vijay <Ta***********@yahoo.com> wrote:
What happens to float variable in loops. For example,
float f=8.7;
if(f<8.7)
printf("less");
else if(f==8.7)
printf("equal");
else if(f>8.7)
printf("more");
prints "less". Shouldn't it print "equal".
No, you can't expect that. The basic problem is that numbers are
stored with a finite number of digits. And numbers like 8.7 are
no simple looking numbers anymore when converted to binary but
have an infinite number of digits (like one third is an infinite
fraction when written in base 10). But since you only have a
finite numbers of bits to store them in, the numbers get truncated.
So most floating point numbers can be stored only as an approxi-
mation and what's stored of your 8.7 is probably something like
8.6999998 instead of 8.7 - you can easily see that effect when
you try to print it out with enough digits, try e.g.
printf( "%20.18f\n", f );
Moreover, since (usually) floats have less bits than doubles,
the same number stored in a float and a double variable will
differ - just try it out with
printf( "%20.18f %20.18f\n", f, 8.7 );
This also indicates that "8.7" isn't treated as a float but
as a double - all calculations in C are done per default in
double and all constants like "8.7" are treated as doubles -
to avoid that you would have to write "8.7f" instead. So what
you do in your program is comparing the value of the float
variable 'f' with the value of 8.7 when stored as a double.
But since these values typically differ you hardly ever will
get "less" printed out (except for numbers that can be stored
using a small number of digits, your result would be different
if you would use e.g. 2.5 instead of 8.7).
So the first thing to keep in mind when dealing with floating
point numbers is that comparing them for equaliy will typically
only work by accident.
Regards, Jens
--
\ Jens Thoms Toerring ___
Je***********@physik.fu-berlin.de
\__________________________
http://www.toerring.de 
