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Bug #8883

Rational canonicalization unexpectedly converts to Fixnum

Added by melquiades (Paul Cantrell) about 7 years ago. Updated almost 7 years ago.

Status:
Closed
Priority:
Normal
Assignee:
-
Target version:
-
ruby -v:
ruby 2.0.0p247 (2013-06-27 revision 41674) [x86_64-darwin12.3.0]
[ruby-core:57092]

Description

The documentation for Rational (http://www.ruby-doc.org/core-2.0.0/Rational.html) states that the result of creating or doing arithmetic on Rationals returns Rationals, as one would expect. Examples from the docs:

Rational(1)      #=> (1/1)
3.to_r           #=> (3/1)
Rational(-2, 9) * Rational(-9, 2)  #=> (1/1)

These all work as documented in 1.9. In 2.0, however, they all return Fixnum:

Rational(1)      #=> 1
3.to_r           #=> 3
Rational(-2, 9) * Rational(-9, 2)  #=> 1

This leads to unexpected behavior:

Rational(2) / Rational(3)  # => 0  ...but returns (2/3) in 1.9

That behavior is potentially dangerous. Math that may usually work, but suddenly start suffering from truncation errors depending on intermediate results. For example:

def should_always_return_one(a, b, c)
(Rational(a, c) + Rational(b, c)) / (a + b) * c
end

Under 1.9:

should_always_return_one(2, 3, 7) #=> (1/1)
should_always_return_one(2, 4, 7) #=> (1/1)
should_always_return_one(2, 5, 7) #=> (1/1)
should_always_return_one(2, 6, 7) #=> (1/1)

Under 2.0:

should_always_return_one(2, 3, 7) #=> 1
should_always_return_one(2, 4, 7) #=> 1
should_always_return_one(2, 5, 7) #=> 0   Oops!
should_always_return_one(2, 6, 7) #=> 1

Either the docs are wrong, or this is a bug. I vote bug. Whether arithmetic expressions truncate the result should not depend on whether intermediate values just happen to be integers! Such behavior renders Rational almost too dangerous to use in situations where exact results are required. (Yes, I realize that requiring 'mathn' fixes this, but even with such a workaround as an option, this is dangerously broken. See also #2121.) Note that floating point arithmetic does not exhibit this behavior.

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