From: rashworth <rashworth@...>
Date: 2001-03-17T13:22:06+09:00
Subject: [ruby-talk:12760] Re: Sum of Squares

thanks for your code.  ERA

---------- Original Message ----------------------------------
From: Luigi Ballabio <luigi.ballabio@riskmap.net>
Reply-To: ruby-talk@ruby-lang.org
Date: Fri, 16 Mar 2001 17:53:59 +0900

>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
>
>