CODEWITHSUNDEEP
rfunctioncall
Mark Heckmann
Mark Heckmann

Reputation: 11441

Double colon operator to specify function in do.call

Suming two vectors using sum and its do.call equivalent would be

sum(1,2)
do.call("sum", list(1,2))

Specifying the sum function using the double colon operator (base::sum) will work in the first but fail in the do.call case.

base::sum(1,2)
do.call("base::sum", list(1,2))
> Error in `base::sum`(1, 2) : could not find function "base::sum"
  1. Is there a way to make do.call work with double colons?
  2. In order to learn: What is happening behind the scenes causing it to fail?

Upvotes: 5

Views: 403

Answers (2)

Thomas Farrar
Thomas Farrar

Reputation: 253

It is possible to use do.call to call a double colon operator with package and function that are specified as characters. Since pkgname::fname is equivalent to `::`(pkgname, fname) (i.e., the double colon operator in backticks is a function), where pkgname and fname can be objects or characters naming objects, we can make `::` the what argument of do.call and pass pkgname and fname (as characters) in a list as the args argument. The arguments of the function named by fname will then come after the do.call call. Thus:

do.call(what = `::`, args = list("base", "sum"))(1, 2)

Upvotes: 3

Eugene Brown
Eugene Brown

Reputation: 4362

According to the documentation the what arg of do.call will take either a function or a non-empty character string naming the function to be called.

If you try implementing the double colon without the quotes it works:

> do.call(base::sum, list(1,2))
[1] 3

So while there is a function named sum in the package base, you can't name the function by also specifying the package. Instead, just remove the quotes.

Upvotes: 6

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