IrrationalExpressions.jl

Make expressions like 2π behave as irrational numbers in Julia
Author jishnub
Popularity
0 Stars
Updated Last
2 Years Ago
Started In
March 2020

IrrationalExpressions.jl

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IrrationalExpressions is a Julia module that makes expressions like behave as irrational numbers, rather than Float64.

The Problem

Julia has a few irrational constants, like π and e. Arbitrary precision packages, like BigFloat, may provide conversion methods that yield these constants with the desired precision. However, any arithmetic operation that happens before conversion defaults to Float64.

julia> BigFloat(π) + BigFloat(-π)
1.224646799147353177226065932275001058209749445923078164062861980294536250318213e-16

julia> typeof(-π)
Float64

This may lead to subtle bugs. For example, 2π*x will only be correct to about 16 decimal places, even if x has higher precision. It must be written as 2(π*x) in order to get the precision of x.

The Solution

Using the IrrationalExpressions module, arithmetic operations don't immediately force conversion to Float64. Instead the expression is kept unevaluated until the target type is known.

julia> using IrrationalExpressions

julia> -pi
-π = -3.1415926535897...

julia> BigFloat(π) + BigFloat(-π)
0.0

Supported Operations

+, -, * and / are currently supported.

Downconversion occurs when a floating point value is encountered. The resulting type is that of the floating point value.

julia> τ = 2π
2π = 6.2831853071795...

julia> τ + 0.0
6.283185307179586

julia> τ + BigFloat(0)
6.283185307179586476925286766559005768394338798750211641949889184615632812572396

Functions in Base typically convert to Float64 when encountering an unknown subtype of Real. They will work as usual.

julia> cos(2π)
1.0

julia> typeof(ans)
Float64

Supported Types

Integer, Rational and Irrational can be used to build irrational expressions. Care must be taken so that floats are not accidentally created. (1//2)π is an IrrationalExpr, but (1/2)π is a Float64.

New floating-point types need not explicitly support conversion from IrrationalExpr. Any subtype of AbstractFloat that has conversions from Integer, Rational and Irrational, promotion from Real and the necessary arithmetic operations is automatically supported.

Caveat

Because this is a quick hack, there's no simplification, or elimination of common subexpressions. If irrational expressions are inadvertently created in a loop, they can grow exponentially

julia> a = π; for i=1:5; global a = a-a/3; end; a
((((π - π / 3) - (π - π / 3) / 3) - ((π - π / 3) - (π - π / 3) / 3) / 3) - (((π - π / 3) - (π - π / 3) / 3) - ((π - π / 3) - (π - π / 3) / 3) / 3) / 3) - ((((π - π / 3) - (π - π / 3) / 3) - ((π - π / 3) - (π - π / 3) / 3) / 3) - (((π - π / 3) - (π - π / 3) / 3) - ((π - π / 3) - (π - π / 3) / 3) / 3) / 3) / 3 = 0.41370767454680...

(The work-around is to convert to the desired floating-point type before entering the loop.)

To-Do-List

  • There needs to be some way to keep expressions from blowing up in a loop. At minimum, the size should be tracked, and an error thrown at some point.
  • It would be possible to extend this to things like sqrt(Integer), Integer^Rational, etc.
  • Support for complex-valued irrational expressions, like pi * im is still missing.