FAQ: Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?
Author Message
FAQ: Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?

This message is one of several periodic postings to comp.lang.perl.misc
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 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.
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 an 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 represent an important
part of the Usenet tradition.  They serve to reduce the volume of
redundant traffic on a news group by providing quality answers to
questions that keep coming up.

If you are some how irritated by seeing these postings you are free
to ignore them or add the sender to your killfile.  If you find
errors or other problems with these postings please send corrections
or comments to the posting email address or to the maintainers as
directed in the perlfaq manual page.

Perl, can be found by pointing your news client to

or to the many thousands of other useful Usenet news groups.

Note that the FAQ text posted by this server may have been modified
from that distributed in the stable Perl release.  It may have been
edited to reflect the additions, changes and corrections provided
by respondents, reviewers, and critics to previous postings of
these FAQ. Complete text of these FAQ are available on request.

The perlfaq manual page contains the following copyright notice.

Copyright (c) 1997-1999 Tom Christiansen and Nathan

This posting is provided in the hope that it will be useful but
does not represent a commitment or contract of any kind on the part
of the contributers, authors or their agents.

04.01
--
This space intentionally left blank

Mon, 23 Feb 2004 13:17:01 GMT

 Page 1 of 1 [ 1 post ]

Relevant Pages