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]

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.

 

[Fred Kleinschmidt]

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.

 

[CBFalconer]

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.

 

[Richard Heathfield]

Fred Kleinschmidt said:

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.