[R] Successive subsets from a vector?

[Prof Brian Ripley]

    embed(VECTOR, 5)[, 5:1]

gives the subsets, so something like

    apply(embed(VECTOR, 5)[, 5:1], 1, paste, collapse="")

does the job.

The following is a bit more efficient

    ind <- 1:(length(VECTOR)-4)
    do.call(paste, c(lapply(0:4, function(j) VECTOR[ind+j]), sep=""))

but by looking at how embed() works it could be made as efficient.

Larger example:

VECTOR <- sample(1:10, 1e5, replace=TRUE)
> system.time(apply(embed(VECTOR, 5)[, 5:1], 1, paste, collapse=""))
[1] 5.73 0.05 5.81   NA   NA
> system.time({ind <- 1:(length(VECTOR)-4)
+ do.call(paste, c(lapply(0:4, function(j) VECTOR[ind+j]), sep=""))
+ })
[1] 1.00 0.01 1.01   NA   NA

The loop method took 195 secs.  Just assigning to an answer of the correct 
length reduced this to 5 secs.  e.g. use

    ADDRESSES <- character(length(VECTOR)-4)

Moral: don't grow vectors repeatedly.

On Tue, 22 Aug 2006, kone wrote:

> I'd like to pick every imbricated five character long subsets from a 
> vector. I guess there is some efficient way to do this without loops...
> Here is a for-loop-version and a model for output:
> 
> VECTOR=c(1,4,2,6,5,0,11,10,4,3,6,8,6);
> 
> ADDRESSES=c();

You do not need the semicolons, and they just confuse readers.

> for(i in 1:(length(VECTOR)-4)){
> 	ADDRESSES[i]=paste(VECTOR[i:(i+4)],collapse="")	
> }
> 
>  > ADDRESSES
> [1] "14265"   "42650"   "265011"  "6501110" "5011104" "0111043" 
> "1110436" "104368"
> [9] "43686"
> 
> 
> Atte Tenkanen
> University of Turku, Finland
> 
> 	[[alternative text/enriched version deleted]]
> 
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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)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595