From: "Ara.T.Howard" <Ara.T.Howard@...>
Date: 2005-07-16T12:34:18+09:00
Subject: Re: Cascading <=> comparisons

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 }
>
> In perl, I can just casade the '<=>' comparisons, because it
> will treat the zero value as "false".  Thus, I could get away
> with a one-liner somewhat similar to:
>
> vals[kb] <=> vals[ka] or a <=> kb

[vals[kb], ka] <=> [vals[ka], kb]

note that the array approach scales for any number of things, not only two.
you can always do

   [a,b,c] <=> [x,y,z]

even if a, b, and c are different types.  so long as you can

   a <=> x
   b <=> y
   c <=> z

hth.

-a
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