CODEWITHSUNDEEP
rtidyeval
xiaodai
xiaodai

Reputation: 16014

How do I capture the argument to a function and apply them as is in another context/environment

I am pretty sure my language is imprecise, but I don't know the correct terms yet. This is what I wanted to achieve:

I have a function abc2. I want to obtain the arguments to them (without evaluating or changing the argument) provided by the user, to use in another function. E.g.

abc2 <- function(...) {
 # what do I have to do here?
}

adf = data.frame(a = 1:3, b = 1:3)
bdf = data.frame(a = 11:13, b = 11:33)

such that calling abc2(a) is the same as calling 

dplyr::select(adf, a)
dplyr::select(bdf, a)

and that calling abc2("a") is the same as calling

dplyr::select(adf, "a")
dplyr::select(bdf, "a")

Basically, I want to transplant the ... from abc2 and apply them without any change from how the user has input it and apply elsewhere.

Base R and tidyeval explanation welcome! I am trying to read up on quo and enquo but I can't seem to get my head around it.

Upvotes: 0

Views: 60

Answers (1)

G. Grothendieck
G. Grothendieck

Reputation: 269664

1) base R Pass ... on. Either "a" or just a can be used. The passing mechanism here uses only base R. The dplyr is only so that we can use select.

library(dplyr)

adf = data.frame(a = 1:3, b = 1:3)
bdf = data.frame(a = 11:13, b = 21:23)

abc2 <- function(...) {
  print(select(adf, ...))
  print(select(bdf, ...))
}

# or: abc2(a)
abc2("a")

giving:

  a
1 1
2 2
3 3
   a
1 11
2 12
3 13

2) rlang This also works but this time we use rlang's enquos and !!! with tidyeval all of which is pulled in by dplyr.

abc2a <- function(...) {
  dots <- enquos(...)
  print(select(adf, !!!dots))
  print(select(bdf, !!!dots))
}
abc2(a)

Upvotes: 4

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