typelevel/spire

Error in rational.toDouble

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#660 opened on Dec 21, 2017

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 (2 comments) (0 reactions) (0 assignees)Scala (1,775 stars) (238 forks)batch import
bughelp wantedlow hanging fruit

Description

The toDouble method on Rational instances returns incorrectly for certain inputs. To reproduce,

import spire.math.Rational

def bld(numStr: String, denomStr: String): (Double, Double) = {
  val num = BigInt(numStr)
  val denom = BigInt(denomStr)

  val expected = (BigDecimal(num) / BigDecimal(denom)).toDouble
  val actual = Rational(num, denom).toDouble

  (expected, actual)
}

println(
  bld(
    "179709322496314735297382168733437804616092100361307360024524978382010425322923559",
    "179709322496314735297382168733483632988275016201074155389238170380281177201180672"
  ))
println(
  bld(
    "-1257965257474203147081675181125850088132776039278857905825424935347911159685302509",
    "1257965257474203147081675181134385430917925113407519087724667192661968240408264704"
  ))
println(bld(
  "3618502788666131106986593281458997943466459808578940660043329292438660832363",
  "3618502788666131106986593281521497120414687020801267626233049500247285301248"
))

spire version: 0.14.1 scala version: 2.12.4 javac: 1.8.0_152 jvm: Java HotSpot(TM) 64-Bit Server VM (build 25.152-b16, mixed mode) OS: Ubuntu 16.04

Contributor guide

Tech stack
scala
Domain
data
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
ScalaSpire libraryrational numbersdouble conversion
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
50
Research direction
Investigate the toDouble method in spire.math.Rational. The issue provides reproduction code. Check the implementation of toDouble for large numerator/denominator causing precision loss. Look at the Rational.scala file in the spire source.