# Multiplying array **URL:** \ **Category:** ruby-talk\ **Created:** [23 January 2007 15:05 UTC](https://rubytalk.org/t/multiplying-array/34726 "2007-01-23T15:05:47Z")\ **Posts on this page:** 11\ **Page:** 1
**Author:** ![WKC\_CCC](https://avatars.discourse-cdn.com/v4/letter/w/65b543/32.png) [@WKC\_CCC](https://rubytalk.org/u/WKC_CCC)\ **Post date:** [23 January 2007 15:05 UTC](https://rubytalk.org/t/multiplying-array/34726/1 "2007-01-23T15:05:47Z")
HI, Does anyone know of a function that multiplies the contents of an array. For example: one = [1,2,3] two = [[2],[3],[4]] output = [[2],[6],[12]] I've written a simple function that does this, however I'm sure there is a better way, instead of casting the item to a float. def multiplyArray(arr1,arr2)   newArr = []   i=0   arr1.each do |x|     x = x.to\_s.to\_f     item = arr2[i].to\_s.to\_f     ele = x \* item     newArr.push(ele)     i = i + 1    end   puts newArr end Thanks, > **···** > > -- > Posted via [http://www.ruby-forum.com/](http://www.ruby-forum.com/). ---
**Author:** ![Robert\_K1](https://yyz1.discourse-cdn.com/flex029/user_avatar/rubytalk.org/robert_k1/32/1830_2.png) [@Robert\_K1](https://rubytalk.org/u/Robert_K1)\ **Post date:** [23 January 2007 15:25 UTC](https://rubytalk.org/t/multiplying-array/34726/2 "2007-01-23T15:25:06Z")
Why do you cast to float? You can multiply integers directly. First with a straightforward array "two": irb(main):001:0\> require 'enumerator' =\> true irb(main):002:0\> one = [1,2,3] =\> [1, 2, 3] irb(main):003:0\> two = [2,3,4] =\> [2, 3, 4] irb(main):004:0\> one.to\_enum(:zip, two).map {|a,b| a\*b} =\> [2, 6, 12] Now with your array: irb(main):005:0\> two.map! {|i| [i]} =\> [[2], [3], [4]] irb(main):006:0\> one.to\_enum(:zip, two).map {|a,b| a \* b[0]} =\> [2, 6, 12] Kind regards   robert > **···** > > On 23.01.2007 16:05, WKC CCC wrote: > > > Does anyone know of a function that multiplies the contents of an array. > > For example: > > > > one = [1,2,3] > > two = [[2],[3],[4]] > > > > output = [[2],[6],[12]] > > > > I've written a simple function that does this, however I'm sure there is > > a better way, instead of casting the item to a float. ---
**Author:** ![Ara.T.Howard6](https://avatars.discourse-cdn.com/v4/letter/a/958977/32.png) [@Ara.T.Howard6](https://rubytalk.org/u/Ara.T.Howard6)\ **Post date:** [23 January 2007 15:36 UTC](https://rubytalk.org/t/multiplying-array/34726/3 "2007-01-23T15:36:04Z")
if you are doing lots of this then check out narray:    harp:~ \> cat a.rb    require 'narray'    a = NArray.to\_na [1,2,3]    b = NArray.to\_na [2,3,4]    p( a \* b )    harp:~ \> ruby a.rb    NArray.int(3):    [2, 6, 12] regards. -a > **···** > > On Wed, 24 Jan 2007, WKC CCC wrote: > > > HI, > > > > Does anyone know of a function that multiplies the contents of an array. > > For example: > > > > one = [1,2,3] > > two = [[2],[3],[4]] > > > > output = [[2],[6],[12]] > > > > I've written a simple function that does this, however I'm sure there is > > a better way, instead of casting the item to a float. > > > > def multiplyArray(arr1,arr2) > > newArr = > > i=0 > > arr1.each do |x| > >    x = x.to\_s.to\_f > >    item = arr2[i].to\_s.to\_f > >    ele = x \* item > >    newArr.push(ele) > >    i = i + 1 > >   end > > puts newArr > > end > > > > Thanks, > > -- > we can deny everything, except that we have the possibility of being better. > simply reflect on that. > - the dalai lama ---
