A Ruby reject! that returns the rejected items

Jamie Hill July 25, 2007

Hash
Reject
Ruby

I often need to do a reject! and return the rejected items instead of the modified collection. This saves having to do a select beforehand.
An example of how accomplish this:

options = { :a =&gt; 1, :b =&gt; 2, :c =&gt; 3 }
rejects = Hash[*options.select { |k, v| k == :b && options.delete(k) }.flatten]
assert_equal { :a =&gt; 1, :c =&gt; 3 }, options
assert_equal { :b =&gt; 2 }, rejects

This could be written as a method of the Hash class and an alternative for Array.
For Hash, the code would look something like:

class Hash
  def extract!
    Hash[*self.select { |k, v| yield(k, v) && self.delete(k) }.flatten]
  end
end
options = { :a =&gt; 1, :b =&gt; 2, :c =&gt; 3 }
rejects = options.extract! { |k, v| k == :b }
assert_equal { :a =&gt; 1, :c =&gt; 3 }, options
assert_equal { :b =&gt; 2 }, rejects

If I am missing something obvious in Ruby that accomplishes the same, please leave a comment.