Group hashes by keys and sum the values

Question

I have an array of hashes:

[{\"Vegetable\"=>10}, {\"Vegetable\"=>5}, {\"Dry Goods\"=>3>}, {\"Dry Goods\"=>2}]

I need to use

Answer 1

If have two hashes with multiple keys:

h1 = { "Vegetable" => 10, "Dry Goods" => 2 }
h2 = { "Dry Goods" => 3, "Vegetable" => 5 }
details = {}
(h1.keys | h2.keys).each do |key|
  details[key] = h1[key].to_i + h2[key].to_i
end
details

Answer 2

ar = [{"Vegetable"=>10}, {"Vegetable"=>5}, {"Dry Goods"=>3}, {"Dry Goods"=>2}]

While the Hash.merge technique works fine, I think it reads better with an inject:

ar.inject({}) { |memo, subhash| subhash.each { |prod, value| memo[prod] ||= 0 ; memo[prod] += value } ; memo }
=> {"Dry Goods"=>5, "Vegetable"=>15}

Better yet, if you use Hash.new with a default value of 0:

ar.inject(Hash.new(0)) { |memo, subhash| subhash.each { |prod, value| memo[prod] += value } ; memo }
=> {"Dry Goods"=>5, "Vegetable"=>15}

Or if inject makes your head hurt:

result = Hash.new(0)
ar.each { |subhash| subhash.each { |prod, value| result[prod] += value } }
result
=> {"Dry Goods"=>5, "Vegetable"=>15}

Answer 3

I'm not sure that a hash is what you want here, because I don't multiple entries in each hash. so I'll start by changing your data representation a little.

ProductCount=Struct.new(:name,:count)
data = [ProductCount.new("Vegetable",10),
        ProductCount.new("Vegetable",5),
        ProductCount.new("Dry Goods",3),
        ProductCount.new("Dry Goods",2)]

If the hashes can have multiple key-value pairs, then what you probably want to do is

data = [{"Vegetable"=>10}, {"Vegetable"=>5}, {"Dry Goods"=>3>}, {"Dry Goods"=>2}]
data = data.map{|h| h.map{|k,v| ProductCount.new(k,v)}}.flatten

Now use the facets gem as follows

require 'facets'
data.group_by(&:name).update_values{|x| x.map(&:count).sum}

The result is

{"Dry Goods"=>5, "Vegetable"=>15}

Answer 4

Simply use:

array = [{"Vegetable"=>10}, {"Vegetable"=>5}, {"Dry Goods"=>3}, {"Dry Goods"=>2}]
array.inject{|a,b| a.merge(b){|_,x,y| x + y}}

Answer 5

ar = [{"Vegetable"=>10}, {"Vegetable"=>5}, {"Dry Goods"=>3}, {"Dry Goods"=>2}]
p ar.inject{|memo, el| memo.merge( el ){|k, old_v, new_v| old_v + new_v}}
#=> {"Vegetable"=>15, "Dry Goods"=>5}

Hash.merge with a block runs the block when it finds a duplicate; inject without a initial memo treats the first element of the array as memo, which is fine here.