Know Your Operations

- 3 mins

I recently saw a new detail of a new release into Ruby 2.7 where the array class would have methods like intersection, union, and difference.

TL;DR These already exist; as &, |, and - — but they have been SUPERCHARGED 🔥

There are many cool additions to Ruby 2.7. Through my readings this morning, I saw that the array class would be getting an additional Array#intersection alias; yet another alias along the lines of Array#union and Array#difference.

While this sounded interesting, I thought “What is the performance benefit of having this method?” So I got the new Ruby 2.7 and gave it a spin.

Sadly, the patch for Array#intersection has not made it into the 2.7 preview build yet.

✔ ~/Downloads/ruby-2.7.0-preview1/install_here/test_programs
10:43 $ ../bin/ruby -v
ruby 2.7.0preview1 (2019-05-31 trunk c55db6aa271df4a689dc8eb0039c929bf6ed43ff) [x86_64-darwin18]
✔ ~/Downloads/ruby-2.7.0-preview1/install_here/test_programs
10:43 $ ../bin/ruby

So I thought to give the other methods a benchmark. This benchmark still uses the Ruby version above, and seeks to compare the timing difference between the method on the object and the operation.

Array Difference a.k.a “-“

The syntax for this is left_array.difference(right_array).

This returns all the elements in left_array which are not present in right_array. The difference operation, unlike all other operations considered here, is not commutative. This means left_array - right_array != right_array - left_array, unless in a special case where the contents of both arrays allow this.


require 'benchmark'

a = (1..8).to_a
b = (2..9).to_a

# a - b = [1]
# b - a = [9]

puts "Time for a - b"
puts Benchmark.measure { 1_000_000.times { a - b } }
puts "Time for a.difference b"
puts Benchmark.measure { 1_000_000.times { a.difference b } }

Results for Difference

Time for a - b
  0.263208   0.000529   0.263737 (  0.264506)
Time for a.difference b
  0.284521   0.002734   0.287255 (  0.289998)

The results here show that the operation is more performant; albeit slightly than the method call.

Note that the test inputs here are two ordered arrays — which I believe is the simplest case — made to overlap themselves with only one integer.

Array Union a.k.a “|”

This returns all unique items in two arrays. This operation is commutative. Despite the position of the operands, the result is always the same


require 'benchmark'

a = (1..8).to_a
b = (2..9).to_a

# a | b = [1, 2, 3, 4, 5, 6, 7, 8, 9]

puts "Time for a | b"
puts Benchmark.measure { 1_000_000.times { a | b } }
puts "Time for a.union b"
puts Benchmark.measure { 1_000_000.times { a.union b } }

Results for (ordered) Union

Time for a | b
  0.514844   0.003135   0.517979 (  0.521104)
Time for a.union b
  0.520425   0.002118   0.522543 (  0.525702)


From the benchmarks, it seems like there isn’t much of a performance hit with the methods on the array object; slightly less performant, but nothing which would bring down your servers in production.

This is still a welcome change; along two lines

  1. Ruby prides itself in fostering developer happiness. With this new additions, code just reads better; the intent of the code is self-evident.
  2. With these new methods, we now have the ability to pass multiple arrays to the operations. e.g. [1, 2, 3].difference([1], [2]) #=> [3]. This was not possible with just the operations.
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