**Author:** ![Gavin\_Kistner2](https://avatars.discourse-cdn.com/v4/letter/g/b9e5f3/32.png) [@Gavin\_Kistner2](https://rubytalk.org/u/Gavin_Kistner2)\ **Post date:** [23 January 2007 16:25 UTC](https://rubytalk.org/t/multiplying-array/34726/4 "2007-01-23T16:25:10Z")
WKC CCC wrote: > Does anyone know of a function that multiplies the contents of an array. module Enumerable   def product     inject{ |piece, prod| prod\*piece }   end   def sum     inject(0){ |piece, total| total+piece }   end end a = (1..10) p a.sum #=\> 55 p a.product #=\> 3628800 ---
**Author:** ![W\_James](https://avatars.discourse-cdn.com/v4/letter/w/e274bd/32.png) [@W\_James](https://rubytalk.org/u/W_James)\ **Post date:** [23 January 2007 21:40 UTC](https://rubytalk.org/t/multiplying-array/34726/5 "2007-01-23T21:40:08Z")
WKC CCC wrote: > HI, > > Does anyone know of a function that multiplies the contents of an array. > For example: > > one = [1,2,3] > two = [[2],[3],[4]] > > output = [[2],[6],[12]] > > I've written a simple function that does this, however I'm sure there is > a better way, instead of casting the item to a float. > > def multiplyArray(arr1,arr2) >   newArr = >   i=0 >   arr1.each do |x| >     x = x.to\_s.to\_f >     item = arr2[i].to\_s.to\_f >     ele = x \* item >     newArr.push(ele) >     i = i + 1 >    end >   puts newArr > end > > Thanks, > > -- > Posted via [http://www.ruby-forum.com/\](http://www.ruby-forum.com/%5C). [1,2,3].zip([2,3,4]).map{|a,b| a\*b}     ==\>[2, 6, 12] ---
**Author:** ![Rob\_Biedenharn1](https://avatars.discourse-cdn.com/v4/letter/r/57b2e6/32.png) [@Rob\_Biedenharn1](https://rubytalk.org/u/Rob_Biedenharn1)\ **Post date:** [23 January 2007 16:33 UTC](https://rubytalk.org/t/multiplying-array/34726/6 "2007-01-23T16:33:10Z")
module Enumerable    def product      inject(1) { |prod, piece| prod\*piece }    end    def sum      inject(0) { |total, piece| total+piece }    end end True, except the block parameters were reversed. In these cases, the final result wasn't affected because the operations are commutative. I also added the argument to the inject within product for symmetry. You could equally remove the 0 argument from the sum method's use of inject. -Rob Rob Biedenharn [http://agileconsultingllc.com](http://agileconsultingllc.com) Rob@AgileConsultingLLC.com > **···** > > On Jan 23, 2007, at 11:25 AM, Phrogz wrote: > > > WKC CCC wrote: > > > > > Does anyone know of a function that multiplies the contents of an array. > > > > module Enumerable > >   def product > >     inject{ |piece, prod| prod\*piece } > >   end > >   def sum > >     inject(0){ |piece, total| total+piece } > >   end > > end > > > > a = (1..10) > > p a.sum > > #=\> 55 > > p a.product > > #=\> 3628800 ---
