[R] x[0]: Can '0' be made an allowed index in R?

[Rui Barradas]

Às 08:55 de 21/04/2024, Hans W escreveu:
> As we all know, in R indices for vectors start with 1, i.e, x[0] is not a
> correct expression. Some algorithms, e.g. in graph theory or combinatorics,
> are much easier to formulate and code if 0 is an allowed index pointing to
> the first element of the vector.
> 
> Some programming languages, for instance Julia (where the index for normal
> vectors also starts with 1), provide libraries/packages that allow the user
> to define an index range for its vectors, say 0:9 or 10:20 or even negative
> indices.
> 
> Of course, this notation would only be feasible for certain specially
> defined vectors. Is there a library that provides this functionality?
> Or is there a simple trick to do this in R? The expression 'x[0]' must
> be possible, does this mean the syntax of R has to be twisted somehow?
> 
> Thanks, Hans W.
> 
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Hello,

I find what you are asking awkward but it can be done with S3 classes.
Write an extraction method for the new class and in the use case below 
it works. The method increments the ndex before calling NextMethod, the 
usual extraction function.


`[.zerobased` <- function(x, i, ...) {
   i <- i + 1L
   NextMethod()
}
as_zerobased <- function(x) {
   class(x) <- c("zerobased", class(x))
   x
}

x <- 1:10
y <- as_zerobased(x)

y[0]
#> [1] 1
y[1]
#> [1] 2
y[9]
#> [1] 10
y[10]
#> [1] NA


Hope this helps,

Rui Barradas


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