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Why Array#length is better than Array#size

Recently Luke and I spotted a subtle bug in our code where we were mistakenly assuming a method returned an Array when it actually returned a Fixnum. We were then calling the #size method on the result, but interestingly Fixnum#size returns the number of bytes in its machine representation.

Previously I had noticed that my colleague Paul Battley tended to use Array#length in preference to Array#size. Now I know why, or at least I know why I’m going to start using Array#length! If we’d used #length in this case we would have seen a NoMethodError instead of the spurious value, 4.

  [5,6].size # => 2
  1.size # => 4
  [5,6].length # => 2
  1.length # => NoMethodError: undefined method `length' for 1:Fixnum

Update: Josh Susser has posted a useful related article about the use of…