Forum: Ruby adding, multiplying array

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051b47f68568dd185e10bc0445a03f0c?d=identicon&s=25 Haris Bogdanoviæ (Guest)
on 2009-06-07 16:17
(Received via mailing list)
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)

and how to add that to c array ?

Thanks
B3881a28fe402dd2d1de44717486cae8?d=identicon&s=25 Michael Kohl (Guest)
on 2009-06-07 17:24
(Received via mailing list)
On Sun, Jun 7, 2009 at 4:15 PM, Haris Bogdanoviæ
<haris.bogdanovic@gmail.com> wrote:
> 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

How about a very simple each_index approach?

>> c.each_index { |i| c[i] += a[i] * b[i] }
=> [7, 7, 7]

Hope that helps,
Michael
8f6f95c4bd64d5f10dfddfdcd03c19d6?d=identicon&s=25 Rick Denatale (rdenatale)
on 2009-06-07 17:32
(Received via mailing list)
On Sun, Jun 7, 2009 at 10:15 AM, Haris
Bogdanoviæ<haris.bogdanovic@gmail.com> wrote:
>
> 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]
--
Rick DeNatale

Blog: http://talklikeaduck.denhaven2.com/
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Fbb4d027695dfdf76bf448b15d7e306a?d=identicon&s=25 matt neuburg (Guest)
on 2009-06-07 18:40
(Received via mailing list)
Rick DeNatale <rick.denatale@gmail.com> wrote:

> > a=[2,2,2]
> a.zip(b) # => [[2, 3], [2, 3], [2, 3]]
> 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.
E0d864d9677f3c1482a20152b7cac0e2?d=identicon&s=25 Robert Klemme (Guest)
on 2009-06-07 19:06
(Received via mailing list)
On 07.06.2009 17:31, Rick DeNatale wrote:
> On Sun, Jun 7, 2009 at 10:15 AM, Haris
> Bogdanović<haris.bogdanovic@gmail.com> wrote:

>> 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]]

This is true only if you do not provide a block:

irb(main):007:0> a = Array.new 5 do |i| i end
=> [0, 1, 2, 3, 4]
irb(main):008:0> b = Array.new 5 do |i| i * 2 end
=> [0, 2, 4, 6, 8]
irb(main):009:0> a.zip(b) {|x,y| printf "%d - %d\n",x,y}
0 - 0
1 - 2
2 - 4
3 - 6
4 - 8
=> nil

Kind regards

  robert
21dd018d2954c5c1f9f2fe00a18f6d75?d=identicon&s=25 Gabriel Medina (rha7dotcom)
on 2009-06-07 22:57
(Received via mailing list)
Haris Bogdanoviæ wrote:
>
>
require 'pp'
d=[]
a.each_with_index{|o, i| d[i] = a[i] + b[i] * c[i] };
pp d
B3881a28fe402dd2d1de44717486cae8?d=identicon&s=25 Michael Kohl (Guest)
on 2009-06-07 23:37
(Received via mailing list)
On Sun, Jun 7, 2009 at 10:55 PM, Rha7 <rha7.com@gmail.com> wrote:
> require 'pp'
> d=[]
> a.each_with_index{|o, i| d[i] = a[i] + b[i] * c[i] };
> pp d

Since you don't use the 'o' block variable, you can also use
Array#each_index (which is what I used in my first reply to this
thread).

c.each_index { |i| c[i] += a[i] * b[i] }
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