FAQ 4.1: Why am I getting long decimals (eg, 19.9499999999999) instead ...

This message is one of several periodic postings to DevShed's Perl forum intended to make it easier for Perl programmers to find answers to common questions. The core of this message represents an excerpt from the documentation provided with every Standard Distribution of Perl.

---------------------------------------------
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 approximate 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.
Floating-point numbers read in from a file or appearing as literals in
your program are converted from their decimal floating-point
representation (eg, 19.95) to the internal binary representation.

However, 19.95 can't be precisely represented as a binary floating-point
number, just like 1/3 can't be exactly represented as a decimal
floating-point number. The computer's binary representation of 19.95,
therefore, isn't exactly 19.95.

When a floating-point number gets printed, the binary floating-point
representation is converted back to decimal. These decimal numbers are
displayed in either the format you specify with printf(), or the current
output format for numbers (see the section on "$#" in the perlvar manpage
if you use print. `$#' has a different default value in Perl5 than it did
in Perl4. Changing `$#' yourself is deprecated.)

This affects all computer languages that represent decimal floating-point
numbers in binary, not just Perl. Perl provides arbitrary-precision
decimal numbers with the Math::BigFloat module (part of the standard Perl
distribution), but mathematical operations are consequently slower.

To get rid of the superfluous digits, just use a format (eg,
`printf("%.2f", 19.95)') to get the required precision. See the section on
"Floating-point Arithmetic" in the perlop manpage.
---------------------------------------------

Documents such as this have been called "Answers to Frequently Asked Questions" or FAQ for short. They serve to reduce the volume of redundant traffic on this bulletin board by providing quality answers to questions that keep comming up. If you find errors or other problems with these postings please send corrections or comments to the posting email address.


------------------
Written by Anonym0us, inspired by the original PerlFAQ Server.