# Complete Newbie - Matrix Question **URL:** \ **Category:** ruby-talk\ **Created:** [10 December 2006 23:10 UTC](https://rubytalk.org/t/complete-newbie-matrix-question/33632 "2006-12-10T23:10:05Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![Jay](https://avatars.discourse-cdn.com/v4/letter/j/ed8c4c/32.png) [@Jay](https://rubytalk.org/u/Jay)\ **Post date:** [10 December 2006 23:10 UTC](https://rubytalk.org/t/complete-newbie-matrix-question/33632/1 "2006-12-10T23:10:05Z")
Hi Folks, This is probably quite easy, but I can't find a Matrix function or way to populate a specific element of a matrix with a given value? ie, I want to tell that element i,j of matrix.a should be 3 My code below is a simple test to populate a 5 by 5 matrix from values in a CSV text file. The loops etc are messy, but it's just an example. require "mathn" i=1 j=1 a = Matrix.zero(5) File.open("TestData.txt").each do |record|     record.split(",").each do |field|     field.chomp!     if i\<5 then       Matrix.a(i,j)=field # \<-------------How do I do this line?     end     if i = 5 then       i = 1       j=j+1     end     i=i+1    end end puts a.to\_s Any Help/pointers would be great. TIA. Jay. ---
**Author:** ![Paul\_Lutus](https://avatars.discourse-cdn.com/v4/letter/p/2bfe46/32.png) [@Paul\_Lutus](https://rubytalk.org/u/Paul_Lutus)\ **Post date:** [10 December 2006 23:35 UTC](https://rubytalk.org/t/complete-newbie-matrix-question/33632/2 "2006-12-10T23:35:06Z")
Jay wrote: > **···** > > > Hi Folks, > > This is probably quite easy, but I can't find a Matrix function or way > > to populate a specific element of a matrix with a given value? ie, I > > want to tell that element i,j of matrix.a should be 3 > > ------------------------------- > > #!/usr/bin/ruby -w > > require 'mathn' > > class Matrix >    def =(i,j,v) >       @rows[i][j] = v >    end > end > > m = Matrix.zero(5) > > m[2,2] = 3 > > p m > > ------------------------------- > > -- > Paul Lutus > [http://www.arachnoid.com](http://www.arachnoid.com) ---
**Author:** ![Brock\_Lee](https://avatars.discourse-cdn.com/v4/letter/b/4da419/32.png) [@Brock\_Lee](https://rubytalk.org/u/Brock_Lee)\ **Post date:** [10 December 2006 23:55 UTC](https://rubytalk.org/t/complete-newbie-matrix-question/33632/3 "2006-12-10T23:55:04Z")
Based on the documentation at ruby-doc.org, I would think that the following would work for versions 1.8.5 and later. But my version of Ruby, 1.8.4, does not seem to support it. Here it is:     a[i, j] = field One potential solution would be to put your data into an Array initially, and then convert it to a Matrix at the end. Assuming a is an Array, the following would work:     m = Matrix.rows(a) By the way, a bit further down you have a conditional:     if i = 5 which should be:     if i == 5 Brock Lee Jay wrote: > **···** > > > Hi Folks, > > This is probably quite easy, but I can't find a Matrix function or way > > to populate a specific element of a matrix with a given value? ie, I > > want to tell that element i,j of matrix.a should be 3 > > My code below is a simple test to populate a 5 by 5 matrix from values > > in a CSV text file. The loops etc are messy, but it's just an example. > > > > require "mathn" > > i=1 > > j=1 > > a = Matrix.zero(5) > > File.open("TestData.txt").each do |record| > >     record.split(",").each do |field| > >     field.chomp! > >     if i\<5 then > >       Matrix.a(i,j)=field # \<-------------How do I > > do this line? > >     end > >     if i = 5 then > >       i = 1 > >       j=j+1 > >     end > >     i=i+1 > >    end > > end > > puts a.to\_s > > > > Any Help/pointers would be great. > > TIA. > > Jay. ---
