[R] the secret (?) language of lists

[Prof Brian Ripley]

Please, these are NOT lists.  They are vectors: the difference should be 
clear even from 'An Introduction to R'.


What Peter's first solution does is to create a matrix and then read it 
out in the default column-major order.  This is again all discussed in
'An Introduction to R'.

Using c() in this way is often frowned on: as.vector() is clearer.


On Tue, 14 Nov 2006, Jeffrey Robert Spies wrote:

> A couple days ago, Mark Leeds asked about a solution that would
> basically stagger two lists, a and b, to return a list in the form of
> a[1], b[1], a[2], b[2], a[3]....  In particular, the summary of his
> question was in reference to lists defined by
>
>  x <- 5
>  tempin <- seq(1,1411, by=30)
>  a <- tempin
>  b <- tempin + x
>
> I offered the following function
>
>  everyOther <- function(tempin, x){
>  	tempout <- array(data=NA, dim=length(tempin)*2)
>  	tempout[seq(1,length(tempin)*2, by=2)]<-tempin
>  	tempout[seq(2,length(tempin)*2, by=2)]<-tempin+x
>  	tempout
>  }
>
> which did what it was supposed to, and Gavin Simpson offered a
> similar function.  Peter Dalgaard, however, supplied a much more
> elegant solution:
>
>  c(rbind(tempin,tempin+5))
>
> or
>
>  rep(tempin, each=2) + c(0,5)
>
> I thought I'd bring this up as a new topic because it's really no
> longer related to what Mark first asked, but is there a way, perhaps
> from the documentation, that a user would know that c() and lists in
> general behave as they do in these two lines?  Or would we just need
> to dig into the code?
>
> I am reminded of quote by Byron Ellis: "Contrary to popular belief
> the speed of R's interpreter is rarely the limiting factor to R's
> speed. People treating R like C is typically the limiting factor. You
> have vector operations, USE THEM."  Not exactly the point, but close.
>
> Thanks!
>
> Jeff Spies
> http://www.nd.edu/~jspies/
>
>
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>
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-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
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