Inverse of which

My own solution (for now): EDIT as per @Marek's suggestion.

invwhich<-function(indices, outlength, useNames = TRUE)
{
    rv<-logical(outlength)
    #rv<-rep(FALSE, outlength) #see Marek's comment
    if(length(indices) > 0)
    {
        rv[indices]<-TRUE
        if(useNames) names(rv)[indices]<-names(indices)
    }
    return(rv)
}

It performs very well (apparently better than @Andrie's oneliner) and, in as much as possible, accounts for useNames. But is it possible to make this into a oneliner?

wrt performance, I simply use:

someindices<-sample(1000000, 500000, replace=FALSE)
system.time(replicate(100, tmp<-invwhich(someindices, 1000000)))

as a very lo-fi performance measurement.

One-liner solution:

invwhich <- function(indices, totlength) is.element(seq_len(totlength), indices)

invwhich(c(2,5), 10)
[1] FALSE  TRUE FALSE FALSE  TRUE FALSE FALSE FALSE FALSE FALSE