$ perldoc q decimal
Found in /opt/perl/lib/5.8.0/pod/perlfaq4.pod
Why am I getting long decimals (eg, 19.9499999999999)
instead of the numbers I should be getting (eg, 19.95)?
The infinite set that a mathematician thinks of as
the real numbers can only be approximated on a
computer, since the computer only has a finite
number of bits to store an infinite number of, um,
numbers.
Internally, your computer represents floating
point numbers in binary. Floatingpoint numbers
read in from a file or appearing as literals in
your program are converted from their decimal
floatingpoint representation (eg, 19.95) to an
internal binary representation.
However, 19.95 can't be precisely represented as a
binary floatingpoint number, just like 1/3 can't
be exactly represented as a decimal floatingpoint
number. The computer's binary representation of
19.95, therefore, isn't exactly 19.95.
When a floatingpoint number gets printed, the
binary floatingpoint representation is converted
back to decimal. These decimal numbers are dis
played in either the format you specify with
printf(), or the current output format for num
bers. (See "$#" in perlvar if you use print. $#
has a different default value in Perl5 than it did
in Perl4. Changing $# yourself is deprecated.)
This affects all computer languages that represent
decimal floatingpoint numbers in binary, not just
Perl. Perl provides arbitraryprecision decimal
numbers with the Math::BigFloat module (part of
the standard Perl distribution), but mathematical
operations are consequently slower.
If precision is important, such as when dealing
with money, it's good to work with integers and
then divide at the last possible moment. For
example, work in pennies (1995) instead of dollars
and cents (19.95) and divide by 100 at the end.
To get rid of the superfluous digits, just use a
format (eg, "printf("%.2f", 19.95)") to get the
required precision. See "Floatingpoint Arith
metic" in perlop.
Abigail
