Showing unstable and stable sorting in Ruby.

stable_sort_example.rb

# This example is flawed, but hopefully useful for demonstration purposes.
def test_stable_sorting
  ary = (1..100).to_a.shuffle + (1..100).to_a.shuffle

  # This associates an ordering with the randomized numbers
  idx = 0
  paired = ary.collect {|value| [value, idx += 1]}
  puts "Now the numbers are paired; the first is the random number 1-100,"
  puts "the second is its sequence within the 200 entries."
  puts paired.inspect
  puts
  puts "#sort is unstable; you'll see many entries with equal first values"
  puts "where the first of them has a higher second (sequence) number, meaning"
  puts "it's out of order now."
  puts paired.sort {|x,y| x.first <=> y.first }.inspect
  puts
  puts "Now we sort exclusively on the value, while preserving ordering;"
  puts "All entries with identical first values should have second values"
  puts "that are also in numerical order."
  puts paired.sort_by.with_index {|x, i| [x.first, i]}.inspect
end