From: Rob Biedenharn <Rob@...>
Date: 2010-10-13T22:58:45+09:00
Subject: Re: sort_by: multiple fields with reverse sort


On Oct 13, 2010, at 8:53 AM, Rahul Kumar wrote:

> When i sat down to code, i realized that i could not just put the
> sort_keys list to sort_by, i would have to unroll the sort_keys array.
> So, i might as well use sort. This seems to work in the small case i
> have. Could someone comment on this, and suggest cleaner better way to
> do. Thanks.
>
> b=[ ["radio", 30, 5], ["radio", 20, 5], ["archie", 20, 5], ["newton",
> 10, 3] ]
> sort_keys=[1,0]
> rev_flag = [true, false]
>
> c=b.sort{|x,y|
>  res = 0
>  sort_keys.each_with_index { |e,i|
>    if rev_flag[i]
>      res = y[e] <=> x[e]
>    else
>      res = x[e] <=> y[e]
>    end
>    break if res != 0
>  }
>  res
> }
> c
>
> --  
> Posted via http://www.ruby-forum.com/.
>


While it is not strictly equivalent to array_of_strings.sort.reverse,  
this little addition to String allows array_of_strings.sort_by{|s|-s}

class String
   RAB_REPLACE = ('a'..'z').to_a.reverse.join.freeze
   def -@
     self.tr('a-z',RAB_REPLACE)
   end
end

Then you can do:

b.sort_by {|ary| [ary[1], -ary[0]] }

irb> b.sort_by {|ary| [ary[1], -ary[0]] }
=> [["newton", 10, 3], ["radio", 20, 5], ["archie", 20, 5], ["radio",  
30, 5]]

If your strings are always simple words, then this may be enough for  
you (or at least give you an idea).

If your array is short (i.e., the difference between sort and sort_by  
is acceptable), then you can always do:

irb> b.sort {|e1,e2| (e1[1] <=> e2[1]).nonzero? || e2[0] <=> e2[0] }
=> [["newton", 10, 3], ["radio", 20, 5], ["archie", 20, 5], ["radio",  
30, 5]]

( which doesn't rely on String#-@ )

-Rob

Rob Biedenharn		
Rob@AgileConsultingLLC.com	http://AgileConsultingLLC.com/
rab@GaslightSoftware.com		http://GaslightSoftware.com/