[R] subscripts in lists

Richard A. O'Keefe ok at cs.otago.ac.nz
Tue Aug 12 00:24:41 CEST 2003

Chris Knight <christopher.knight at plant-sciences.oxford.ac.uk> has
	lis<-list(c("a","b","next","want1","c"),c("d", "next", "want2", "a"))
and wants c("want1","want2")

Step 1:
    inx <- sapply(lis, function(x) which(x == "next")) + 1
==> 4 3

Step 2:
    sapply(1:length(lis), function(i) lis[[i]][inx[i]])
==> "want1" "want2"

Think about this for a bit and restructure it:

    sapply(1:length(lis), function (i) {v <- lis[[i]]; v[which(v=="next")+1]})

Wrap it up:

    after <- function(lis, what="next") {
	sapply(1:length(lis), function (i) {
	    v <- lis[[i]]
	    v[which(v == what)+1]

Of course, from my point of view, a call to sapply() *is* a loop, just
packaged slightly differently.  I think this is reasonably clear.

More information about the R-help mailing list