Greetings,
Occasionally, I desire to call a function with one argument set to equal
to another. Here is a toy example:
?? f <- function(x, y) {
? ? ?? x + y
?? }
?? f(x = 3, y = x) # Error in f(x = 3, y = x) : object 'x' not found
So far, the most concise way I found to accomplish this is:
?? f(x = 3, y = local(sys.frame(1)$x)) # evaluates to 6
but I dislike this solution because local() creates a new environment.
Surely there must be a better way?
Note: I'm not interested in solutions that require modifying or currying f.
Regards,
Ben
Hi Ben,
How about something like this:
f <- function(x, y = NULL) {
if (is.null(y))
y <- x
x + y
}
> f(3, 4)
[1] 7
> f(3)
[1] 6
Regards,
Marc Schwartz
> On Jun 20, 2020, at 7:15 AM, Benjamin Tyner <btyner at gmail.com>
wrote:
>
> Greetings,
>
> Occasionally, I desire to call a function with one argument set to equal to
another. Here is a toy example:
>
> f <- function(x, y) {
>
> x + y
> }
>
> f(x = 3, y = x) # Error in f(x = 3, y = x) : object 'x' not
found
>
> So far, the most concise way I found to accomplish this is:
>
> f(x = 3, y = local(sys.frame(1)$x)) # evaluates to 6
>
> but I dislike this solution because local() creates a new environment.
Surely there must be a better way?
>
> Note: I'm not interested in solutions that require modifying or
currying f.
>
> Regards,
> Ben
>
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On 20/06/2020 7:15 a.m., Benjamin Tyner wrote:> Greetings, > > Occasionally, I desire to call a function with one argument set to equal > to another. Here is a toy example: > > ?? f <- function(x, y) { > > ? ? ?? x + y > ?? } > > ?? f(x = 3, y = x) # Error in f(x = 3, y = x) : object 'x' not found > > So far, the most concise way I found to accomplish this is: > > ?? f(x = 3, y = local(sys.frame(1)$x)) # evaluates to 6 > > but I dislike this solution because local() creates a new environment. > Surely there must be a better way? > > Note: I'm not interested in solutions that require modifying or currying f.How about g <- function(x, y = x) { f(x, y) } g(x = 3) or even yEqualsX <- function(f) function(x, y = x) f(x, y) yEqualsX(f)(x = 3) These are a lot like currying, but aren't currying, so they may be acceptable to you. Personally I'd choose the first one. Duncan Murdoch
On 6/20/20 9:00 AM, Duncan Murdoch wrote:> How about > > g <- function(x, y = x) { > ? f(x, y) > } > g(x = 3) > > or even > > yEqualsX <- function(f) function(x, y = x) f(x, y) > > yEqualsX(f)(x = 3) > > These are a lot like currying, but aren't currying, so they may be > acceptable to you.? Personally I'd choose the first one. > > Duncan Murdoch >Thank you Duncan; I should have been more explicit that I was also trying to avoid defining any new functions, but yes, it's hard to argue with the wrapper approach as a longstanding best practice. Basically I'm wondering if it would be theoretically possible to construct a function "g" (possibly it would have to be primitive/internal) such that f(x = 3, y = g(expr)) would evaluate expr in the evaluation environment of f? After rereading section 4.3.3 of the R Language Definition, it's clear that supplied arguments are evaluated in the calling environment; though trickery in f()'s body may be used to evaluate elsewhere, such options seem limited from within the argument list itself. Ben