Feature #8850: Convert Rational to decimal string - Ruby trunk - Ruby Issue Tracking System

Feature #8850

Convert Rational to decimal string

Added by Yui NARUSE over 1 year ago. Updated about 1 year ago.

Status:	Assigned
Priority:	Normal
Assignee:	Yukihiro Matsumoto

Description

On Ruby 2.1.0, decimal literal is introduced.
It generates Rational but it cannot easily convert to decimal string.
You know, Rational#to_f is related to this but
* Float is not exact number
** 0.123456789123456789r.to_f.to_s #=> "0.12345678912345678"
* it can't handle recursive decimal
** (151/13r).to_f.to_s #=> "11.615384615384615"

the method name ** to_decimal ** to_decimal_string ** to_s(format: :decimal) ** extend sprintf
how does it express recursive decimal ** (151/13r).to_decimal_string #=> "11.615384..." ** (151/13r).to_decimal_string #=> "11.(615384)"

Example implementation is following.
Its result is
** 0.123456789123456789r.to_f.to_s #=> "0.123456789123456789"
** (151/13r).to_f.to_s #=> "11.(615384)"

class Rational
  def to_decimal_string(base=10)
    n = numerator
    d = denominator
    r, n = n.divmod d
    str = r.to_s(base)
    return str if n == 0
    h = {}
    str << '.'
    n *= base
    str.size.upto(Float::INFINITY) do |i|
      r, n = n.divmod d
      if n == 0
        str << r.to_s(base)
        break
      elsif h.key? n
        str[h[n], 0] = '('
        str << ')'
        break
      else
        str << r.to_s(base)
        h[n] = i
        n *= base
      end
    end
    str
  end
end