Count number of arguments passed to function

user_1655567 0
Viewed 3146

I'm interested in counting a number of arguments passed to a function. length can't be used for that purpose:

>> length(2,2,2,2,2)
Error in length(2, 2, 2, 2, 2) : 
  5 arguments passed to 'length' which requires 1

This is obvious as length takes 1 argument so:

length(c(2,2,2,2,2))

would produce the desired result - 5.

Solution

I want to call my function like that myFunction(arg1, arg2, arg3). This can be done with use of an ellipsis:

myCount <- function(...) {length(list(...))}

myCount would produce the desired result:

>> myCount(2,2,2,2,2)
[1] 5

Problem

This is awfully inefficient. I'm calling this function on substantial number of arguments and creating lists just to count number of objects is wasteful. What's the better way of returning the number of arguments passed to a function?

ellipsis r function
3 Answers
user_36446890

Here is a somewhat elegant way using length() with purrr::lift_*() familiy functions.

Generally you are passing multiple arguments to length(), which is not working because length() takes a vector or a list as input.

So what we need is to convert the input from a vector/list to ... (dots). purrr::lift_*() family provides a series of functions that do so.

One option can be converting from vector to dots:

> lift_vd(length)(2, 2, 2, 2, 2)
[1] 5

Another option can be converting from list to dots:

> lift_ld(length)(2, 2, 2, 2, 2)
[1] 5

Both options are working perfectly well, and what you need is using one of the purrr::lift_*() functions on length() before passing spliced arguments to it.

user_137193290
Related