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Bug #7829

closed

Rounding error in Ruby Time

Added by loirotte (Philippe Dosch) over 9 years ago. Updated over 8 years ago.

Status:
Closed
Priority:
Normal
Target version:
ruby -v:
ruby 1.9.3p194 (2012-04-20 revision 35410) [i686-linux]
Backport:
[ruby-core:52130]

Description

Even if I know the precision errors related to the implementation of IEEE 754 floating values, I'm very surprised of:

irb(main):001:0> Time.utc(1970,1,1,0,0,12.860).strftime("%H:%M:%S,%L")
=> "00:00:12,859"

The fact is that I obtain:

irb(main):002:0> Time.utc( 1970, 1, 1, 0, 0, 12.860 ).subsec
=> (60517119992791/70368744177664)
irb(main):003:0> Time.utc( 1970, 1, 1, 0, 0, 12.860 ).subsec.to_f
=> 0.8599999999999994

If I well understand the precision error that is reported for the 12th or 14th digit after the comma, I don't understand why the rounding process gives an unexpected result for this value. In this case, the last significant digit of my value is impacted, and it appears to be a embarrassing behavior. For other values, the obtained result is as expected:

irb(main):001:0> Time.utc(1970,1,1,0,0,12.880).strftime("%H:%M:%S,%L")
=> "00:00:12,880"

Moreover, this is a part of the Time class and I don't know any way to fix it in a program (and I don't know the full list of values reproducing this issue...)

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