# Re: [R] help on establishing a matrix

From: Jim Lemon <jim_at_bitwrit.com.au>
Date: Thu, 31 Jan 2008 19:57:44 +1100

Gator Connection wrote:
> I am a beginner and this is a naive question.
> I have the following data set.
> row column height
> 1 2 96
> 3 7 67
> 9 25 77
> ......
> I have a matrix of 50*100 data points and about 60% of them are zeros.
> I want to put the height data into the matrix according to their row and column numbers. does anybody have experience in setting up such matrix?
> Your help is highly appreciated.
> Jack
Hi Jack,
Well, what do you know? I thought that this:

x<-matrix(c(rep(1:5,each=3),rep(1:3,5),sample(10:100,15)),ncol=3) x
newmat<-matrix(0,nrow=5,ncol=3)
newmat[x[,1],x[,2]]<-x[,3]

would do it, but it doesn't. However,

# scramble the matrix
x1<-x[sample(1:15,15),]
# create a new matrix by ordering it by row and column newmat<-matrix(x1[order(x1[,1],x1[,2]),3],ncol=3,byrow=TRUE) newmat

works for me

Jim

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