From: Su Zhang <su.comp.lang.ruby@...>
Date: 2011-07-02T00:44:27+09:00
Subject: Re: How to order Structs based on two fields

On 7/1/2011 7:30 AM, I単aki Baz Castillo wrote:
> I'll do some benchmarks and select the fastest one.

The efficiency depends on how expensive it is to compute the key of the 
elements. If the computation is non-trivial, then Array#sort_by would be 
much faster because it caches the keys before the sort so that no 
unnecessary computation is done (i.e., the key of each element is 
computed exactly once).

If, like it is in your case, the computation only involves several 
Fixnum comparisons, Array#sort_by will most likely be slower as it needs 
to allocate (cache) the key set prior to the actual sorting.

For example, there should be a difference if the accessors themselves 
are expensive. Something like:

   def a
     sleep 0.01
     @a
   end

As a side note, this idiom used by Array#sort_by is known as the 
Schwartzian transform in the Perl world.