Ruby: summing up values within an array of hashes between certain dates

Question

I have an array of hashes in Ruby:

array = [
  {:date => Wed, 04 May 2011 00:00:00 PDT -07:00,
   :value => 200}
  {:date => Wed, 04 May 2011 01:00:00 PDT -07:00,
   :value => 100}
  {:date => Tue, 03 May 2011 01:00:00 PDT -07:00,
   :value => 300}
  {:date => Tue, 03 May 2011 01:00:00 PDT -07:00,
   :value => 150}
]

I'd like to be able to combine the values within each day so that I have a new array like this:

array = [
  {:date => Wed, 04 May 2011 00:00:00 PDT -07:00,
   :value => 300}
  {:date => Tue, 03 May 2011 00:00:00 PDT -07:00,
   :value => 450}
]

What's the most elegant way to search the array by day and sum up the values for each day?

This is what I initially tried:

entries = [
  {:date => Wed, 04 May 2011 00:00:00 PDT -07:00,
   :value => 200}
  {:date => Wed, 04 May 2011 01:00:00 PDT -07:00,
   :value => 100}
  {:date => Tue, 03 May 2011 01:00:00 PDT -07:00,
   :value => 300}
  {:date => Tue, 03 May 2011 01:00:00 PDT -07:00,
   :value => 150}
]

first_day = 29.days.ago.beginning_of_day
total_days = 30

day_totals = (0...total_days).inject([]) do |array, num|
    startat = first_day + num.day
    endat = startat.end_of_day

    total_value_in_day = entries.where("date >= ? and date <= ?", startat, endat).sum(:value)

    array << {:date => startat, :value => total_value_in_day}
end

I realized my mistake was with the where method which is a Rails method for searching objects, and can't be used on arrays. So my main question, is there a way to search arrays or hashes with conditions.

The most elegant way really is for you to write some code yourself, if you don't mind my being elegantly blunt. Elegance in code is a value, and it is good to reach for it, but you cannot reach mastery without (painful) exercise. Perhaps read a good book — sehe
– sehe, Commented May 4, 2011 at 20:44
edited the post to show the code I tried on my own, and where I was stuck. — Chanpory
– Chanpory, Commented May 4, 2011 at 20:55

Dylan Markow · Accepted Answer · 2011-05-04 20:59:21Z

5

You can iterate over the entries to create a new array:

totals = Hash.new(0)
array.each do |entry|
  totals[entry[:date]] += entry[:value]
end

# Now totals will be something like this:
# => {"Wed, 04 May 2011" => 300, "Tue, 03 May 2011" => 450...}

# If you then want this in the same array format you started with:
new_array = totals.collect{ |key, value| {:date => key, :value => value} }
# => [{:date => "Wed, 04 May 2011", :value => 300}, {....}]

answered May 4, 2011 at 20:59

Dylan Markow

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1 Comment

Chanpory Over a year ago

This worked great! I also had to use entry[:date].beginning_of_day to add up entries within varying times of the day. Thanks!

Michael Kohl · Accepted Answer · 2011-05-04 21:09:00Z

3

For 1.9.2:

>> array.each_with_object(Hash.new(0)) { |el, hash| hash[el[:date]] += el[:value] } 
#=> {"Wed, 04 May 2011 00:00:00 PDT -07:00"=>200, "Wed, 04 May 2011 01:00:00 PDT -07:00"=>100, "Tue, 03 May 2011 01:00:00 PDT -07:00"=>450}

Also works with 1.8:

>> array.inject(Hash.new(0)) { |hash, el| hash[el[:date]] += el[:value] ; hash } 
#=> {"Wed, 04 May 2011 00:00:00 PDT -07:00"=>200, "Wed, 04 May 2011 01:00:00 PDT -07:00"=>100, "Tue, 03 May 2011 01:00:00 PDT -07:00"=>450}

answered May 4, 2011 at 21:09

Michael Kohl

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