Gist shows benchmark code for adding prefix to string in Ruby

benchmark_prefix_for_string.rb

require "benchmark/ips"

def use_array_join
  str = "world"
  prefix = "Hello, "
  [prefix, str].join # => "Hello, world"
end

def use_insert
  str = "world"
  str.insert(0, "Hello, ") # => Hello, world
end

def use_squarebrackets1
  str = "world"
  str[/\A/] = "Hello, " # => "Hello, "
  str                   # => "Hello, world"
end

def use_squarebrackets2
  str = "world"
  str[0,0] = "Hello, " # => "Hello, "
  str                  # => "Hello, world"
end

def use_prepend
  str = "world"
  str.prepend("Hello, ") # => "Hello, world"
end

Benchmark.ips do |x|
  x.report("Array#join")       { use_array_join }
  x.report("String#insert")    { use_insert }
  x.report('String#[\/A\]')    { use_squarebrackets1 }
  x.report("String#[0,0]")     { use_squarebrackets2 }
  x.report("String#prepend")   { use_prepend }
  x.compare!
end

What about that?

def use_arrows
  str = "world"
  "Hello, " << str # => "Hello, world"
end

Comparison:
           String#<<:  2836395.5 i/s
       String#insert:  2599252.8 i/s - 1.09x slower
      String#prepend:  2504770.5 i/s - 1.13x slower
        String#[0,0]:  2407843.9 i/s - 1.18x slower
          Array#join:  1244613.4 i/s - 2.28x slower
       String#[\/A\]:   710030.8 i/s - 3.99x slower

Also:

def use_arr_index
  str = "world"
  str[0] = "Hello, " # => "Hello, world"
end