Rails sum hashes inside an array

Question

Basically I have an array of hashes like so :

[
  { :id => 20, :total => 1, :total2 => 0 },
  { :id => 21, :total => 1, :total2 => 0 },
  { :id => 22, :total => 2, :total2 => 0 },
  { :id => 23, :total => 1, :total2 => 0 },
  { :id => 20, :total => 1, :total2 => 0 },
  { :id => 21, :total => 1, :total2 => 0 },
  { :id => 22, :total => 1, :total2 => 1 },
  { :id => 23, :total => 1, :total2 => 0 }
]

I want the array to sum the last two hash columns like so, keeping the first (:id) as an identifier:

[
  { :id => 20, :total => 2, :total2 => 0 },
  { :id  => 21, :total => 2, :total2 => 0 },
  { :id  => 22, :total => 3, :total2 => 1 }
]

I have looked around and it seems that the .inject() method is used in this instance but I cannot really figure out the syntax/how to use this.

What I am looking for is to keep the first column (:id) as an ID field; if there is another hash with this ID, like in my example above, the two hashes should be added together.

MrYoshiji · Accepted Answer · 2013-08-22 15:52:03Z

1

Can you try this?

array = [{:stemp=>20, :vtotal=>1, :avg=>0}, {:stemp=>21, :vtotal=>1, :avg=>0},{:stemp=>22, :vtotal=>2, :avg=>0}, {:stemp=>23, :vtotal=>1, :avg=>0},  {:stemp=>20, :vtotal=>1, :avg=>0}, {:stemp=>21, :vtotal=>1, :avg=>0}, {:stemp=>22, :vtotal=>1, :avg=>1}, {:stemp=>23, :vtotal=>1, :avg=>0}]

result = array.group_by{|h| h[:stemp]}.map do |stemp, hashes|
  { stemp: stemp, vtotal: hashes.map{|h| h[:vtotal]}.inject(:+), avg: hashes.map{|h| h[:avg]}.inject(:+) }
end

Just copy-pasted it in the IRB console with ruby 1.9.3, outputs this:

[
  {:stemp=>20, :vtotal=>2, :avg=>0},
  {:stemp=>21, :vtotal=>2, :avg=>0},
  {:stemp=>22, :vtotal=>3, :avg=>1},
  {:stemp=>23, :vtotal=>2, :avg=>0}
]

answered Aug 22, 2013 at 15:52

MrYoshiji

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Comments

Benjamin Manns · Accepted Answer · 2013-08-22 15:55:53Z

I reformatted the data in the question and in my answer so that it is a little easier for others to see what is going on.

data = [
  { :stemp => 20, :vtotal => 1, :avg => 0 },
  { :stemp => 21, :vtotal => 1, :avg => 0 },
  { :stemp => 22, :vtotal => 2, :avg => 0 },
  { :stemp => 23, :vtotal => 1, :avg => 0 },
  { :stemp => 20, :vtotal => 1, :avg => 0 },
  { :stemp => 21, :vtotal => 1, :avg => 0 },
  { :stemp => 22, :vtotal => 1, :avg => 1 },
  { :stemp => 23, :vtotal => 1, :avg => 0 }
]

First, group your hashes by the stemp.

data = data.group_by { |datum| datum[:stemp] }

Then iterate over each stemp and its entries.

data = data.map do |stemp, entries|
  # this pulls out each hash's :vtotal entry and then combines it with the + operator
  vtotal = entries.map { |entry| entry[:vtotal] }.inject(&:+)
  # this does the same as above, but for the avg entry
  avg = entries.map { |entry| entry[:avg] }.inject(&:+)
  { :stemp => stemp, :vtotal => vtotal, :avg => avg }
end

This outputs

[
  { :stemp => 20, :vtotal => 2, :avg => 0 },
  { :stemp => 21, :vtotal => 2, :avg => 0 },
  { :stemp => 22, :vtotal => 3, :avg => 1 },
  { :stemp => 23, :vtotal => 2, :avg => 0 }
]

Kyle · Accepted Answer · 2013-08-22 16:05:17Z

0

This solution suffers from readability but I wanted to provide it for reference.

Hash#merge accepts a block that will be executed when a colliding key is found.

arr = [ {:id => 20, :total => 1, :total2 => 0} ... ]

arr.group_by{ |h| h[:id] }.map do |_, hash|
  hash.reduce do |hash_a, hash_b|
    hash_a.merge(hash_b){ |key, old, new| key == :id ? old : old + new }
  end
end

answered Aug 22, 2013 at 16:05

Kyle

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Comments

joe nayyar · Accepted Answer · 2013-08-22 16:25:27Z

0

This one also works

arr.group_by{|t| t[:stemp]}.map {|key, value| value.inject({}) { |hash, values| values.merge(hash){ |key, v1, v2| key == :stemp ? v1 : v1+v2  }}}

change to :id

arr.group_by{|t| t[:id]}.map {|key, value| value.inject({}) { |hash, values| values.merge(hash){ |key, v1, v2| key == :id ? v1 : v1+v2  }}}

answered Aug 22, 2013 at 16:25

joe nayyar

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Comments

Nikolay Kostov · Accepted Answer · 2016-12-22 09:53:17Z

0

[{"4"=>"20.0"}, {"4"=>"20.0"}, {"4"=>"10.0"}, {"4"=>"10.0", "5"=>"10.0"}, {"4"=>"10.0", "5"=>"0.0"}, {"4"=>"10.0"}, {"4"=>"10.0"}, {"4"=>"0.0", "5"=>"10.66"}, {"4"=>"20.0"}, {"4"=>"10.0"}, {"4"=>"10.0"}, {"4"=>"0.0"}].map{|m| m.map{|k,v| v.to_f}.sum()}.sum()

edited Dec 22, 2016 at 9:53

Nikolay Kostov

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answered Dec 22, 2016 at 9:29

Ankit Varshney

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