[Rd] Multiple Assignment built into the R Interpreter?
Duncan Murdoch
murdoch@dunc@n @end|ng |rom gm@||@com
Sat Mar 11 14:37:24 CET 2023
I think the standard way to do this in R is given by list2env(), as
described in a couple of answers on the SO page you linked.
The syntax you proposed would be likely to be confusing in complex
expressions, e.g.
f(A, C, Q, R = init_matrices(X, Y, Z))
would obviously not work but wouldn't trigger a syntax error, and
f((A, C, Q, R = init_matrices(X, Y, Z)))
could work, but looks too much like the previous one. So I think R
would want Javascript-like
[A, C, Q, R] <- init_matrices(X, Y, Z)
instead. But then the question would come up about how to handle the
RHS. Does the function have to return a list? What if the length of
the list is not 4? Or is it just guaranteed to be equivalent to
temp <- init_matrices(X, Y, Z)
A <- temp[[1]]
C <- temp[[2]]
Q <- temp[[3]]
R <- temp[[4]]
which would work for other vector types besides lists?
BTW, here's a little hack that almost works:
`vals<-` <- function(x, ..., value) {
others <- substitute(list(...))
if (length(others) > 1)
for (i in seq_along(others)[-1])
assign(as.character(others[[i]]), value[[i]], envir =
parent.frame())
value[[1]]
}
You call it as
vals(a, b, c) <- 1:3
and it assigns 1 to a, 2 to b, and 3 to c. It doesn't quite do what you
want because it requires that a exists already, but b and c don't have to.
Duncan Murdoch
On 11/03/2023 4:04 a.m., Sebastian Martin Krantz wrote:
> Dear R Core,
>
> working on my dynamic factor modelling package, which requires several
> subroutines to create and update several system matrices, I come back to
> the issue of being annoyed by R not supporting multiple assignment out of
> the box like Matlab, Python and julia. e.g. something like
>
> A, C, Q, R = init_matrices(X, Y, Z)
>
> would be a great addition to the language. I know there are several
> workarounds such as the %<-% operator in the zeallot package or my own %=%
> operator in collapse, but these don't work well for package development as
> R CMD Check warns about missing global bindings for the created variables,
> e.g. I would have to use
>
> A <- C <- Q <- R <- NULL
> .c(A, C, Q, R) %=% init_matrices(X, Y, Z)
>
> in a package, which is simply annoying. Of course the standard way of
>
> init <- init_matrices(X, Y, Z)
> A <- init$A; C <- init$C; Q <- init$Q; R <- init$R
> rm(init)
>
> is also super cumbersome compared to Python or Julia. Another reason is of
> course performance, even my %=% operator written in C has a non-negligible
> performance cost for very tight loops, compared to a solution at the
> interpretor level or in a primitive function such as `=`.
>
> So my conclusion at this point is that it is just significantly easier to
> implement such codes in Julia, in addition to the greater performance it
> offers. There are obvious reasons why I am still coding in R and C, thanks
> to the robust API and great ecosystem of packages, but adding this could be
> a presumably low-hanging fruit to make my life a bit easier. Several issues
> for this have been filed on Stackoverflow, the most popular one (
> https://stackoverflow.com/questions/7519790/assign-multiple-new-variables-on-lhs-in-a-single-line)
> has been viewed 77 thousand times.
>
> But maybe this has already been discussed here and already decided against.
> In that case, a way to browse R-devel archives to find out would be nice.
>
> Best regards,
>
> Sebastian
>
> [[alternative HTML version deleted]]
>
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