[R] subscripts in lists

[Liaw, Andy]

> sapply(lis, function(x) x[which(x == "next") + 1])
[1] "want1" "want2"

HTH,
Andy

> From: Chris Knight 
> 
> I am tying myself in knots over subscripts when applied to lists
> 
> I have a list along the lines of:
> 
> lis<-list(c("a","b","next","want1","c"),c("d", "next", "want2", "a"))
> 
> >From which I want to extract the values following "next" in each
> member of the list, i.e. something along the lines of answer<-c( 
> "want1", "want2"). Is this possible without using loops? The elements 
> of lis are of different lengths and "next" occurs once per element 
> somewhere in the middle.
> 
> The thought process behind this is:
> 
> It's easy enough to do it for an individual element of the 
> list: lis[[1]][match("next",lis[[1]])+1]
> 
> but how to do that to all elements of the list? I can get their 
> indices e.g. as a list using lapply:
> 
> lapply(lapply(lis,match,x="next"),"+",y=1)
> 
> or return a particular subscript using:
> lapply(lis,"[", i=3)
> 
> but don't see how one could combine the two to get answer<-c("want1", 
> "want2") without resorting to:
> 
> answer<-character
> for(s in 1:length(lis)){
> answer<-c(answer,lis[[s]][match("next",lis[[s]])+1])
> }
> 
> Am I missing something obvious (or non-obvious)? I suppose the 
> secondary question is 'should I care?'. I am intending to use this on 
> hundreds of lists sometimes with tens of thousands of elements, with 
> more than one version of "next" in each, so felt that  the lower 
> efficiency of looping was likely to matter.
> Any help much appreciated,
> 
> Chris
> -- 
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> Department of Plant Sciences              +44 (0)1865 275 790
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