Feature #9834
closed
Float#{next_float,prev_float}
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?
Files
Updated by phasis68 (Heesob Park) over 11 years 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 
Updated by akr (Akira Tanaka) over 11 years 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.)
Updated by matz (Yukihiro Matsumoto) over 11 years ago
Accepted.
Matz.
Updated by marcandre (Marc-Andre Lafortune) over 11 years ago
Float#next or Float#next_float?
Yukihiro Matsumoto wrote:
Accepted.
Matz.
Updated by matz (Yukihiro Matsumoto) over 11 years ago
- Assignee set to akr (Akira Tanaka)
Float#next_float definitely. Float#next is too short and too simple for this method.
Matz.
Updated by akr (Akira Tanaka) over 11 years ago
- Status changed from Open to Closed
- % Done changed from 0 to 100
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.
[ruby-core:62562] [Feature #9834]