String concatenation in Ruby

Jonas Elfström — Tue, 02 Feb 2010 00:04:00 +0100

There's no StringBuilder class in Ruby because the String class has the << for appending. The problem is that not every Ruby programmer seems to be aware of it. Recently I've seen += being used to append to strings where << would have been a much better choice.

The problem with using += is that it creates a new String instance and if you do that in a loop you can get really horrible performance.

If you are dealing with an array you don't even have to use << because Array#join is even faster and shows intent in a nice way.

require 'benchmark'

array_of_rnd_strings=(0...262144).map{65.+(rand(25)).chr}
                                 .join.scan(/.{1,8}/m)

Benchmark.bm do |benchmark|
  benchmark.report do
    str=array_of_rnd_strings.join
  end
  benchmark.report do
    str2=""
    array_of_rnd_strings.each do |s|
      str2<<s
    end
  end
  benchmark.report do
    str3=""
    array_of_rnd_strings.each do |s|
      str3+=s
    end
  end
end

The array_of_rnd_strings is an array of 32768 8 characters long random strings.

user	system	total	real
0.030000	0.000000	0.030000	( 0.027184)
0.160000	0.010000	0.170000	( 0.190277)
106.020000	0.300000	106.320000	(113.457793)

The performance of += was even worse than I imagined!

"String concatenation in Ruby" by Jimmy

Tue, 06 Apr 2010 15:10:42 +0200

Sweet

Alice, Bob and Mallory: String concatenation in Ruby

String concatenation in Ruby

"String concatenation in Ruby" by Jimmy