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

[David Barron]

Well, as I understand it, with a list [] doesn't return the actual
contents of that element of the list, whereas [[]] does.  Compare, for
example rnds[1] + 1 with rnds[[1]] + 1.  So, with a list you have to
use the double bracket notation to get to the actual contents of that
element of the list, as opposed to a pointer to the element.  With a
vector or a matrix, [[]] doesn't seem to have much use, though you can
use it with a single number, in which case you will get the same
result as [].

There is a separate method for data frames, in which [[]] will give
you a column.  So, I think that data[[1]] is the same as data[,1].

Anyway, these rules have always worked for me, though I'm sure others
can give a more sophisticated answer!

On 24/10/06, Joe Byers <joe-byers at utulsa.edu> wrote:
> 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.
> >>
> >
> >
>
>
>


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