CODEWITHSUNDEEP
rfunctional-programmingparameter-passingpass-by-reference
Qbik
Qbik

Reputation: 6157

Basics of argument passing in R's functions

How R interpret line : arg.list <- list(x, y) in below definition of function, does it copy x and y into arg.list object when execution happen or they are passed by reference ?

fplot <- function(x, y, add=FALSE){
  arg.list <- list(x, y)
  if(!add){
    plot(arg.list))
  }else{
    lines(arg.list)
  }
}

Upvotes: 1

Views: 1849

Answers (2)

R Yoda
R Yoda

Reputation: 8760

The variables are embedded into the list by reference (at least if you use vectors).

Proof:

library(pryr)

x <-   1:100
y <- 201:200

arg.list <- list(x,y)

al.x <- arg.list[[1]]
al.y <- arg.list[[2]]

Now look at the memory addresses (they are the same):

> address(x)
[1] "0x37598c0"
> address(y)
[1] "0x40fd6f8"
> address(al.x)
[1] "0x37598c0"
> address(al.y)
[1] "0x40fd6f8"

If you change one item a copy will be created ("copy on modification"):

> x[1]=42
> address(x)
[1] "0x417a470"
> al.x <- arg.list[[1]]
> address(al.x)
[1] "0x37598c0"

Edit:

As @HongOoi said: R semantically never uses references (except for objects in the environment class) but copies for variables. It "is clever enough to avoid copies until they are really required" ("copy on [first] modification"). Function parameters are passed "by value" semantically (even though references are used until a modification occurs).

Upvotes: 4

Hong Ooi
Hong Ooi

Reputation: 57696

The semantics of R is that function arguments are always passed by value. The underlying implementation may not necessarily make new copies of the arguments, so as to save memory. But your function will behave as if it has brand-new copies to work with.

This means you don't have to worry about changing a variable outside a function because you changed it inside:

x <- 1
f <- function(z) {
    z <- z + 1
    z
}

y <- f(x)
print(y)  # y now contains 2
print(x)  # but x still contains 1

If R was pass-by-reference, then modifying the argument of f would also modify the variable that was passed in. This doesn't happen.

Upvotes: 3

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