Suppose I have a list where I want to extract only the elements that occur
in every component. For instance in the list foo I want to know that the
numbers 2 and 3 occur in every component. The solution I have seems
unnecessarily clunky. TIA, Andy
foo <- list(x = 1:10, y=2:11, z=1:3)
bar <-unlist(foo)
bartab <- table(bar)
as.numeric(names(bartab)[bartab==length(foo)])
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Here is one solution: > all(unlist(lapply(foo, function(x) c(2,3) %in% x))) [1] TRUE This doesn't have the restriction of assuming that the components of the list have unique elements, as the original solution does. Patrick Burns patrick at burns-stat.com +44 (0)20 8525 0696 http://www.burns-stat.com (home of S Poetry and "A Guide for the Unwilling S User") Andy Bunn wrote:>Suppose I have a list where I want to extract only the elements that occur >in every component. For instance in the list foo I want to know that the >numbers 2 and 3 occur in every component. The solution I have seems >unnecessarily clunky. TIA, Andy > > foo <- list(x = 1:10, y=2:11, z=1:3) > bar <-unlist(foo) > bartab <- table(bar) > as.numeric(names(bartab)[bartab==length(foo)]) > > [[alternative HTML version deleted]] > >______________________________________________ >R-help at stat.math.ethz.ch mailing list >https://stat.ethz.ch/mailman/listinfo/r-help >PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html > > > >
> The original post is ambiguous: do you want to find the intersection or do > you want to find whether a prespecified set is in the > intersection? Patrick > provided you an answer to the latter while you provided an answer to the > former. Actually, I thought using table as you did (mod the need for no > replicates) was clever. A more direct but I think considerably slower > approach would be to use intersect() in a loop: > > inall<-intersect(foo[[1]],foo[[,2]]) > for(i in seq(3, to=length(foo))inall<-intersect(inall,foo[[i]]) > > I suspect you already thought of this and rejected it. Other than > transparency, I think the only advantage it has is that it will work for > something other than lists of numerics, e.g. it will work for lists of > factors, which the table() solution would not.Indeed, the first post was ambiguous. I was after the type of solution that I posted: the intersection rather than a prespecified set. Given that, I suppose my use of table is satisfactory. Thanks Patrick and Bert. -Andy
I guess I am coming a bit late to this but here is a one-line zoo solution. The lapply creates a zoo object from each foo component with the constant value 1 and the contents as the times of that object. Merging the objects together using cbind gives columns of 1s and NAs and removing the NAs leaves a zoo object at only the intersection of times, which we then extract. library(zoo) time(na.omit(do.call(cbind, lapply(foo, zoo, x = 1)))) # 2, 3 On 4/8/06, Andy Bunn <abunn at nova.whrc.org> wrote:> > The original post is ambiguous: do you want to find the intersection or do > > you want to find whether a prespecified set is in the > > intersection? Patrick > > provided you an answer to the latter while you provided an answer to the > > former. Actually, I thought using table as you did (mod the need for no > > replicates) was clever. A more direct but I think considerably slower > > approach would be to use intersect() in a loop: > > > > inall<-intersect(foo[[1]],foo[[,2]]) > > for(i in seq(3, to=length(foo))inall<-intersect(inall,foo[[i]]) > > > > I suspect you already thought of this and rejected it. Other than > > transparency, I think the only advantage it has is that it will work for > > something other than lists of numerics, e.g. it will work for lists of > > factors, which the table() solution would not. > > > Indeed, the first post was ambiguous. I was after the type of solution that I posted: the intersection rather than a prespecified set. Given that, I suppose my use of table is satisfactory. > > Thanks Patrick and Bert. > > -Andy > > ______________________________________________ > R-help at stat.math.ethz.ch mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html >