Treat arithmetic operators as functions

Question

I've read that everything in R is function. So I wonder if "+" is a function too and if we can write something like that:

xx <- c(1,2,3)
yy <- c(1,2,3,4,5,6)

# zz is the sum of the two lengths
zz <- +(if(exists("xx")) length(xx), if(exists("yy")) length(yy))

Answer 1

Yes, you can:

xx <- c(1,2,3)
yy <- c(1,2,3,4,5,6)

# zz is the sum of the two lengths
zz <- `+`(if(exists("xx")) length(xx), if(exists("yy")) length(yy))
#[1] 9

To call objects that don't have syntactically valid names (such as the function + that gets called implicitly if you do something like 1 + 2) you need to enclose the name in backticks (`) or quotes (" or ').

See also Section 3.1.4 of the R Language Definition:

Except for the syntax, there is no difference between applying an operator and calling a function. In fact, x + y can equivalently be written `+`(x, y). Notice that since ‘+’ is a non-standard function name, it needs to be quoted.

In your code you get the error:

Error: unexpected ',' in "zz <- +(if(exists("xx")) length(xx),"

This is because you don't call the (binary) function "+", but the unary operator +, which doesn't expect function arguments and thus interprets the parentheses as the "arithmetic" operator. A comma is not allowed between those.