Feature #9834

Float#{next_float,prev_float}

Added by Akira Tanaka about 1 year ago. Updated about 1 year ago.

[ruby-core:62562]
Status:Closed
Priority:Normal
Assignee:Akira Tanaka

Description

I'd like to add Float#next_float and Float#prev_float which
returns next representable floating-point number and
previous representable floating-point number.

p 3.0.next_float #=> 3.0000000000000004
p 3.0.prev_float #=> 2.9999999999999996

These methods can be useful to examine the behavior of floating-point numbers.

For example, they can be used to examine floating-point error in 0.1 + 0.1 + ... + 0.1.

f = 0.0
100.times { f += 0.1 }
p f                            #=> 9.99999999999998       # should be 10.0 in the ideal world.
p 10-f                         #=> 1.9539925233402755e-14 # the floating-point error.
p(10.0.next_float-10)          #=> 1.7763568394002505e-15 # 1 ulp (units in the last place).
p((10-f)/(10.0.next_float-10)) #=> 11.0                   # the error is 11 ulp.
p "%a" % f                     #=> "0x1.3fffffffffff5p+3" # the last hex digit is 5.  16 - 5 = 11 ulp.

The methods are implemented using nextafter() function described in
IEEE 754 (Appendix), C99 and POSIX.
It seems the function is pretty portable on Unix variants.

However I implemented missing/nextafter.c for environments which don't have the function.

Any idea?

next_float-and-prev_float.patch Magnifier (11.9 KB) Akira Tanaka, 05/13/2014 09:47 AM

Associated revisions

Revision 45982
Added by Akira Tanaka about 1 year ago

  • configure.in: Check nextafter() availability.

  • include/ruby/missing.h (nextafter): New optional declaration.

  • missing/nextafter.c: New file.

  • numeric.c: Float#next_float and Float#prev_float implemented.

[Feature #9834]

Revision 45982
Added by Akira Tanaka about 1 year ago

  • configure.in: Check nextafter() availability.

  • include/ruby/missing.h (nextafter): New optional declaration.

  • missing/nextafter.c: New file.

  • numeric.c: Float#next_float and Float#prev_float implemented.

[Feature #9834]

Revision 45992
Added by Nobuyoshi Nakada about 1 year ago

Makefile.sub: fix nextafter

  • win32/Makefile.sub (MISSING, CONFIG_H): msvcr120.dll provides nextafter() as well as other mathematic functions. finite() and isnan() used in missing/nextafter.c are not by older runtimes. [Feature #9834]

Revision 45992
Added by Nobuyoshi Nakada about 1 year ago

Makefile.sub: fix nextafter

  • win32/Makefile.sub (MISSING, CONFIG_H): msvcr120.dll provides nextafter() as well as other mathematic functions. finite() and isnan() used in missing/nextafter.c are not by older runtimes. [Feature #9834]

History

#1 Updated by Heesob Park about 1 year ago

Here is a pure ruby implementation of Float#{next_float,prev_float} (adopted from http://golang.org/src/pkg/math/nextafter.go)

class Float
  def dbl2num(dbl)
    [dbl].pack('d').unpack('Q')[0]
  end

  def num2dbl(num)
    [num].pack('Q').unpack('d')[0]
  end

  def nextafter(y)
    y = y.to_f
    if (self.nan? || y.nan?)
      Float::NAN
    elsif self == y
      y
    elsif self == 0
      num2dbl(1) * (y<=>0.0)
    elsif (y > self) == (self > 0)
      num2dbl(dbl2num(self) + 1)
    else
      num2dbl(dbl2num(self) - 1)
    end
  end

  def prev_float
    nextafter(-Float::INFINITY)
  end

  def next_float
    nextafter(Float::INFINITY)
  end
end

#2 Updated by Akira Tanaka about 1 year ago

Thank you for an interesting implementation.

Heesob Park wrote:

      if (self==Float::NAN || y==Float::NAN)

This doesn't work. Float#nan? should be used.

      elsif self == y
        r = self

This should be "r = y" to to follow C99's nextafter() behavior.
(It doesn't affect next_float and prev_float, though.)

#3 Updated by Yukihiro Matsumoto about 1 year ago

Accepted.

Matz.

#4 Updated by Marc-Andre Lafortune about 1 year ago

Float#next or Float#next_float?

Yukihiro Matsumoto wrote:

Accepted.

Matz.

#5 Updated by Yukihiro Matsumoto about 1 year ago

  • Assignee set to Akira Tanaka

Float#next_float definitely. Float#next is too short and too simple for this method.

Matz.

#6 Updated by Akira Tanaka about 1 year ago

  • % Done changed from 0 to 100
  • Status changed from Open to Closed

Applied in changeset r45982.


  • configure.in: Check nextafter() availability.

  • include/ruby/missing.h (nextafter): New optional declaration.

  • missing/nextafter.c: New file.

  • numeric.c: Float#next_float and Float#prev_float implemented.

[Feature #9834]

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