Floating-point cheat sheet for Perl

Floating-Point Types

Perl supports platform-native floating-point as scalar values; in practice this usually means IEEE 754 double precision.

Exact Types

Perl can also store decimal numbers as strings, but the builtin arithmetic operators will convert them to integer or floating-point values to perform the operation.

The Math::BigFloat extension provides an arbitrary-precision decimal type:

	use Math::BigFloat ':constant'
	my $f = 0.1 + 0.2; # returns exactly 0.3

The Number::Fraction extension provides a fraction type that overloads the arithmetic operators with symbolic fraction arithmetic:

	use Number::Fraction ':constants';
	my $f = '1/2' - '1/3'; # returns 1/6

The Math::BigRat extension provides similar functionality. Its advantage is compatibility with the Math::BigInt and Math::BigFloat extensions, but it does not seem to support fraction literals.

How to Round

To get a string:

	$result = sprintf("%.2f", 1.2345); # returns 1.23

To format output:

	printf("%.2f", 1.2); # prints 1.20

Note that this implicitly uses round-to-even. The variable $# contains the default format for printing numbers, but its use is considered deprecated.

The Math::Round extension provides various functions for rounding floating-point values:

	use Math::Round qw(:all);
	$result = nearest(.1, 4.567) # prints 4.6
	$result = nearest(.01, 4.567) # prints 4.57

The Math::BigFloat extension also supports various rounding modes:

	use Math::BigFloat;
	my $n = Math::BigFloat->new(123.455);
	my $f1 = $n->round('','-2','common'); # returns 123.46
	my $f2 = $n->round('','-2','zero'); # returns 123.45

Resources

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