Count number of arguments passed to function

问题:

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?

回答1:

How about

myCount <- function(...) {length(match.call())-1}

This just inspects the passed call (and removes 1 for the function name itself)