From: Joel VanderWerf <vjoel@...>
Date: 2007-11-02T11:05:10+09:00
Subject: Re: Fastest way to compute dot product (inner product) in Ruby?

theosib@gmail.com wrote:
> ...Anyhow, so I'm trying to find the fastest way to
> compute a dot product...

require 'narray' # not pure ruby
NVector[1,2,3] * NVector[3,6,9]
=> 42

Here's a benchmark:

require 'benchmark'
require 'narray'

def dot_product_e l1, l2
     sum = 0
     for i in 0...l1.size
         sum += l1[i] * l2[i]
     end
     sum
end

Benchmark.bmbm(12) do |x|

   n = 100_000
   u = NVector[1..20]
   v = NVector[21..40]

   x.report("u*v") do
     n.times do
       u*v
     end
   end

   x.report("dot_product_e") do
     n.times do
       dot_product_e u, v
     end
   end
end

__END__

Output:

Rehearsal -------------------------------------------------
u*v             0.380000   0.020000   0.400000 (  0.396224)
dot_product_e   3.370000   0.000000   3.370000 (  3.382755)
---------------------------------------- total: 3.770000sec

                     user     system      total        real
u*v             0.390000   0.000000   0.390000 (  0.398509)
dot_product_e   3.380000   0.010000   3.390000 (  3.385840)

-- 
       vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407