**Author:** ![Brock\_Lee](https://avatars.discourse-cdn.com/v4/letter/b/4da419/32.png) [@Brock\_Lee](https://rubytalk.org/u/Brock_Lee)\ **Post date:** [10 December 2006 23:55 UTC](https://rubytalk.org/t/complete-newbie-matrix-question/33632/4 "2006-12-10T23:55:05Z")
Based on the documentation at ruby-doc.org, I would think that the following would work for versions 1.8.5 and later. But my version of Ruby, 1.8.4, does not seem to support it. Here it is:     a[i, j] = field One potential solution would be to put your data into an Array initially, and then convert it to a Matrix at the end. Assuming a is an Array, the following would work:     m = Matrix.rows(a) By the way, a bit further down you have a conditional:     if i = 5 which should be:     if i == 5 Brock Lee Jay wrote: > **···** > > > Hi Folks, > > This is probably quite easy, but I can't find a Matrix function or way > > to populate a specific element of a matrix with a given value? ie, I > > want to tell that element i,j of matrix.a should be 3 > > My code below is a simple test to populate a 5 by 5 matrix from values > > in a CSV text file. The loops etc are messy, but it's just an example. > > > > require "mathn" > > i=1 > > j=1 > > a = Matrix.zero(5) > > File.open("TestData.txt").each do |record| > >     record.split(",").each do |field| > >     field.chomp! > >     if i\<5 then > >       Matrix.a(i,j)=field # \<-------------How do I > > do this line? > >     end > >     if i = 5 then > >       i = 1 > >       j=j+1 > >     end > >     i=i+1 > >    end > > end > > puts a.to\_s > > > > Any Help/pointers would be great. > > TIA. > > Jay. ---
**Author:** ![Jose\_Augusto1](https://avatars.discourse-cdn.com/v4/letter/j/e68b1a/32.png) [@Jose\_Augusto1](https://rubytalk.org/u/Jose_Augusto1)\ **Post date:** [11 December 2006 00:23 UTC](https://rubytalk.org/t/complete-newbie-matrix-question/33632/5 "2006-12-11T00:23:09Z")
Hi Once I had the same problem and proposed the following functions to be appended to standard matrix and vector classes (but they weren't ;-( ): > **···** > > # > # Extending standard Matrix and Vector Classes to write directly > # > class Matrix > >     # setting one single element >     def =(i, j, val) >         @rows[i][j] = val >     end > >     # setting one row >     def set\_row(i,\*row) >         case row[0] >             when Vector >                 @rows[i] = row[0].to\_a >             when Array >                 @rows[i] = row[0] >             else # list of scalars >                 @rows[i] = row >         end >     end > >     # setting one column >     def set\_column(j,\*column) >         case column[0] >             when Vector, Array >                 @rows.collect{|row| >                     row[j] = column[0][j] >                 } >             else >                 @rows.collect{|row| >                     row[j] = column[j] >                 } >         end >     end > > end > > class Vector >     # setting one single element >     def =(i, val) >         @elements[i] = val >     end > >     # clear vector >     def Vector.zero(n) >         new(:init\_elements, Array.new(n,0), >             copy = false) >     end > end > > Hope this helps... > > JA > > On 12/10/06, Jay \ wrote: > > > Hi Folks, > > This is probably quite easy, but I can't find a Matrix function or way > > to populate a specific element of a matrix with a given value? ie, I > > want to tell that element i,j of matrix.a should be 3 > > My code below is a simple test to populate a 5 by 5 matrix from values > > in a CSV text file. The loops etc are messy, but it's just an example. > > > > require "mathn" > > i=1 > > j=1 > > a = Matrix.zero(5) > > File.open("TestData.txt").each do |record| > >     record.split(",").each do |field| > >     field.chomp! > >     if i\<5 then > >       Matrix.a(i,j)=field # \<-------------How do I > > do this line? > >     end > >     if i = 5 then > >       i = 1 > >       j=j+1 > >     end > >     i=i+1 > >    end > > end > > puts a.to\_s > > > > Any Help/pointers would be great. > > TIA. > > Jay.