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 [mre] with debugging questions. — Eric Postpischil, Jun 15 '22 at 23:42
@EricPostpischil dear sir, added #included and the int main (void) just now. Thank you — Alpaca Ma, Jun 15 '22 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`](https://stackoverflow.com/questions/58403537). — Steve Summit, Jun 16 '22 at 00:11

Eric Postpischil · Answer 1 · 2022-06-15T23:48:21.303

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.

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! — Alpaca Ma, Jun 16 '22 at 00:00

score 1 · Answer 2 · answered Jun 16 '22 at 01:21

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.

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! — Alpaca Ma, Jun 16 '22 at 05:31

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

2 Answers2