Bug #16612: Enumerator::ArithmeticSequence#last with float range produces incorrect value

• Status: Assigned
• Priority: Normal
• Assignee: mrkn (Kenta Murata)
• Target version: -
• ruby -v: 2.7.0
• Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN

[bjfish (Brandon Fish)]

Steps to reproduce

% ruby -e 'p ((0.24901079128550474)...(340.25008088980684)).step(34.00010700985213).last'
340.2500808898068

Expected result

Last value should match the value produced by the following:

ruby -e 'p ((0.24901079128550474)...(340.25008088980684)).step(34.00010700985213).to_a[-1]'
306.24997387995467

Environment
This was observed in versions 2.7.0 and 2.6.5.

[Eregon (Benoit Daloze)]

I'm unsure if the bug is in last or in each.

# last seems correct
[7] pry(main)> (0.24901079128550474r + 34.00010700985213r * 10).to_f
=> 340.2500808898068
[8] pry(main)> 340.25008088980684
=> 340.25008088980684

# Computing with 10 += seems correct too
[18] pry(main)> n=0.24901079128550474;
[19] pry(main)> 10.times {p n+=34.00010700985213}
34.249117801137636
68.24922481098977
102.2493318208419
136.24943883069403
170.24954584054615
204.24965285039826
238.24975986025038
272.2498668701025
306.2499738799546
340.2500808898067
=> 10
[20] pry(main)> 340.2500808898067 < 340.25008088980684
=> true

# But each/to_a gives different results:
[17] pry(main)> ((0.24901079128550474)...(340.25008088980684)).step(34.00010700985213).to_a
=> [0.24901079128550474,
 34.249117801137636,
 68.24922481098976,
 102.24933182084189,
 136.24943883069403,
 170.24954584054615,
 204.2496528503983,
 238.24975986025044,
 272.24986687010255,
 306.24997387995467]

So I think last is correct, and each misses to yield the last element, probably due to accumulated errors.

[nobu (Nobuyoshi Nakada)]

err in ruby_float_step_size seems too big in this case.

[mrkn (Kenta Murata)]

• Assignee set to mrkn (Kenta Murata)
• Status changed from Open to Assigned

Since past few days/weeks, the Redmine threading is broken. Can somebody
please look into this?

Thx.

Vít

Dne 07. 02. 20 v 9:58 [e-mail] napsal(a):

Issue #16612 has been updated by mrkn (Kenta Murata).

Assignee set to mrkn (Kenta Murata)
Status changed from Open to Assigned

---

Bug #16612: Enumerator::ArithmeticSequence#last with float range produces incorrect value
https://bugs.ruby-lang.org/issues/16612#change-84193

• Author: bjfish (Brandon Fish)
• Status: Assigned
• Priority: Normal
• Assignee: mrkn (Kenta Murata)
• ruby -v: 2.7.0

* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN

Steps to reproduce

% ruby -e 'p ((0.24901079128550474)...(340.25008088980684)).step(34.00010700985213).last'
340.2500808898068

Expected result

Last value should match the value produced by the following:

ruby -e 'p ((0.24901079128550474)...(340.25008088980684)).step(34.00010700985213).to_a[-1]'
306.24997387995467

Environment
This was observed in versions 2.7.0 and 2.6.5.