Change default arguments of an R function at runtime

E

4

13

Is it possible to change the default values of formal parameters in an R function at runtime?

Let's assume, we have the function

f <- function(x=1) { 
    ...
}

can I somehow change the default value of x from 1 to, say, 2?

Thanks in advance,
Sven

Euryale answered 18/4, 2012 at 15:3 Comment(3)

An interesting question, but it sounds a bit dangerous. Why would you want to do that? – Dracula 18/4, 2012 at 15:5

I am trying to implement a VM for R, therefore I need to know whether function signatures can be assumed to be immutable. – Euryale 18/4, 2012 at 15:7

#9896311 – Hotze 19/4, 2012 at 2:57

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7

UPDATE: 2020-12-13

This method is no longer available

Yes, the Defaults package allows you to do this.

Gigantic answered 18/4, 2012 at 15:9 Comment(5)

I found another way to accomplish this task: formals(f) = pairlist(x=1) – Euryale 18/4, 2012 at 16:7

@SvenHager: that's essentially what Defaults does, and you might want to use alist instead of list or pairlist. – Gigantic 18/4, 2012 at 16:25

Cool. I might use this to change paste( , sep=" ") to paste( , sep=""). (Yes, I know about paste0 in R 2.15.0) – Philippopolis 18/4, 2012 at 18:24

@KevinWright: be careful doing that with widely-used support functions like paste. You may break things that will be difficult to diagnose. – Gigantic 18/4, 2012 at 18:34

It looks like Defaults is no longer available from CRAN. You can still do this using the formals function, see: #9896311 – Magel 10/2, 2016 at 18:16

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2

An alternative (shown in a different SO post) is to use the formals function, e.g.:

formals(f) <- 2

Magel answered 10/2, 2016 at 18:23 Comment(1)

This errors using R 4.3.2. You should set the value explicitly with formals(f)$x <- 2. – Sprit 29/3 at 18:19

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1

As the Defaults package is no longer available from CRAN, you can use default.

As an example:

x <- list(a = 1, b = 2, c = 3)
default::default(unlist) <- list(use.names = FALSE)
unlist(x)
#> [1] 1 2 3

unlist <- default::reset_default(unlist)
unlist(x)
#> a b c 
#> 1 2 3

Created on 2019-03-22 by the reprex package (v0.2.0.9000).

Discriminating answered 22/3, 2019 at 14:57 Comment(0)

C

1

I tried to do the same argument wrapping for the packagefinder library which has an alias of fp() pointing to findPackage(). I tried all sorts of methods, including using formals(), but in the end, the only thing that worked for me were the following 3 variations:

#--------------------------------------
# packagefinder
#--------------------------------------
# fp = findPackage
# Set default to use: 
#   fp(... , display = "console", return.df = TRUE)
#--------------------------------------
fp <- function(...) {
  packagefinder::fp(..., display="console", return.df=TRUE)
}

fp <- function(...) invisible(findPackage(..., display="console", return.df=TRUE))
fp <- function(..., display="console", return.df=TRUE) packagefinder::fp(...,display=display, return.df=return.df)

The formals() method I could not get to work.

# Fail-1
formals(fp) <- alist(... = , display="console", return.df=TRUE)

# Fail-2
MY_ARGS <- list(display="console", return.df=TRUE)
formals(fp)[names(MY_ARGS)] <- MY_ARGS

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