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

On Tue, 24 Oct 2006, Joe W. Byers wrote:

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.

Yes, this can be confusing.  One reason that it is confusing is that the 
rules appear to be different (though they aren't) for vectors and lists.

The single bracket [ extracts a subvector, and the double bracket [[ 
extracts an element.  That is, with
  a<-list(b=1,c=2,d=3)
you can extracts the first element of a,

a[[1]]

[1] 1
or a sublist with the first two elements

a[1:2]

$b
[1] 1

$c
[1] 2
or a sublist with just the first element

a[1]

$b
[1] 1

The same is true for numeric or character vectors, but there an element 
and a subvector of length one are the same, so the distinction between [[ 
and [ is harder to understand.

b<-1:10
b[1:2]

[1] 1 2

b[1]

[1] 1

b[[1]]

[1] 1

 	-thomas