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

Joe W. Byers Joe-Byers at utulsa.edu
Tue Oct 24 12:58:10 CEST 2006

```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

```