From: "David A. Black" <dblack@...>
Date: 2005-07-16T11:53:23+09:00
Subject: Re: Cascading <=> comparisons

Hi --

On Sat, 16 Jul 2005, Garance A Drosehn wrote:

> Let's say I have a hash with some values in it, and I want to
> print out that hash in some sort order.  I can:
>
> korder = vals.keys.sort { |ka, kb|
>    vals[kb] <=> vals[ka]
> }
> korder.each { |key| printf " %3d - %s\n", vals[key], key }
>
> That's pretty easy, but what if I want to cascade comparisons
> in that sort, such that if the data-values are equal, then I want
> the sort order to be based on some other comparison?  It
> seems to me that I have to:
>
> korder = vals.keys.sort { |ka, kb|
>    sres = vals[kb] <=> vals[ka]
>    sres = ka <=> kb    if sres == 0
>    sres
> }
> korder.each { |key| printf " %3d - %s\n", vals[key], key }

You can use sort_by and put your sort keys in an array.  For example,
this sorts a hash by value and then by key:

   hash = { "a" => 1, "b" => 4, "c" => 3, "d" => 1 }
   p hash.sort_by {|k,v| [v,k] }
     => [["a", 1], ["d", 1], ["c", 3], ["b", 4]]


David

-- 
David A. Black
dblack@wobblini.net