**Author:** ![Gavin\_Kistner2](https://avatars.discourse-cdn.com/v4/letter/g/b9e5f3/32.png) [@Gavin\_Kistner2](https://rubytalk.org/u/Gavin_Kistner2)\ **Post date:** [23 January 2007 16:50 UTC](https://rubytalk.org/t/multiplying-array/34726/7 "2007-01-23T16:50:12Z")
Rob Biedenharn wrote: > module Enumerable >    def product >      inject(1) { |prod, piece| prod\*piece } >    end >    def sum >      inject(0) { |total, piece| total+piece } >    end > end > > True, except the block parameters were reversed. In these cases, the > final result wasn't affected because the operations are commutative. Man, I swear that every time I use #inject I get it backwards, and think that I've fixed the answer in my mind for the next time. Thanks for the correction. (Anyone got a good \_why-like mnemonic they use for remembering the order?) > I also added the argument to the inject within product for symmetry. > You could equally remove the 0 argument from the sum method's use of > inject. I thought about that, but in my mind, the sum of an empty array is zero, but the product of an empty array is nil (or 0?), not 1. Of course, totally up the the OP as to how s/he wants to handle this edge case. ---
**Author:** ![WKC\_CCC](https://avatars.discourse-cdn.com/v4/letter/w/65b543/32.png) [@WKC\_CCC](https://rubytalk.org/u/WKC_CCC)\ **Post date:** [23 January 2007 16:57 UTC](https://rubytalk.org/t/multiplying-array/34726/8 "2007-01-23T16:57:12Z")
How can the inject function work on 2 arrays? > **···** > > -- > Posted via [http://www.ruby-forum.com/](http://www.ruby-forum.com/). ---
**Author:** ![Patrick\_Gundlach13](https://avatars.discourse-cdn.com/v4/letter/p/43a26b/32.png) [@Patrick\_Gundlach13](https://rubytalk.org/u/Patrick_Gundlach13)\ **Post date:** [23 January 2007 16:57 UTC](https://rubytalk.org/t/multiplying-array/34726/9 "2007-01-23T16:57:36Z")
[inject confusion] (Anyone got a good \_why-like mnemonic they use for > remembering the order?) not good, but it helps me: "memo" comes first (alphabetically), so it is inject(memo, obj) P. > **···** > > -- > Posted via [http://www.ruby-forum.com/\](http://www.ruby-forum.com/%5C). ---
**Author:** ![Gavin\_Kistner2](https://avatars.discourse-cdn.com/v4/letter/g/b9e5f3/32.png) [@Gavin\_Kistner2](https://rubytalk.org/u/Gavin_Kistner2)\ **Post date:** [23 January 2007 17:15 UTC](https://rubytalk.org/t/multiplying-array/34726/10 "2007-01-23T17:15:09Z")
WKC CCC wrote: > How can the inject function work on 2 arrays? It can't; I only read the first line of your post (which described a single array) and rushed off my answer. As penance, I give you another solution: a = [1, 2, [3], [4], 5] b = [6, [7], [8], 9, 10 ] p a.flatten.zip(b.flatten).map{ |a,b| a\*b } #=\> [6, 14, 24, 36, 50] ---
**Author:** ![Bertram\_Scharpf](https://avatars.discourse-cdn.com/v4/letter/b/43a26b/32.png) [@Bertram\_Scharpf](https://rubytalk.org/u/Bertram_Scharpf)\ **Post date:** [23 January 2007 20:07 UTC](https://rubytalk.org/t/multiplying-array/34726/11 "2007-01-23T20:07:44Z")
Hi, > **···** > > Am Mittwoch, 24. Jan 2007, 01:57:36 +0900 schrieb Patrick Gundlach: > > > [inject confusion] > > > > \> (Anyone got a good \_why-like mnemonic they use for > > \> remembering the order?) > > > > not good, but it helps me: > > > > "memo" comes first (alphabetically), so it is > > > > inject(memo, obj) > > I'm trying to help myself with the order invariance in > >   a == a.inject() { |r,e| r+[e] } >   a == a.inject() { |r,e| r \<\< e } > > Bertram > > -- > Bertram Scharpf > Stuttgart, Deutschland/Germany > [http://www.bertram-scharpf.de](http://www.bertram-scharpf.de)