scanf() is getting skipped

Question

Hi Just started learning c for uni (usually use objective c) and have run into a strange issue with scanf, i have the following code

   while(stringCheck == 0){
    scanf("%c",&computerType);
    computerType = toupper(computerType);

    if ( computerType == 'L') {
        /*set stringCheck to 1 so the scanf while loop breaks*/
        stringCheck = 1; 
        counter = 0;
    } else {
        printf("ERROR\n");
    }
}

This i printing out "ERROR" then asking for input (so it is skipping the scanf statment the first time). If i change the it to another variable that is a string it works fine, it stops on the first time.

The rest of the code works fine, its just the fact that it print an error as soon as it enters the loop that is annoying.

I have tried getChar() and it does the same thing.

Thank you for any help you have to offer.

It isn't skipping for me. However, it is printing "ERROR\n" once for each character I enter if it is not 'L'. — Tyler Crompton, Mar 24 '11 at 01:10
Can you give us the entire source code? I think there is another input function being called before the code shown here which is leaving characters in the buffer. — Agnel Kurian, Mar 24 '11 at 01:13

score 3 · Accepted Answer · edited May 23 '17 at 12:08

If it's printing an error the first time you enter the loop, then there's already something in the input buffer. I'll guarantee (assuming your compiler is not brain-dead) that it is not skipping the scanf. You should check what it's actually receiving by changing:

printf("ERROR\n");

to:

printf("ERROR, code = %02x\n", computerType);

I would suggest that it's the newline from the last time your program ran that code (you say it's the first time but it's unclear as to whether you mean the first time into that loop ever (since program started) or first time entering that loop but you've been through it before on this program run.

When you enter LENTER the first time, your code will pull out the L but not the ENTER. If you then call that code again, it will get the ENTER key.

You should either understand and allow for what's actually entered or use a safe and sound input function like this one.

Thanks, it was the new line, fixed it by: `computerType = getchar(); scanf("%c",&computerType);` works fine now — Darc, Mar 24 '11 at 03:03

Jonathan Leffler · Answer 2 · 2011-03-24T01:36:49.673

You should always check the return value from scanf(); it tells you how many of the conversions succeeded. In this context, if you don't get back 1, you have a problem. The first time around the loop, scanf() reads a character - but not an ell (l or L) because you say you get an error message. The next iteration attempts to read the newline or whatever else follows the previous erroneous character, and the newline is certainly not an ell, and the other characters quite likely aren't an ell either, thus producing at least one more error message. You would get an error printed for each non-ell character.

Generally, if you use scanf(), it is fairly hard to recover from errors. You're likely to be better off reading a line into a buffer (character array) and use sscanf() to parse it.

score 1 · Answer 3 · edited Nov 07 '12 at 18:34

1

You should just need to add a space before %c. I am unsure why it works, but it does. This also happens with other data types.

Replace your scanf statement with scanf(" %c",&computerType);

edited Nov 07 '12 at 18:34

Troy Alford

26,660
10
64
82

answered Nov 07 '12 at 18:02

kishan

11
1

Adding the space character causes `scanf` to skip white space (i.e., spaces, tabs, newlines) before reading more input. Without the space character in the format, %c reads the next character from the input, white space or not. – prprcupofcoffee Nov 07 '12 at 18:28

scanf() is getting skipped

3 Answers3

Linked