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Doubt about getchar() and scanf() for a character?

Hello,

the following code does not work:
"
....
void main(void)
{
char option;

printf("Choose an option: ");

option = toupper(getchar());

printf("Chosse another option: ");

scanf("%c", &option);

...
"

It shows "Choose an option: Choose another option ". Why?
Whether I put two lines "fflush(stdin)" before the inputs, the same
above occurs.

my gcc: "$ gcc --version
gcc (GCC) 3.3.3 20040412 (Red Hat Linux 3.3.3-7)"

TIA, Vinicius.
Nov 14 '05 #1
3 2468
Vinicius wrote:
Hello,

the following code does not work:
"
...
void main(void) Yep, this could be one reason.
The main() function returns int. Always.
Anything else may lead to undefined behavior.

{
char option;

printf("Choose an option: ");

option = toupper(getchar());

printf("Chosse another option: ");

scanf("%c", &option);

...
"

It shows "Choose an option: Choose another option ". Why? Because you typed in the code instead of pasting it.
I show:
Choose an option: Chosse another option:
This depends on your operating system and how it handles
input and output to the user. Read the C language FAQ
below for more information.

Whether I put two lines "fflush(stdin)" before the inputs, the same
above occurs. The fflush() is not defined for input streams. Again,
another FAQ. You could try flushing the output buffer,
echoing your input character or other platform dependent
solutions. Again, read the FAQ.


my gcc: "$ gcc --version
gcc (GCC) 3.3.3 20040412 (Red Hat Linux 3.3.3-7)"

TIA, Vinicius.

--
Thomas Matthews

C++ newsgroup welcome message:
http://www.slack.net/~shiva/welcome.txt
C++ Faq: http://www.parashift.com/c++-faq-lite
C Faq: http://www.eskimo.com/~scs/c-faq/top.html
alt.comp.lang.learn.c-c++ faq:
http://www.comeaucomputing.com/learn/faq/
Other sites:
http://www.josuttis.com -- C++ STL Library book

Nov 14 '05 #2


"Vinicius" <cv********@click21.com.br> wrote in message
news:87*************************@posting.google.co m...
Hello,

the following code does not work:
"
...
void main(void)
{
char option;

printf("Choose an option: ");

option = toupper(getchar());

printf("Chosse another option: ");

scanf("%c", &option);

...
"

It shows "Choose an option: Choose another option ". Why?
Whether I put two lines "fflush(stdin)" before the inputs, the same
above occurs.

my gcc: "$ gcc --version
gcc (GCC) 3.3.3 20040412 (Red Hat Linux 3.3.3-7)"

TIA, Vinicius.


Please read the FAQ { http://www.eskimo.com/~scs/c-faq/top.html }, and
specifically read about, topics on

void main()
fflush(stdin)
If you are careful, you will also find an answer to your problem.I am too
lazy to find a solution to your problem.

Tip: It's nothing to do with the version of your compiler.
--
Imanpreet Singh Arora
Zmoc.Zliamg@Zteerpnami
Remove Z to mail
"Things may come to those who wait, but only the things left by those who
hustle."
Abraham Lincoln




Nov 14 '05 #3
Vinicius <cv********@click21.com.br> wrote:
the following code does not work:
What exactly does not work? I think it works as designed, but perhaps
not as expected by you.
Have you tried to print out `option' after each read?
"
...
void main(void) int main(void); {
char option; printf("Choose an option: ");

option = toupper(getchar()); printf("Chosse another option: "); scanf("%c", &option); ...
" It shows "Choose an option: Choose another option ". Why?
On my system the two messages are on two different lines,
unless you run them with stdin redirected, like this:
./a.out <In
Whether I put two lines "fflush(stdin)" before the inputs, the same
above occurs.


Don't do it, it's undefined. What possibly could it do?
If stdin is your terminal, then getchar() will read the first character
entered, and scanf() will read the next character that is in the *stream*,
which is possibly a carriage-return - depending on how you enter the
characters:
a<CR>b<CR> - first `option' is 'a', second time it is '\n'
ab<CR> - first it is 'a', then it is 'b'
a b<CR> - first read 'a', then ' '
a<CR>b<CR> - first read ' ', then 'a'
etc...

If the scanf line looked like this:
scanf(" %c", &option); /* note the extra space */
then scanf would skip any white-space characters before reading
next character:
a<CR>b<CR> - read 'a' (getchar()), then skip '\n' and read 'b' (scanf())
a<CR> <TAB>b<CR> - after reading 'a', '\n', ' ' and '\t' are ignored
and 'b' gets into `option'
a<CR>b<CR> - getchar() reads first ' ', scanf() skips the second ' '
and reads 'a'

Note, on top of all above, the terminals are line-buffered, so you can't
enter any characters without entering (pressing) <CR>. The situation
is a little different when your stdin is a file.

scanf() is a useful function, but not easy to understand, and if you're
a beginner, leave it for later. Better use fgets() for reading; atoi()
and strtol() families for conversions, but maybe sscanf() would prove
easy to use here as well.

To know scanf() you have to _carefully_ read its description. It is
not an "inverse" of printf(), and the format string similarities with
that of printf() are only apparent.

--
Stan Tobias
sed 's/[A-Z]//g' to email
Nov 14 '05 #4

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