PERLFAQ4(1) Perl Programmers Reference Guide PERLFAQ4(1)
NAME
perlfaq4 - Data Manipulation ($Revision: 1.37 $, $Date: 2002/11/13 06:04:00 $)
DESCRIPTION
This section of the FAQ answers questions related to manipulating numbers, dates, strings, arrays, hashes, and miscellaneous data issues.
Data: Numbers
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.
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 perlop for more details.
printf "%.2f", 10/3;
my $number = sprintf "%.2f", 10/3;
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 conver-
sion 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 644, rather than the intended octal literal 0644. The problem can be
seen with:
printf("%#o",644); # prints 01204
Surely you had not intended "chmod(01204, $file);" - did you? If you want to use numeric literals as arguments to chmod() et al. then
please try to express them as octal constants, that is with a leading zero and with the following digits restricted to the set 0..7.
Does Perl have a round() function? What about ceil() and floor()? Trig functions?
Remember that int() merely truncates toward 0. For rounding to a certain number of digits, sprintf() or printf() is usually the easiest
route.
printf("%.3f", 3.1415926535); # prints 3.142
The POSIX module (part of the standard Perl distribution) implements ceil(), floor(), and a number of other mathematical and trigonometric
functions.
use POSIX;
$ceil = ceil(3.5); # 4
$floor = floor(3.5); # 3
In 5.000 to 5.003 perls, trigonometry was done in the Math::Complex module. With 5.004, the Math::Trig module (part of the standard Perl
distribution) implements the trigonometric functions. Internally it uses the Math::Complex module and some functions can break out from the
real axis into the complex plane, for example the inverse sine of 2.
Rounding in financial applications can have serious implications, and the rounding method used should be specified precisely. In these
cases, it probably pays not to trust whichever system rounding is being used by Perl, but to instead implement the rounding function you
need yourself.
To see why, notice how you'll still have an issue on half-way-point alternation:
for ($i = 0; $i < 1.01; $i += 0.05) { printf "%.1f ",$i}
0.0 0.1 0.1 0.2 0.2 0.2 0.3 0.3 0.4 0.4 0.5 0.5 0.6 0.7 0.7
0.8 0.8 0.9 0.9 1.0 1.0
Don't blame Perl. It's the same as in C. IEEE says we have to do this. Perl numbers whose absolute values are integers under 2**31 (on
32 bit machines) will work pretty much like mathematical integers. Other numbers are not guaranteed.
How do I convert between numeric representations?
As always with Perl there is more than one way to do it. Below are a few examples of approaches to making common conversions between num-
ber representations. This is intended to be representational rather than exhaustive.
Some of the examples below use the Bit::Vector module from CPAN. The reason you might choose Bit::Vector over the perl built in functions
is that it works with numbers of ANY size, that it is optimized for speed on some operations, and for at least some programmers the nota-
tion might be familiar.
How do I convert hexadecimal into decimal
Using perl's built in conversion of 0x notation:
$int = 0xDEADBEEF;
$dec = sprintf("%d", $int);
Using the hex function:
$int = hex("DEADBEEF");
$dec = sprintf("%d", $int);
Using pack:
$int = unpack("N", pack("H8", substr("0" x 8 . "DEADBEEF", -8)));
$dec = sprintf("%d", $int);
Using the CPAN module Bit::Vector:
use Bit::Vector;
$vec = Bit::Vector->new_Hex(32, "DEADBEEF");
$dec = $vec->to_Dec();
How do I convert from decimal to hexadecimal
Using sprint:
$hex = sprintf("%X", 3735928559);
Using unpack
$hex = unpack("H*", pack("N", 3735928559));
Using Bit::Vector
use Bit::Vector;
$vec = Bit::Vector->new_Dec(32, -559038737);
$hex = $vec->to_Hex();
And Bit::Vector supports odd bit counts:
use Bit::Vector;
$vec = Bit::Vector->new_Dec(33, 3735928559);
$vec->Resize(32); # suppress leading 0 if unwanted
$hex = $vec->to_Hex();
How do I convert from octal to decimal
Using Perl's built in conversion of numbers with leading zeros:
$int = 033653337357; # note the leading 0!
$dec = sprintf("%d", $int);
Using the oct function:
$int = oct("33653337357");
$dec = sprintf("%d", $int);
Using Bit::Vector:
use Bit::Vector;
$vec = Bit::Vector->new(32);
$vec->Chunk_List_Store(3, split(//, reverse "33653337357"));
$dec = $vec->to_Dec();
How do I convert from decimal to octal
Using sprintf:
$oct = sprintf("%o", 3735928559);
Using Bit::Vector
use Bit::Vector;
$vec = Bit::Vector->new_Dec(32, -559038737);
$oct = reverse join('', $vec->Chunk_List_Read(3));
How do I convert from binary to decimal
Perl 5.6 lets you write binary numbers directly with the 0b notation:
$number = 0b10110110;
Using pack and ord
$decimal = ord(pack('B8', '10110110'));
Using pack and unpack for larger strings
$int = unpack("N", pack("B32",
substr("0" x 32 . "11110101011011011111011101111", -32)));
$dec = sprintf("%d", $int);
# substr() is used to left pad a 32 character string with zeros.
Using Bit::Vector:
$vec = Bit::Vector->new_Bin(32, "11011110101011011011111011101111");
$dec = $vec->to_Dec();
How do I convert from decimal to binary
Using unpack;
$bin = unpack("B*", pack("N", 3735928559));
Using Bit::Vector:
use Bit::Vector;
$vec = Bit::Vector->new_Dec(32, -559038737);
$bin = $vec->to_Bin();
The remaining transformations (e.g. hex -> oct, bin -> hex, etc.) are left as an exercise to the inclined reader.
Why doesn't & work the way I want it to?
The behavior of binary arithmetic operators depends on whether they're used on numbers or strings. The operators treat a string as a
series of bits and work with that (the string "3" is the bit pattern 00110011). The operators work with the binary form of a number (the
number 3 is treated as the bit pattern 00000011).
So, saying "11 & 3" performs the "and" operation on numbers (yielding 3). Saying "11" & "3" performs the "and" operation on strings
(yielding "1").
Most problems with "&" and "|" arise because the programmer thinks they have a number but really it's a string. The rest arise because the
programmer says:
if ("