What is the logic of this function in R?

Question

I'm studying about arguments in R functions, but I have some problem to understand the logic of it.

h <- function(a = 1, b = d){
    d <- (a + 1)^2
    c(a, b)
}

h()
# [1] 1 4

I expected the error message would be return because there is no value of b. d is created under h function but there is no code like b = d which assign a value to b in function h .

However, the result is [1] 1 4.

How were b and d linked?

Answer 1

Default function arguments values are lazily evaluated in R (i.e. evaluated only when they're needed):

See the output of this code for an example :

printme <- function(name,x){cat('evaluating',name,'\n');x}

h <- function(a = printme('a',1), b = printme('b',d)){
  cat('computing d...\n')
  d <- (a + 1)^2
  cat('d computed\n')
  cat('concatenating a and b...\n')
  c(a, b)
  cat('a and b concatenated\n')
}

h()

Console output :

computing d...
evaluating a 
d computed
concatenating a and b...
evaluating b 
a and b concatenated

As you can see, d is calculated before evaluating the default value of b

EDIT :

Furthermore, as correctly pointed out by @BrodieG in the comments, default arguments are evaluated in the function environment; in fact, in the example above, b can be initialized to the value of variable d that is defined inside the function environment.

Instead, when you specify a argument (without using the default), the expression that assigns the parameter is still lazily evaluated, but this time in the calling environment e.g. :

# same functions as above, but this time we specify the parameters in the call     
h(a=printme('a',123),b=printme('d',d))

Console output :

computing d...
evaluating a 
d computed
concatenating a and b...
evaluating d 
Error in printme("d", d) : object 'd' not found

Note the error when argument b is evaluated because d cannot be found in the calling environment.