NAME

perlfaq4 - Data Manipulation ($Revision: 10394 $)

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.

the perlnumber manpage shows 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 "Floating Point Arithmetic" for more details.

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

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

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 answer to "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.

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 hexadecimal (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, such as 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/bases/radixes?

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 number representations. This is intended to be representational rather than exhaustive.

Some of the examples later in the perlfaq4 manpage 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 notation might be familiar.

How do I convert hexadecimal into decimal

Using perl's built in conversion of 0x notation:

        $dec = 0xDEADBEEF;

Using the hex function:

        $dec = hex("DEADBEEF");

Using pack:

        $dec = unpack("N", pack("H8", substr("0" x 8 . "DEADBEEF", -8)));

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 sprintf:

        $hex = sprintf("%X", 3735928559); # upper case A-F
        $hex = sprintf("%x", 3735928559); # lower case a-f

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:

        $dec = 033653337357; # note the leading 0!

Using the oct function:

        $dec = oct("33653337357");

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