From: usa@garbagecollect.jp
To: ruby-core@ruby-lang.org
Subject: [ruby-core:103234] [Ruby master Bug#17218] Range#step sometimes behaves unexpectedly with Rational endpoints and increment
Date: Mon, 05 Apr 2021 01:22:31 +0000 (UTC) [thread overview]
Message-ID: <redmine.journal-91314.20210405012231.44964@ruby-lang.org> (raw)
In-Reply-To: redmine.issue-17218.20201006154650.44964@ruby-lang.org
Issue #17218 has been updated by usa (Usaku NAKAMURA).
backported into ruby_2_6 at r67936
----------------------------------------
Bug #17218: Range#step sometimes behaves unexpectedly with Rational endpoints and increment
https://bugs.ruby-lang.org/issues/17218#change-91314
* Author: rorymolinari (Rory Molinari)
* Status: Closed
* Priority: Normal
* Assignee: mrkn (Kenta Murata)
* ruby -v: ruby 2.7.2p137 (2020-10-01 revision 5445e04352) [x86_64-darwin19]
* Backport: 2.5: DONTNEED, 2.6: REQUIRED, 2.7: DONE
----------------------------------------
I am using the latest stable version. The same behavior exists in 2.6.6.
```
11:28:25 $ ruby -v
ruby 2.7.2p137 (2020-10-01 revision 5445e04352) [x86_64-darwin19]
```
When creating an `Enumerator::ArithmeticSequence` with Rational endpoints and increment, sometimes inconsistent behavior results due to floating-point approximation in `#last`.
``` ruby
x = Rational(10997, 10000)
y = Rational(11, 10)
s = Rational(1, 10000)
puts "#{[x, y, s].map(&:to_f)}" # -> [1.0997, 1.1, 0.0001]
# intention: this contains exactly the precise Rational representations of 1.0997, 1.0998, 1.0999, 1.1
arith_seq = (x..y).step(s)
puts arith_seq.class # -> Enumerator::ArithmeticSequence
# Things look OK
puts arith_seq.first # -> 10997/10000
puts arith_seq.end # -> 11/10
puts arith_seq.step # -> 1/10000
# But the array that we get from #to_a is missing the last element, (11/10)
puts arith_seq.to_a.to_s # -> [(10997/10000), (5499/5000), (10999/10000)]
# This is apparently due to the value of #last
puts arith_seq.last # -> 1.0999999999999999
# The object itself is confused
puts arith_seq.size # -> 4
puts arith_seq.to_a.size # -> 3
```
The issue is in the `Enumerator::ArithmeticSequence` instance we get when we call `Range#step` without a block. The `Range#step` method passes the right things when it gets a block.
``` ruby
block_vals = []
(x..y).step(s) { |v| block_vals << v }
puts block_vals.to_s # -> [(10997/10000), (5499/5000), (10999/10000), (11/10)]
```
I would expect `arith_seq.last` to be the exact value `Rational(11, 10)`. After all, `arith_seq` was created from a `Range` with `Rational` endpoints and given a rational step size.
--
https://bugs.ruby-lang.org/
prev parent reply other threads:[~2021-04-05 1:22 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-06 15:46 [ruby-core:100312] [Ruby master Bug#17218] Range#step sometimes behaves unexpectedly with Rational endpoints and increment due to silent floating-point approximation rorymolinari
2020-10-06 16:06 ` [ruby-core:100313] [Ruby master Bug#17218] Range#step sometimes behaves unexpectedly with Rational endpoints and increment mame
2020-11-24 22:09 ` [ruby-core:101056] " merch-redmine
2020-12-09 6:37 ` [ruby-core:101331] " muraken
2021-03-20 6:35 ` [ruby-core:102950] " nagachika00
2021-04-05 1:22 ` usa [this message]
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