R help - Oct 2006 - Help with understanding [[]] [] array, list, matrix referencing

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

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
>
> ______________________________________________
> R-help at 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.
>

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

David,

Yes, I did.  I just still do not get it.  That is why I ask here.  
Hoping someone knew a step by step guide that I could look at.  My trial 
and error approach takes me hours some days.  I currently move most 
things in and out of data.frames where I can name the columns and 
reference with the $ and a subsetting function, but that is not always 
efficient.  If I could understand the [ referencing better, my code 
would be more efficient and I think faster.  Part of my problem is my 
SAS background where everything is a flat table and coding is really 
sloppy.  A data step with a bunch of if-then-else to perform the 
calculation where as in matrix format like in R you do things more 
compactly.  Not always easy to read but efficient and fast.

I appreciate you help.

Thank you
Joe




David Barron wrote:
> 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
>>
>> ______________________________________________
>> R-help at 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.
>>
>
>

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