Scanf skips every other while loop in C

Question

I'm trying to develop a simple text-based hangman game, and the main game loop starts with a prompt to enter a guess at each letter, then goes on to check if the letter is in the word and takes a life off if it isn't. However, when I run the game the prompt comes up twice each time, and the program doesn't wait for the user's input. It also takes off a life (one life if it was the right input, two if it wasn't), so whatever it's taking in isn't the same as the previous input. Here's my game loop, simplified a bit:

while (!finished)
{
    printf("Guess the word '%s'\n",covered);

    scanf("%c", &currentGuess);

    i=0;
    while (i<=wordLength)
    {
        if (i == wordLength)
        {
            --numLives;
            printf("Number of lives: %i\n", numLives);
            break;
        } else if (currentGuess == secretWord[i]) {
            covered[i] = secretWord[i];
            secretWord[i] = '*';
            break;
        }
        ++i;
    }

    j=0;
    while (j<=wordLength)
    {
        if (j == (wordLength)) {
            finished = 1;
            printf("Congratulations! You guessed the word!\n");
            break;
        } else {
            if (covered[j] == '-') {
                break;
            }
        }
        ++j;

        if (numLives == 0) {
            finished = 1;
        }

    }
}

I assume the problem is scanf thinking it's taken something in when it hasn't, but I have no idea why. Does anyone have any idea? I'm using gcc 4.0.1 on Mac OS X 10.5.

Answer 1

When you read keyboard input with scanf(), the input is read after enter is pressed but the newline generated by the enter key is not consumed by the call to scanf(). That means the next time you read from standard input there will be a newline waiting for you (which will make the next scanf() call return instantly with no data).

To avoid this, you can modify your code to something like:

scanf("%c%*c", &currentGuess);

The %*c matches a single character, but the asterisk indicates that the character will not be stored anywhere. This has the effect of consuming the newline character generated by the enter key so that the next time you call scanf() you are starting with an empty input buffer.

Caveat: If the user presses two keys and then presses enter, scanf() will return the first keystroke, eat the second, and leave the newline for the next input call. Quirks like this are one reason why scanf() and friends are avoided by many programmers.