arrays,ruby,csv , How to pivot array into another array in Ruby


How to pivot array into another array in Ruby

Question:

Tag: arrays,ruby,csv

I have a multidimensional array like this one :

myArray = [["Alaska","Rain","3"],["Alaska","Snow","4"],["Alabama","Snow","2"],["Alabama","Hail","1"]]

I would like to end up with CSV output like this.

State,Snow,Rain,Hail
Alaska,4,3,nil
Alabama,2,nil,1

I know that to get this outputted to CSV the way I want it I have to have output array like this:

outputArray =[["State","Snow","Rain","Hail"],["Alaska",4,3,nil],["Alabama",2,nil,1]]

but I don't know how to get to this stage. I've tried using group_by but with no success.


Answer:

Here is a way using an intermediate hash-of-hash

The h ends up looking like this

{"Alaska"=>{"Rain"=>"3", "Snow"=>"4"}, "Alabama"=>{"Snow"=>"2", "Hail"=>"1"}}

myArray = [["Alaska","Rain","3"],["Alaska","Snow","4"],["Alabama","Snow","2"],["Alabama","Hail","1"]]
myFields = ["Snow","Rain","Hail"]

h = Hash.new{|h, k| h[k] = {}} 
myArray.each{|i, j, k| h[i][j] = k }
p [["State"] + myFields] + h.map{|k, v| [k] + v.values_at(*myFields)}

output

[["State", "Snow", "Rain", "Hail"], ["Alaska", "4", "3", nil], ["Alabama", "2", nil, "1"]]

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