Alice, Bob and Mallory

String concatenation in Ruby

Posted by Jonas Elfström Mon, 01 Feb 2010 23:04:00 GMT

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.

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require 'benchmark'

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

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

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