# 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

- Semantics of numbers and numeric operations in Perl
- sprintf function
- Math::Round extension
- Number::Fraction extension
- Math::BigRat extension
- Math::BigFloat extension

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