Unexpected input when using getchar()

Question

I am creating a program that returns true if a character I input is an upper case letter and the loop stops when I input the char '0'. The code is as follows:

#include <stdio.h>

int main (void)
{
    char c;
    do 
    {
        printf ("Please enter character to check if uppercase: ");
        c = getchar ();

        if ( (c >= 'A') && (c <= 'Z') )
        {
            printf ("true\n");
        }
        else
        {
            printf ("false\n");
        }
    } while ( c != '0');


    return 0;
}

However, I get weird behavior (Output):

Please enter character to check if uppercase: a

false

Please enter character to check if uppercase: false

Please enter character to check if uppercase: b

false

Please enter character to check if uppercase: false

Please enter character to check if uppercase: A

true

Please enter character to check if uppercase: false

Please enter character to check if uppercase: 0

false

- The "false" thats comes after the prompt is not what I typed. For example:

1. Prompt appears
2. I type in the character 'a'
3. Console prints false
4. Prompt appears but also printed is the word 'false' next to prompt
5. Prompt appears again

So it seems that getchar() is taking input that is not coming from me. Any ideas?

You mention scanf repeatedly, but your code only uses `getchar`. Please use more care in your questions to avoid confusing answerers. — Chris Hayes, Aug 05 '13 at 06:59
@ChrisHayes SO allows you to edit other people's questions. Since, as you point out, the code uses only `getchar()` for input, I have edited the question to replace the mentioned of `scanf()` with `getchar()`. — This isn't my real name, Aug 06 '13 at 21:08
The return type of `getchar` is `int` and you should store it into a variable of type `int` and compare with `EOF`; See also [what happens if you use `char c = getchar()` instead of `int`](http://stackoverflow.com/questions/35356322/difference-between-int-and-char-in-getchar-and-putchar) — Antti Haapala -- Слава Україні, Feb 13 '16 at 09:21

P.P · Answer 1 · 2013-08-05T07:05:23.600

2

getchar() leaves a newline character which is consumed in the next iteration. Add another getchar() to ignore the newline:

c = getchar ();
getchar(); // To consume the newline

Or read out all the until newline:

int c=0;
while ((c = getchar()) != '\n' && c != EOF) ;

edited Aug 05 '13 at 07:05

answered Aug 05 '13 at 06:58

P.P

117,907
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This is correct, although somewhat dangerous if the next character is not a newline. If you want to preserve non-newline characters you can use [ungetc](http://stackoverflow.com/questions/13993742/is-there-anyway-to-peek-at-the-stdin-buffer). (For example, if the user enters 'aaa\n'.) – Chris Hayes Aug 05 '13 at 07:01
@ChrisHayes ungetc() is guaranteed to push back only character. So that's not safe either. getchar(), scanf() are all prone errors in one way or other. I'd suggect to fgets() + sscanf(). But that seems to be an overkill for this. – P.P Aug 05 '13 at 07:07
1

I don't understand what you mean. ungetc is perfectly safe in this context, because you should only need to push back characters, one at a time. That is, pop off a character with `getchar`, and if it's not a newline, use `ungetc` and continue your loop. – Chris Hayes Aug 05 '13 at 07:09

score 2 · Answer 2 · edited May 23 '17 at 12:10

2

You are not handling newlines.

See this answer for clarification.

edited May 23 '17 at 12:10

Community

1
1

answered Aug 05 '13 at 07:00

Henrik

4,254
15
28

Unexpected input when using getchar()

2 Answers2