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## 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?

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