[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
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