[R] Help with understanding [[]] [] array, list, matrix referencing

[David Barron]

Have you tried help("["), which gives a good explanation.


On 24/10/06, Joe W. Byers <Joe-Byers at utulsa.edu> wrote:
> Hi all,
>
> I would greatly appreciate some help understanding how R references
> arrays, matrices, lists, and objects using [[]] and [].  I have read the
> R guides and several tutorials but I am not the fastest kid on the block
> so I am still having difficulty understanding this.  For examples the
> following code produces a 5 element list of 2X5 random numbers that I
> then convert to a 2X5X5 matrix.
> cov<-matrix(c(.4,-.1,-.1,.3),nrow=2,ncol=2)
> rnds<-NULL;
> for (i in 1:5){
>         t1<-rnorm(5,cov)
>         t2<-rnorm(5,cov)
>         t3<-rbind(t1,t2)
>         rnds[i]<-list(t3)
> }
>
> rnds.matrix<-array(unlist(rnds),dim=c(2,5,5));
>
> To access the matrix rnds.matrix I use rnds.matrix[x,y,z].  This I
> understand.
>
> To access the list I user [[z]][x,y].  This I do not understand.  I
> found by chance this reference notation in an old mailing list that
> helped me.
>
> I could use some help in knowing when to use [[]] referencing and when
> to use [] referencing.  If there is a really good book, webpage, or link
> with explanation and examples I would appreciate you forwarding the the
> citation.
>
> Thank you
> Joe
>
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>


-- 
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David Barron
Said Business School
University of Oxford
Park End Street
Oxford OX1 1HP