Inverse of which

Question

Am I missing something obvious here? It appears the inverse function of which is missing from base R (googling and even a search on SO for "R inverse which" returns a myriad of unrelated links)?

Well, not that I can't write one, but just to relieve my frustration with it being missing and as an R-muscle flexing challenge: how would you go about writing one?

What we need is a function like:

invwhich<-function(indices, totlength)

that returns a logical vector of length totlength where each element in indices is TRUE and the rest is FALSE.

There's bound to be a lot of ways of accomplishing this (some of which are really low hanging fruit), so argue why your solution is 'best'. Oneliner anyone?

If it takes into account some of the other parameters of which (arr.ind??), that's obviously even better...

Answer 1

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