Summit, Commented Jun 16, 2022 at 0:11

Eric Postpischil · Accepted Answer · 2022-06-15 23:48:21Z

3

When scanf sees “.”, it stops scanning and leaves the “.” in the input stream. Your program receives 300 in items, for which it prints the message and loops. The next iteration of the loop calls scanf again. The “.” is still in the input stream, so scanf stops without changing items and returns zero to indicate it assigned zero items. Your program does not check the return result of scanf, so it fails to see this. It only checks the value of items, which has not changed.

Change the program to check the return value of scanf and to handle issues if the return value is not one, meaning that one item was assigned. To continue processing, you may need to use getchar or another input routine to read characters from the input until a new-line character is seen, marking the end of the line that the user typed.

Also, even if one item is assigned, you may wish to read the rest of the input line to see if the user entered any other notable characters, such as anything other than spaces. If they did, you may wish to report an error even if the assigned value is within range.

edited Jun 15, 2022 at 23:48

answered Jun 15, 2022 at 23:42

Eric Postpischil

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1 Comment

Alpaca Ma Over a year ago

What do I have to do to escape the loop in the repeating "item" in this case which it ignores scanf forever? Do I need any break statement? Sorry I am really new to programming. Thanks a lot!

NoDakker · Accepted Answer · 2022-06-16 01:21:37Z

1

To answer your comment about escaping the loop you might check out the following link:

Clear scanf Buffer

Using that advice, I added a catch-all integer variable to your code.

int items, catchall; /* Added variable "catchall" */

Then, I enclosed your error message and added the code similar to the link to your program.

    if (items < 1 || items > 10)
    {
        printf("ERROR: It must between 1 - 10.\n");
        while ((catchall = getchar()) != '\n' && catchall != EOF) /* Clean up the buffer */
        {}
    }

That effectively clears out the "scanf" buffer so that your program can get out of the endless loop, and I did not see any undesired knock-on effects.

Hope that helps.

Regards.

answered Jun 16, 2022 at 1:21

NoDakker

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1 Comment

Alpaca Ma Over a year ago

Thanks you very much! It works like a charm!! Is the getchar() function call included in stdio.h? I don't know if I am allowed to put this in my code sicne it is just a beginner course, but it surely will help me in the future coding!

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