does_sort_preserve_order

does_sort_preserve_order.rb

ary = [
  {:sort_me => 1, :expected_order => 1},
  {:sort_me => 2, :expected_order => 7},
  {:sort_me => 2, :expected_order => 8},
  {:sort_me => 1, :expected_order => 2},
  {:sort_me => 1, :expected_order => 3},
  {:sort_me => 1, :expected_order => 4},
  {:sort_me => 1, :expected_order => 5},
  {:sort_me => 1, :expected_order => 6},
].sort! { |a,b| a[:sort_me] <=> b[:sort_me] }

ary.map { |e| e[:expected_order] }
# [1, 6, 5, 2, 4, 3, 8, 7]
# therefore, it is not order-preserving. This is not surprising, since it is simply quicksort.

ary = [
  {:sort_me => 1, :expected_order => 1},
  {:sort_me => 2, :expected_order => 7},
  {:sort_me => 2, :expected_order => 8},
  {:sort_me => 1, :expected_order => 2},
  {:sort_me => 1, :expected_order => 3},
  {:sort_me => 1, :expected_order => 4},
  {:sort_me => 1, :expected_order => 5},
  {:sort_me => 1, :expected_order => 6},
].sort_by { |e| e[:sort_me] }

ary.map { |e| e[:expected_order] }
# [1, 6, 5, 2, 4, 3, 8, 7]
# sort_by is also not order-preserving.