# Adding, multiplying array **URL:** \ **Category:** Ruby\ **Created:** [June 7, 2009, 4:17pm UTC](https://www.ruby-forum.com/t/adding-multiplying-array/169713 "2009-06-07T16:17:42Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![Haris\_BogdanoviS\_S](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/a8b319/32.png) [@Haris\_BogdanoviS\_S](https://www.ruby-forum.com/u/Haris_BogdanoviS_S)\ **Post date:** [June 7, 2009, 4:17pm UTC](https://www.ruby-forum.com/t/adding-multiplying-array/169713/1 "2009-06-07T16:17:42Z")
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 ---
**Author:** ![Haris\_BogdanoviS\_S](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/a8b319/32.png) [@Haris\_BogdanoviS\_S](https://www.ruby-forum.com/u/Haris_BogdanoviS_S)\ **Post date:** [June 7, 2009, 5:24pm UTC](https://www.ruby-forum.com/t/adding-multiplying-array/169713/2 "2009-06-07T17:24:33Z")
On Sun, Jun 7, 2009 at 4:15 PM, Haris Bogdanoviæ [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) 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 ---
**Author:** ![Haris\_BogdanoviS\_S](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/a8b319/32.png) [@Haris\_BogdanoviS\_S](https://www.ruby-forum.com/u/Haris_BogdanoviS_S)\ **Post date:** [June 7, 2009, 5:32pm UTC](https://www.ruby-forum.com/t/adding-multiplying-array/169713/3 "2009-06-07T17:32:11Z")
On Sun, Jun 7, 2009 at 10:15 AM, Haris Bogdanoviæ[removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) 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/](http://talklikeaduck.denhaven2.com/) Twitter: [http://twitter.com/RickDeNatale](http://twitter.com/RickDeNatale) WWR: [http://www.workingwithrails.com/person/9021-rick-denatale](http://www.workingwithrails.com/person/9021-rick-denatale) LinkedIn: [http://www.linkedin.com/in/rickdenatale](http://www.linkedin.com/in/rickdenatale) ---
**Author:** ![Haris\_BogdanoviS\_S](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/a8b319/32.png) [@Haris\_BogdanoviS\_S](https://www.ruby-forum.com/u/Haris_BogdanoviS_S)\ **Post date:** [June 7, 2009, 6:40pm UTC](https://www.ruby-forum.com/t/adding-multiplying-array/169713/4 "2009-06-07T18:40:50Z")
Rick DeNatale [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) 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. ---
**Author:** ![Haris\_BogdanoviS\_S](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/a8b319/32.png) [@Haris\_BogdanoviS\_S](https://www.ruby-forum.com/u/Haris_BogdanoviS_S)\ **Post date:** [June 7, 2009, 10:57pm UTC](https://www.ruby-forum.com/t/adding-multiplying-array/169713/5 "2009-06-07T22:57:24Z")
Haris Bogdanoviæ wrote: > require ‘pp’ d=[] a.each\_with\_index{|o, i| d[i] = a[i] + b[i] \* c[i] }; pp d ---
**Author:** ![Haris\_BogdanoviS\_S](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/a8b319/32.png) [@Haris\_BogdanoviS\_S](https://www.ruby-forum.com/u/Haris_BogdanoviS_S)\ **Post date:** [June 7, 2009, 7:06pm UTC](https://www.ruby-forum.com/t/adding-multiplying-array/169713/6 "2009-06-07T19:06:46Z")
On 07.06.2009 17:31, Rick DeNatale wrote: > On Sun, Jun 7, 2009 at 10:15 AM, Haris > Bogdanović[removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) 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 ---
**Author:** ![Haris\_BogdanoviS\_S](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/a8b319/32.png) [@Haris\_BogdanoviS\_S](https://www.ruby-forum.com/u/Haris_BogdanoviS_S)\ **Post date:** [June 7, 2009, 11:37pm UTC](https://www.ruby-forum.com/t/adding-multiplying-array/169713/7 "2009-06-07T23:37:14Z")
On Sun, Jun 7, 2009 at 10:55 PM, Rha7 [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) 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] }