ruby: sum corresponding members of two or more arrays

Question

I've got two (or more) arrays with 12 integers in each (corresponding to values for each month). All I want is to add them together so that I've got a single array with summed values for each month. Here's an example with three values: [1,2,3] and [4,5,6] => [5,7,9]

The best I could come up with was:

[[1,2,3],[4,5,6]].transpose.map{|arr| arr.inject{|sum, element| sum+element}} #=> [5,7,9]

Is there a better way of doing this? It just seems such a basic thing to want to do.

Answer 1

Here's the transpose version Anurag suggested:

[[1,2,3], [4,5,6]].transpose.map {|x| x.reduce(:+)}

This will work with any number of component arrays. reduce and inject are synonyms, but reduce seems to me to more clearly communicate the code's intent here...

Answer 2

Now we can use sum in 2.4

nums = [[1, 2, 3], [4, 5, 6]]
nums.transpose.map(&:sum) #=> [5, 7, 9]

Answer 3

For clearer syntax (not the fastest), you can make use of Vector:

require 'matrix'
Vector[1,2,3] + Vector[4,5,6]
=> Vector[5, 7, 9]

For multiple vectors, you can do:

arr = [ Vector[1,2,3], Vector[4,5,6], Vector[7,8,9] ]
arr.inject(&:+)
=> Vector[12, 15, 18]

If you wish to load your arrays into Vectors and sum:

arrays = [ [1,2,3], [4,5,6], [7,8,9] ]
arrays.map { |a| Vector[*a] }.inject(:+)
=> Vector[12, 15, 18]

Answer 4

here's my attempt at code-golfing this thing:

// ruby 1.9 syntax, too bad they didn't add a sum() function afaik
[1,2,3].zip([4,5,6]).map {|a| a.inject(:+)} # [5,7,9]

zip returns [1,4], [2,5], [3,6], and map sums each sub-array.