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Bug #17218

Range#step sometimes behaves unexpectedly with Rational endpoints and increment

Added by rorymolinari (Rory Molinari) about 2 months ago. Updated 5 days ago.

Status:
Assigned
Priority:
Normal
Target version:
-
ruby -v:
ruby 2.7.2p137 (2020-10-01 revision 5445e04352) [x86_64-darwin19]
[ruby-core:100312]

Description

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.

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.

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.

#1

Updated by rorymolinari (Rory Molinari) about 2 months ago

  • Subject changed from Range#step sometimes behaves unexpectedly with Rational endpoints and increment due to silent floating-point approximation to Range#step sometimes behaves unexpectedly with Rational endpoints and increment

Updated by mame (Yusuke Endoh) about 2 months ago

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

Updated by jeremyevans0 (Jeremy Evans) 5 days ago

This is caused by calling rb_int_plus/rb_int_mul/rb_int_minus directly instead of calling Ruby methods in arith_seq_last. I'm guessing the use of Rational begin/end/step was not anticipated. Calling Ruby methods seems to fix the issue. I've submitted a pull request with that approach: https://github.com/ruby/ruby/pull/3810

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