From: Robert Klemme <shortcutter@...>
Date: 2007-08-24T22:52:15+09:00
Subject: Re: Combining Array Elements

2007/8/24, Yossef Mendelssohn <ymendel@pobox.com>:
> On Aug 24, 7:50 am, gregarican <greg.kuj...@gmail.com> wrote:
> > I have an array that I would like to combine elements. Here's a sample
> > array:
> >
> > [["Value A", "Value B", 3], ["Value A", "Value C", 2],["Value A",
> > Value B", 1]]
> >
> > What I would like to do is find the elements where the first two items
> > are the same and then combine the third. The resulting array would
> > consist of:
> >
> > [["Value A", "Value B", 4],["Value A", "Value C", 2]]
> >
> > This is something that is out in left field in terms of how I've used
> > arrays in the past. Anyone know of a quick bit of script I can whip up
> > that will suit the task?
>
>
> irb(main):001:0> orig_array = [["Value A", "Value B", 3], ["Value A",
> "Value C", 2],["Value A", "Value B", 1]]
> => [["Value A", "Value B", 3], ["Value A", "Value C", 2], ["Value A",
> "Value B", 1]]
> irb(main):002:0> hash = Hash.new(0)
> => {}
> irb(main):003:0> orig_array.each { |elem|  hash[ elem[0,2] ] +=
> elem[2] }
> => [["Value A", "Value B", 3], ["Value A", "Value C", 2], ["Value A",
> "Value B", 1]]
> irb(main):004:0> hash.collect { |k, v|  k + [v] }
> => [["Value A", "Value C", 2], ["Value A", "Value B", 4]]
>
>
> or, more concisely
>
>
> irb(main):001:0> orig_array = [["Value A", "Value B", 3], ["Value A",
> "Value C", 2],["Value A", "Value B", 1]]
> => [["Value A", "Value B", 3], ["Value A", "Value C", 2], ["Value A",
> "Value B", 1]]
> irb(main):002:0> orig_array.inject(Hash.new(0)) { |hash, elem|
> hash[ elem[0,2] ] += elem[2]; hash }.collect { |k, v|  k + [v] }
> => [["Value A", "Value C", 2], ["Value A", "Value B", 4]]

I believe there is an even simpler solution:

irb(main):001:0> arr=[["Value A", "Value B", 3], ["Value A", "Value
C", 2], ["Value A", "Value B", 1]]
=> [["Value A", "Value B", 3], ["Value A", "Value C", 2], ["Value A",
"Value B", 1]]

irb(main):003:0> arr.inject(Hash.new(0)) {|ha,(a,b,c)| ha[[a,b]]+=c;ha}
=> {["Value A", "Value C"]=>2, ["Value A", "Value B"]=>4}

irb(main):004:0> arr.inject(Hash.new(0)) {|ha,(a,b,c)|
ha[[a,b]]+=c;ha}.inject([]) {|re,v| re<<v.flatten}
=> [["Value A", "Value C", 2], ["Value A", "Value B", 4]]

Of course, the most elegant solution uses #inject - in this case two
injects. :-)

Although, this one might be even better:

irb(main):007:0> arr.inject(Hash.new(0)) {|ha,(a,b,c)|
ha[[a,b]]+=c;ha}.map {|x| x.flatten}
=> [["Value A", "Value C", 2], ["Value A", "Value B", 4]]

At least 1 #inject. :-)

Kind regards

robert