Reverse a string in Ruby

There’s already an inplace reverse method, called “reverse!”:

$ a = "abc"
$ a.reverse!
$ puts a
cba

If you want to do this manually try this (but it will probably not be multibyte-safe, eg UTF-8), and it will be slower:

class String
  def reverse_inplace!
    half_length = self.length / 2
    half_length.times {|i| self[i], self[-i-1] = self[-i-1], self[i] }
    self
  end
end

This swaps every byte from the beginning with every byte from the end until both indexes meet at the center:

$ a = "abcd"
$ a.reverse_inplace!
$ puts a
dcba