source: trunk/essentials/dev-lang/perl/pod/perlfaq4.pod@ 3397

=head1 NAME

perlfaq4 - Data Manipulation ($Revision: 1.73 $, $Date: 2005/12/31 00:54:37 $)

=head1 DESCRIPTION

This section of the FAQ answers questions related to manipulating
numbers, dates, strings, arrays, hashes, and miscellaneous data issues.

=head1 Data: Numbers

=head2 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?

Internally, your computer represents floating-point numbers
in binary. Digital (as in powers of two) computers cannot
store all numbers exactly.  Some real numbers lose precision
in the process.  This is a problem with how computers store
numbers and affects all computer languages, not just Perl.

L<perlnumber> show the gory details of number
representations and conversions.

To limit the number of decimal places in your numbers, you
can use the printf or sprintf function.  See the
L<"Floating Point Arithmetic"|perlop> for more details.

        printf "%.2f", 10/3;

        my $number = sprintf "%.2f", 10/3;

=head2 Why is int() broken?

Your int() is most probably working just fine.  It's the numbers that
aren't quite what you think.

First, see the above item "Why am I getting long decimals
(eg, 19.9499999999999) instead of the numbers I should be getting
(eg, 19.95)?".

For example, this

    print int(0.6/0.2-2), "\n";

will in most computers print 0, not 1, because even such simple
numbers as 0.6 and 0.2 cannot be presented exactly by floating-point
numbers.  What you think in the above as 'three' is really more like
2.9999999999999995559.

=head2 Why isn't my octal data interpreted correctly?

Perl only understands octal and hex numbers as such when they occur as
literals in your program.  Octal literals in perl must start with a
leading "0" and hexadecimal literals must start with a leading "0x".
If they are read in from somewhere and assigned, no automatic
conversion takes place.  You must explicitly use oct() or hex() if you
want the values converted to decimal.  oct() interprets hex ("0x350"),
octal ("0350" or even without the leading "0", like "377") and binary
("0b1010") numbers, while hex() only converts hexadecimal ones, with
or without a leading "0x", like "0x255", "3A", "ff", or "deadbeef".
The inverse mapping from decimal to octal can be done with either the
"%o" or "%O" sprintf() formats.

This problem shows up most often when people try using chmod(), mkdir(),
umask(), or sysopen(), which by widespread tradition typically take
permissions in octal.

    chmod(644,  $file); # WRONG
    chmod(0644, $file); # right

Note the mistake in the first line was specifying the decimal literal