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I need to sum values in an array hashes and I found a way to do it here

but it sure seems like there should be a more elegant way in Ruby.

Here is what works;

sales = [{"sale_price"=>210000, "deed_type"=>"Warranty Deed"}, {"sale_price"=>268300, "deed_type"=>"Warranty Deed Joint"}]

total_sales = sales.inject(0) {|sum, hash| sum + hash["sale_price"]}

The totals line is not very readable. It would be nice if something like this worked;

total_sales = sales.sum("sale_price")

Is this just wishful thinking or am I overlooking a better solution?

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    Its a correct way to do it. Crude way you can do: sum = 0;sales.each{|x| sum=sum+x["sale_price"]};sum Commented Feb 1, 2013 at 20:51
  • This belongs on codereview.stackexchange.com Commented Feb 1, 2013 at 21:06
  • Thanks for the tip Tin Man, I wasn't aware of the code review site. Commented Feb 1, 2013 at 21:10
  • "It would be nice if something like this worked". This is Ruby, add the Enumerable#sum you'd like and problem solved. Maybe you should call it Enumerable#hash_sum though, Ruby is a OOP language and sum should be calling methods. Commented Feb 1, 2013 at 21:16

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I like using the map/reduce metaphor like so:

total_sales = sales.map {|s| s['sale_price']}.reduce(0, :+)

The reduce method is a synonym for the inject method, I find the name inject to be somewhat confusing with the memo component. It has another form I use above to take the initial value and a reference to a method call used for the combination/reduction process.

I think the overall pattern of mapping the values and then reducing them to an aggregate is well known and self-documenting.

EDIT: Use symbol :+ instead of proc reference &:+

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6 Comments

Great answer. The Symbol.to_proc (&:) used as the second reduce argument is around an order-of-magnitude slower than just writing a plain block .reduce(0){|t,i| t + i }, only worth noting if your array may contain large numbers of elements. See igvita.com/2008/07/08/6-optimization-tips-for-ruby-mri #5.
Wow, good to know. It's amazing what you find when you profile code or dig under the covers. Thanks for the info.
Thanks for the explanation. I am still trying to get my head around map/reduce which is probably why the intent of the code is not intuitive to me. Do you have any recommendations for a good explanation?
The way I read the code is: map a list of sales objects onto a list of sale prices and then reduce them by combining the prices into a total.
Under Rails, you can just go sales.map {|s| s['sale_price']}.sum. If s was an object with a sale_price method, it could be even terser: sales.sum(&:sale_price) :D
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You can make it work:

sales = [{"sale_price"=>210000, "deed_type"=>"Warranty Deed"}, {"sale_price"=>268300, "deed_type"=>"Warranty Deed Joint"}]

def sales.sum(by)
  inject(0){|sum, h| sum + h[by]}
end

p sales.sum("sale_price") #=> 478300

Note this sum method (sum_by might be a better name) is not defined on Array, but only on the specific sales array.

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