Ruby Array concat versus + speed?

Question

I did small performance test of Ruby\'s array concat() vs + operation and concat() was way too fast.

I however am not clear on why

Answer 1

If you're going to run benchmarks, take advantage of prebuilt tools and reduce the test to the minimum necessary to test what you want to know.

Starting with Fruity, which provides a lot of intelligence to its benchmarking:

require 'fruity'

compare do
  plus { [] + [4, 5] }
  concat { [].concat([4, 5]) }
end
# >> Running each test 32768 times. Test will take about 1 second.
# >> plus is similar to concat

When things are close enough to not really worry about, Fruity will tell us they're "similar".

At that point Ruby's built-in Benchmark class can help:

require 'benchmark'

N = 10_000_000
3.times do
  Benchmark.bm do |b|
    b.report('plus')  { N.times { [] + [4, 5] }}
    b.report('concat') { N.times { [].concat([4,5]) }}
  end
end
# >>        user     system      total        real
# >> plus  1.610000   0.000000   1.610000 (  1.604636)
# >> concat  1.660000   0.000000   1.660000 (  1.668227)
# >>        user     system      total        real
# >> plus  1.600000   0.000000   1.600000 (  1.598551)
# >> concat  1.690000   0.000000   1.690000 (  1.682336)
# >>        user     system      total        real
# >> plus  1.590000   0.000000   1.590000 (  1.593757)
# >> concat  1.680000   0.000000   1.680000 (  1.684128)

Notice the different times. Running a test once can result in misleading results, so run them several times. Also, make sure your loops result in a time that isn't buried in background noise caused by processes kicking off.