From: Luigi Ballabio <luigi.ballabio@...>
Date: 2001-03-16T17:53:59+09:00
Subject: [ruby-talk:12734] Re: Sum of Squares

At 02:14 AM 3/16/01 +0900, you wrote:
>On Tue, 13 Mar 2001, rashworth wrote:
> > Thank you for your note. The new coding worked just fine.
> > Do you have some code for the Sum of the Cross-Products?
> > Sumxy = (x1 * y1) + (x2 * y2) + ....
>
>That is called a dot product, not a "sum of cross-products". Unfortunately
>you can't run two iterators at once easily, but you can still write:
>
>class Array
>         def dot(other)
>                 # other.type == Array
>                 # other.length == self.length
>                 sum = 0
>                 length.times {|i| sum += self[i]*other[i] }
>                 sum
>         end
>end

To follow up on the Haskell-functional programming thing, this is where
[1,2,3].zip([4,5,6])                # --> [[1,4],[2,5],[3,6]]
and
[1,2,3].inject(0) { |a,b| a+b }     # --> 1+2+3 = 6
could come handy - the above would be
class Array
     def dot(other)
         self.zip(other).map { |a| a[0]*a[1] } .inject(0) { |a,b| a+b }
     end
end

Bye,
         Luigi