From: Raf Coremans <rrafje@...>
Date: 2007-05-04T04:38:59+09:00
Subject: Re: array slicing

2007/5/3, Josselin <josselin@wanadoo.fr>:
> On 2007-05-03 17:38:57 +0200, "Raf Coremans" <rrafje@gmail.com> said:
>
> > 2007/5/3, Josselin <josselin@wanadoo.fr>:
> >> given an array
> >>
> >> anArray = [["a", 1, 1], ["b", 1, 2], ["c", 1, 5], ["z", 1, 1], ["e", 1,
> >> 5], ["f", 1, 6], ["g", 1, 1], ["h", 1, 3], ["i", 1, 1], ["j", 1, 2],
> >> ["k", 1, 1], ["y", 1, 6], ["m", 1, 1], ["u", 1, 2], ["v", 1, 1], ["p",
> >> 1, 4], ["q", 1, 5], ["r", 1, 1], ["n", 1, 1], ["t", 2, 3], ["s", 2, 2],
> >> ["o", 2, 1], ["w", 2, 1], ["x", 2, 1], ["l", 2, 1], ["d", 2, 1]]
> >>
> >> I would like to get it  sliced in groups of element having the same 2nd
> >> and 3rd values
> >>
> >> sliced_array = [
> >> [ ["a", 1, 1],["g", 1, 1], ["i", 1, 1], ["k", 1, 1], ["m", 1, 1], ["n",
> >> 1, 1],["r", 1, 1],["v", 1, 1],["z", 1, 1] ],
> >>  [ ["b", 1, 2],["j", 1, 2], ["u", 1, 2] ],
> >> [ ["c", 1, 5],["e", 1, 5],["q", 1, 5] ],
> >> [ ["d", 2, 1],["l", 2, 1], ["l", 2, 1], ["o", 2, 1],["w", 2, 1],["x", 2, 1] ],
> >> [ ["f", 1, 6],["y", 1, 6] ],
> >> [ ["h", 1, 3] ],
> >> [ ["p", 1, 4] ],
> >> [ ["s", 2, 2] ],
> >> [ ["t", 2, 3] ]
> >>  ]
> >
> > Applying a trick I learned today:
> >
> > anArray.inject( Hash.new{ |hash, key| hash[key] =[] } ){ |hash,
> > subarray| hash[subarray[1, 2]] << subarray; hash }.values
> >
> > Or if you need a sorted result:
> >
> > anArray.inject( Hash.new{ |hash, key| hash[key] =[] } ){ |hash,
> > subarray| hash[subarray[1, 2]] << subarray; hash }.values.sort_by{
> > |subarray| subarray[0][1,2] }
> >
> >
> > Best regards,
> > Raf
>
> good to know , what about the partition_by , it seems simple ?
>
> module Enumerable
>   def partition_by
>     r = Hash.new{ |h,k| h[k]=[] }
>     each do |e|
>       r[ yield(e) ] << e
>     end
>     return r
>   end
> end
>
> And then :
>
> require 'enumerator'
> sArray = anArray.sort_by {|item| [item[1], item[2] ] }
>
> sArray.partition_by {|ary| ary[1..2]}.to_enum(:each_value).to_a=>
> [[["j", 1, 2], ["u", 1, 2], ["b", 1, 2]], [["l", 2, 1], ["x", 2, 1],
> ["o", 2, 1], ["w", 2, 1], ["d", 2, 1]], [["a", 1, 1], ["z", 1, 1],
> ["v", 1, 1], ["r", 1, 1], ["i", 1, 1], ["m", 1, 1], ["g", 1, 1], ["k",
> 1, 1], ["n", 1, 1]], [["f", 1, 6], ["y", 1, 6]], [["t", 2, 3]], [["c",
> 1, 5], ["e", 1, 5], ["q", 1, 5]], [["s", 2, 2]], [["p", 1, 4]], [["h",
> 1, 3]]]
>
> and that's it.....
>
>
>

Yes. I think it could be improved though:

module Enumerable
 def partition_by
   inject( Hash.new{ |h,k| h[k]=[] } ){ |h, e| h[ yield( e)] << e; h }.values
 end
end

p anArray.partition_by { |e| e[1, 2] } =>
[[["b", 1, 2], ["j", 1, 2], ["u", 1, 2]], [["o", 2, 1], ["w", 2, 1],
["x", 2, 1], ["l", 2, 1], ["d", 2, 1]], [["a", 1, 1], ["z", 1, 1],
["g", 1, 1], ["i", 1, 1], ["k", 1, 1], ["m", 1, 1], ["v", 1, 1], ["r",
1, 1], ["n", 1, 1]], [["f", 1, 6], ["y", 1, 6]], [["t", 2, 3]], [["c",
1, 5], ["e", 1, 5], ["q", 1, 5]], [["s", 2, 2]], [["p", 1, 4]], [["h",
1, 3]]]

Or if you want it sorted:
p anArray.sort.partition_by{ |subarray| subarray[1, 2] }.sort_by{ |e|
e[0][1,2] } => [[["a", 1, 1], ["g", 1, 1], ["i", 1, 1], ["k", 1, 1],
["m", 1, 1], ["n", 1, 1], ["r", 1, 1], ["v", 1, 1], ["z", 1, 1]],
[["b", 1, 2], ["j", 1, 2], ["u", 1, 2]], [["h", 1, 3]], [["p", 1, 4]],
[["c", 1, 5], ["e", 1, 5], ["q", 1, 5]], [["f", 1, 6], ["y", 1, 6]],
[["d", 2, 1], ["l", 2, 1], ["o", 2, 1], ["w", 2, 1], ["x", 2, 1]],
[["s", 2, 2]], [["t", 2, 3]]]


Regards,
Raf