nAiN nAiN - 2 months ago 14
C Question

scanf not working on invalid input

On a character input in the first

scanf()
, the second one doesn't run.
getchar()
isn't working either. It seems that
your_choice
is supposed to take the character and check it afterward but the character is actually being taken by
ch
. What is causing it to work like this and how to resolve the issue. I'm tried re-initializing the variables but doesn't work.

#include <stdio.h>

void choice(int);

int main() {
char ch;
int random, your_choice;

do {
srand(time(NULL));
system("cls");
printf("** 0 is for Rock **\n");
printf("** 1 is for Scissors **\n");
printf("** 2 is for Lizard **\n");
printf("** 3 is for Paper **\n");
printf("** 4 is for Spock **\n");

printf("\nEnter your choice here:");
scanf("%d", &your_choice);

random = rand() % 5; //random number between 0 & 4
if ((your_choice >= 0) && (your_choice <= 4)) {
//choice printer omitted for this post

if ((random == ((your_choice + 1) % 5)) || (random == ((your_choice + 2) % 5)))
printf("\n\n... and you win!!!\n");
else if ((random == ((your_choice + 3) % 5)) || (random == ((your_choice + 4) % 5)))
printf("\n\n... and you lose!!!\n");
else if (random == your_choice)
printf("\n\nUnfortunately, it's a tie!\n");
} else
printf("\nWell, this is wrong! Try again with a number from 0 to 4!!\n");

printf("\nWould you like to play again? (Y/N)?: ");
scanf(" %c", &ch);

} while (ch == 'y' || ch == 'Y');

return 0;
}

Answer

If the user enters characters that cannot be converted to a number, scanf("%d", &your_choice); returns 0 and your_choice is left unmodified, so it is uninitialized. The behavior is undefined.

You should test for this and skip the offending input this way:

    if (scanf("%d", &your_choice) != 1) {
        int c;
        /* read and ignore the rest of the line */
        while ((c = getchar()) != EOF && c != '\n')
            continue;
        if (c == EOF) {
            /* premature end of file */
            return 1;
        }
        your_choice = -1;
    }

Explanation:

  • scanf() returns the number of successful conversions. If the user types a number, it is converted and stored into your_choice and scanf() returns 1, if the user enters something that is not a number, such as AA, scanf() leaves the offending input in the standard input buffer and returns 0, finally if the end of file is reached (the user types ^Z enter in windows or ^D in unix), scanf() returns EOF.

  • if the input was not converted to a number, we enter the body of the if statement: input is consumed one byte at a time with getchar(), until either the end of file or a linefeed is read.

  • if getchar() returned EOF, we have read the entire input stream, no need to prompt the user for more input, you might want to output an error message before returning an error code.

  • otherwise, set your_choice to -1, an invalid value so the read of the code complains and prompts for further input.

Reading and discarding the offending input is necessary: if you do not do that, the next input statement scanf(" %c", &ch); would read the first character of the offending input instead of waiting for user input in response to the Would you like to play again? (Y/N)?: prompt. This is the explanation for the behavior you observe.