From: "Peña, Botp" <botp@...>
Date: 2007-08-25T15:29:33+09:00
Subject: Re: Combining Array Elements

From: Stefan Rusterholz [mailto:apeiros@gmx.net] 
# What makes me slightly sad is that all those examples try to 
# squeeze as much as possible into a single statement. Frankly, 
# in my opinion the resulting code is horrible to read again a few
# months later.

# array  = [["Value A", "Value B", 3], ["Value A", "Value C", 
# 2],["Value A", "Value B", 1]]
# groups = array.group_by { |element| element.first(2) }
# result = groups.map { |key, values|
#   values.map! { |a,b,c| c } 
#   key+[values.sum]
# }

glad you mention group_by :)  anyone who's been making reports and transposing rows/columns can see its value.

i think we can still squeeze to one line and still be readable, now that you mention group_by.

note that i modified your sum. i like passing what i want to add.

eg,

> a
=> [["A", "B", 3], ["A", "C", 2], ["A", "B", 1]]
> a.group_by{|x|x.first(2)}
=> {["A", "B"]=>[["A", "B", 3], ["A", "B", 1]], ["A", "C"]=>[["A", "C", 2]]}

basically, we get the key, then merge the sum of the last elements
thus,

> a.group_by{|x|x.first(2)}.map{|x,y| x<<y.sum{|z|z.last}}
=> [["A", "B", 4], ["A", "C", 2]]

or

> a.group_by{|x|x.first(2)}.map{|x,y| x+[y.sum{|z|z.last}]}
=> [["A", "B", 4], ["A", "C", 2]]

or just use plain inject instead of sum

> a.group_by{|x|x.first(2)}.map{|x,y|x<<y.inject(0){|sum,z|sum+z.last}}
=> [["A", "B", 4], ["A", "C", 2]]

kind regards -botp