[R] interaction of shingles and tapply()

Roger Levy rlevy at ucsd.edu
Wed Nov 28 02:56:03 CET 2007

I'm interested in a version of tapply() that operates with shingles 
instead of factors.  For instance:

x <- c(1,1,2,2,3,3)
y <- c(1,1,1,0,0,0)

s <- shingle(x,intervals=cbind(c(0.5,1.5),c(2.5,3.5)))

# the following function should exist!
tapply.shingle(x,s,mean)  # returns the vector c(0.75,0.25)

I've written such a function as follows:

tapply.shingle <- function(x,s,fn,...) {
   result <- c()
   for(l in levels(s)) {
     x1 <- x[s > l[1] & s < l[2]]
     result <- c(result, fn(x1,...))

However, I'm not thrilled with the for() loop, and I don't see any way 
to generalize this function to handle lists of shingles instead of 
individual shingles, except to use recursion.  Does anyone have any 
suggestions or thoughts?

Many thanks,


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