From: Mike Fletcher <lemurific+rforum@...>
Date: 2005-12-13T23:56:38+09:00
Subject: Re: stable sort_by?

Frederick Ros wrote:
>  h.sort_by {|k,v| [v[:a],v[:b],v[:c],v[:d]]}.reverse

OK, this or something like it should be in the rdoc for sort_by.  I was 
doing something similar and wound up writing a similar sort with 
multiple if tests like the OP because I didn't remember that Array 
implements a sane <=>.

Also, would there be a similar cool way to do this in one step where you 
want a mix of ascending and descending sorts on different parts?  Say I 
had a hash (in YAML):

Bobby:
  age: 11
  lastname: Smith
Suzy:
  age: 13
  lastname: Jones
Ted:
  age 12
  lastname: Smith

And I wanted to sort
  - alphabetically by lastname
  - then age oldest to youngest

So in Perl I'd do something like:

my @sorted_keys = sort { $a->{lastname} cmp $b->{lastname}
                                        || $b->{age} <=> $a->{age} } 
keys %data;

In this case I could use

sorted_keys = data.sort_by { |k,v| [ v["lastname"], -1*v["age"] ] }

But what if I wanted reverse alphabetically by lastname (Z-A)?  In perl 
I'd swap $a and $b in the first comparison, but I can't think of a 
spiffy way to do the same using sort_by.

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