From: Thomas Lumley <tlumley_at_u.washington.edu>

Date: Tue 24 Oct 2006 - 14:21:25 GMT

R-help@stat.math.ethz.ch mailing list

https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code. Received on Wed Oct 25 07:41:03 2006

Date: Tue 24 Oct 2006 - 14:21:25 GMT

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

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https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code. Received on Wed Oct 25 07:41:03 2006

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