infinite loop with scanf

[Michael Goerz]

Hi,

why does the following result in an infinite loop when the user does
*not* enter a number?

do{
printf("Enter Number: ");
} while ( ! scanf("%i", &input) );
Thanks,
Michael Goerz

[Jack Klein]

On Tue, 17 Oct 2006 22:14:06 +0200, Michael Goerz
<ne****@8439.e4ward.comwrote in comp.lang.c:

Hi,

why does the following result in an infinite loop when the user does
*not* enter a number?

do{
printf("Enter Number: ");
} while ( ! scanf("%i", &input) );
Thanks,
Michael Goerz

This is a very frequently asked question by beginning C programmers,
and as such it is in the FAQ for this group.

See http://c-faq.com/stdio/scanfjam.html

And you will very probably find a lot of useful information in the
rest of the FAQ as well. The link to its main page is in my signature
block.

Hint, always check to see if a newsgroup has a FAQ, and check for an
answer to your question if it does, before posting a question to a
group.

--
Jack Klein
Home: http://JK-Technology.Com
FAQs for
comp.lang.c http://c-faq.com/
comp.lang.c++ http://www.parashift.com/c++-faq-lite/
alt.comp.lang.learn.c-c++
http://www.contrib.andrew.cmu.edu/~a...FAQ-acllc.html

[Fred Kleinschmidt]

"Michael Goerz" <ne****@8439.e4ward.comwrote in message
news:4p************@uni-berlin.de...

Hi,

why does the following result in an infinite loop when the user does
*not* enter a number?

do{
printf("Enter Number: ");
} while ( ! scanf("%i", &input) );
Thanks,
Michael Goerz

Think about where the file pointer will be after the scanf call
when a non-integer is entered.

--
Fred L. Kleinschmidt
Boeing Associate Technical Fellow
Technical Architect, Software Reuse Project

[CBFalconer]

Michael Goerz wrote:

>
why does the following result in an infinite loop when the user
does *not* enter a number?

do{
printf("Enter Number: ");
} while ( ! scanf("%i", &input) );

Because nothing flushes the input data that does not form a
number. If something beginning with 'z', for example, does not
form a number, it still doesn't form a number on rescan.

Don't use scanf for interactive input. It bites.

--
Chuck F (cbfalconer at maineline dot net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home.att.net>

[Richard Heathfield]

Fred Kleinschmidt said:

<snip>

Think about where the file pointer will be after the scanf call
when a non-integer is entered.

Right where it was, of course, but I think you mean the file position
indicator.

--
Richard Heathfield
"Usenet is a strange place" - dmr 29/7/1999
http://www.cpax.org.uk
email: rjh at above domain (but drop the www, obviously)