What's the efficient way to multiply two arrays and get sum of multiplied values in Ruby?

Question

What\'s the efficient way to multiply two arrays and get sum of multiply values in Ruby? I have two arrays in Ruby:

array_A = [1, 2, 1, 4, 5, 3, 2, 6, 5, 8, 9]
a         


        

Answer 1

EDIT: Vector is not fastest (Marc Bollinger is totally right).

Here is the modified code with vector and n-times:

require 'benchmark'
require 'matrix'

array_A = [1, 2, 1, 4, 5, 3, 2, 6, 5, 8, 9]
array_B = [3, 2, 4, 2, 5, 1, 3, 3, 7, 5, 4]

vector_A = Vector[*array_A]
vector_B = Vector[*array_B]

def matrix_method a1, a2
  (Matrix.row_vector(a1) * Matrix.column_vector(a2)).element(0,0)
end

def vector_method a1, a2
  a1.inner_product(a2)
end

n = 100000

Benchmark.bmbm do |b|
  b.report('matrix method') { n.times { matrix_method(array_A, array_B) } }
  b.report('array walking') { n.times { (0...array_A.count).to_a.inject(0) {|r, i| r + array_A[i]*array_B[i]} } }
  b.report('array walking without to_a') { n.times { (0...array_A.count).inject(0) {|r, i| r + array_A[i]*array_B[i]} } }
  b.report('array zip') { n.times { array_A.zip(array_B).map{|i,j| i*j }.inject(:+) } }
  b.report('array zip 2') { n.times { array_A.zip(array_B).inject(0) {|r, (a, b)| r + (a * b)} } }
  b.report('while loop') do
    n.times do
      sum, i, size = 0, 0, array_A.size
      while i < size
        sum += array_A[i] * array_B[i]
        i += 1
      end
      sum
    end
  end
  b.report('vector') { n.times { vector_method(vector_A, vector_B) } }
end

And the results:

Rehearsal --------------------------------------------------------------
matrix method                0.860000   0.010000   0.870000 (  0.911755)
array walking                0.290000   0.000000   0.290000 (  0.294779)
array walking without to_a   0.190000   0.000000   0.190000 (  0.215780)
array zip                    0.420000   0.010000   0.430000 (  0.441830)
array zip 2                  0.340000   0.000000   0.340000 (  0.352058)
while loop                   0.080000   0.000000   0.080000 (  0.085314)
vector                       0.310000   0.000000   0.310000 (  0.325498)
----------------------------------------------------- total: 2.510000sec

                                 user     system      total        real
matrix method                0.870000   0.020000   0.890000 (  0.952630)
array walking                0.290000   0.000000   0.290000 (  0.340443)
array walking without to_a   0.220000   0.000000   0.220000 (  0.240651)
array zip                    0.400000   0.010000   0.410000 (  0.441829)
array zip 2                  0.330000   0.000000   0.330000 (  0.359365)
while loop                   0.080000   0.000000   0.080000 (  0.090099)
vector                       0.300000   0.010000   0.310000 (  0.325903)
------

Too bad. :(