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rubystringin-place

How to write an in-place method for strings

I want to write a method to trim characters from the end of a string. This is simple enough to do:

class String
  def trim(amount)
    self[0..-(amount+1)]
  end
end

my_string = "Hello"
my_string = my_string.trim(1) # => Hell

I would rather have this be an in-place method. The naive approach, 

class String
  def trim(amount)
    self[0..-(amount+1)]
  end

  def trim!(amount)
    self = trim(amount)
  end
end

throws the error "Can't change the value of self: self = trim(amount)".

What is the correct way of writing this in-place method? Do I need to manually set the attributes of the string? And if so, how do I access them?

Solution

  • Using String#[]=

    class String
  def trim(amount)
    self[0..-1] = self[0..-(amount+1)]
  end
end

s = 'asdf'
s.trim(2) # => "as"
s # => "as"