From: Rick DeNatale <rick.denatale@...>
Date: 2007-03-20T07:43:57+09:00
Subject: Re: Sorting Dates and Times in an array

On 3/19/07, Phrogz <gavin@refinery.com> wrote:

> If you write:
>   my_array.sort{ |a,b| a.long_calc <=> b.long_calc }
> then you might have to perform that complex calculation 1,000,000
> times to sort your array. Less than ideal.
>
> If instead you write:
>   my_array.sort{ |person| person.long_calc }
> then Ruby will first loop through your items and run long_calc for
> each one, saving the values. Exactly 1,000 calls to long_calc. THEN it
> will use those values to sort your original array, able to quickly
> compare the saved result for each.
>
>
> That's the performance reason to use sort_by. I personally use it
> because it's much less typing. :)


And of course, as the documentation for sort_by points out, it's not
always wise to use it for performance reasons.  According to the doc

a = (1..100000).map {rand(100000)}

a.sort runs an order of magnitude faster than:
a.sort_by {|a| a}

It's also not in general easy to come up with a sort_by block which
does the kind of multi-attribute sort with different sort orders for
each attribute that the OP was looking for.

-- 
Rick DeNatale

My blog on Ruby
http://talklikeaduck.denhaven2.com/