Validating input using scanf's return value isn't working

Question

I'm working on a tic-tac-toe final project. I ask the user to print the box number he would like to fill in, and then i use integer err to obtain the return value of scanf. Scanf should return the number of integers it has read in this case, and I am asking for one integer to be read, so as long as err != 1, it should go into the while loop. This, however, isn't working.

        printf("\nBox number: ");
        int boxNumber, err;
        err = scanf("%d", &boxNumber);

        while (err != 1) {
            printf("\nWrong input.");
            printf("\nBox number: ");
            err = scanf("%d", &boxNumber);
        }

When a non-integer is inputted, the output is:

Wrong Input.
Box number:

In a non-stop loop. The line where scanf is then called again for the user to input a value is as though never run.
I have no idea what the problem is. Please help.

score 7 · Accepted Answer · answered May 10 '18 at 12:03

7

After the first scanf() fails, the non-integer remains in the input stream and prevents any following from succeeding, while staying forever.

You need to get rid of the non-scannable lingering input.

See these articles for choosing your favorite method:
http://sekrit.de/webdocs/c/beginners-guide-away-from-scanf.html
How to read / parse input in C? The FAQ

answered May 10 '18 at 12:03

Yunnosch

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The OP should also check for end of file, ie `scanf()` returning `EOF` to prevent an infinite loop. – chqrlie May 10 '18 at 12:05
1

I have checked the first site and it is great, thank you. – royabousamra May 10 '18 at 13:07

Validating input using scanf's return value isn't working

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