Bug #11907: sort_by does not preserve order with arrays with 7+ items and a block that returns the same value

• Status: Rejected
• Assignee: -
• Target version: -
• ruby -v: 2.3.0
• Backport: 2.0.0: UNKNOWN, 2.1: UNKNOWN, 2.2: UNKNOWN, 2.3: UNKNOWN

[barelyknown (Sean Devine)]

TL;DR Arrays with more than 6 items do not have their order preserved if sorted with sort_by and the sort_by block returns the same value for each.

Let’s start with a simple array of 6 items and sort it by the same value using the sort_by method. In Ruby 2.3 and Ruby 2.2.3, the order of the list will remain the same.

list = [1,2,3,4,5,6]
list.sort_by { |item| nil }
=> [1,2,3,4,5,6]

But, let’s add one item.

list = [1,2,3,4,5,6,7]

# with Ruby 2.2.3
list.sort_by { |item| nil }
=> [1, 2, 3, 4, 5, 6, 7]

# with Ruby 2.3.0
list.sort_by { |item| nil }
=> [4, 2, 3, 1, 5, 6, 7]

Whoa! In Ruby 2.3, the order of the first and fourth items will be swapped. And, if we have a list of 8 or more items, the order of the first and last items will be swapped.

list = [1,2,3,4,5,6,7,8]
list.sort_by { |item| nil }
=> [8, 2, 3, 4, 5, 6, 7, 1]

[barelyknown (Sean Devine)]

• Description updated (diff)

[sawa (Tsuyoshi Sawada)]

I am pretty sure this proposal will be rejected. Ruby's sort is not stable. If you want to do stable sort, use with_index.

[jeremyevans0 (Jeremy Evans)]

• Status changed from Open to Rejected