From: matt@... (matt neuburg)
Date: 2009-06-08T01:40:07+09:00
Subject: Re: adding, multiplying array

Rick DeNatale <rick.denatale@gmail.com> wrote:

> On Sun, Jun 7, 2009 at 10:15 AM, Haris
> Bogdanovi?<haris.bogdanovic@gmail.com> wrote:
> > Hi.
> >
> > I want to multply all elements of two arrays and add that to all elements of
> > the third array like this:
> >
> > c+=a*b
> >
> > c=[1,1,1]
> > a=[2,2,2]
> > b=[3,3,3]
> >
> > c=[7,7,7]  # (1+2*3)=7
> >
> > I tried (found on google):
> >
> > a.zip(b).map {|i,j| i*j} (don't really know how that works)
> 
> zip weaves two enumerables together and creates a new enumerable:
> 
> a.zip(b) # => [[2, 3], [2, 3], [2, 3]]
> 
> [[2, 3], [2, 3], [2, 3]].map {|i,j| i*j}
> 
> takes each sub-array, multiplies the two elements and returns a new
> array with the products which produces [6, 6, 6]
> 
> >
> > and how to add that to c array ?
> >
> 
> Just use zip and map again with a block  which adds the elements.
> 
> c = c.zip(a.zip(b).map {|i, j| i*j}).map {|i, j|i + j}
> 
> Which changes the variable c to reference a new array which turns out
> to be [7, 7, 7]

And if you're going to be doing this kind of thing a lot, you might like
to generalize the repeated zip -> map operation:

class Array
  def combine(other, method)
    zip(other).map {|x,y| x.send(method,y)}
  end
end

c = [1,1,1]
a = [2,2,2]
b = [3,3,3]

c = a.combine(b, :*).combine(c, :+) 
#=> [7,7,7]

:) I'm sure there's a cleaner way... m.


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