R does not report error when an argument of a function is not provided but used for subsetting a vector

Question

Why there is no error reported when b is not supplied but required inside the function? Thanks!

f2 <- function(a,b) {a[b]}; f2(a=rep(1, 2))

I understand that there is no error in this function:

f <- function(x) {
  10
}
f(stop("This is an error!"))

due to lazy evaluation But this

f <- function(x) {
  force(x)
  10
}
f(stop("This is an error!"))

or this

f <- function(x) {
  x
  10
}
f(stop("This is an error!"))

will produce an error. Because in both cases x is used within the function. Both the above two examples are from http://adv-r.had.co.nz/Functions.html. Since b is also used within f2, should it be necessary to add force inside f2? Thanks!

I'm guessing this is some lazy evaluation quirk that I don't fully understand since if you `debug(f2)` you can see that `b` is returned from `ls()` inside the function's environment. `f2 — thelatemail, Apr 26 '18 at 02:16
But I think in the link above, the original function f is not **used**. https://stackoverflow.com/questions/29733257/can-you-more-clearly-explain-lazy-evaluation-in-r-function-operators — kevin, Apr 26 '18 at 02:36

score 6 · Accepted Answer · answered Apr 26 '18 at 11:56

x[b] returns (a duplicate of) x if b is missing. From the R source:

static SEXP VectorSubset(SEXP x, SEXP s, SEXP call)
{
    R_xlen_t stretch = 1;
    SEXP indx, result, attrib, nattrib;

    if (s == R_MissingArg) return duplicate(x);

https://github.com/wch/r-source/blob/ec2e89f38a208ab02449b706e13f278409eff16c/src/main/subset.c#L169

From the documentation, in which 'empty' means 'missing', not NULL:

An empty index selects all values: this is most often used to replace all the entries but keep the attributes.

score 3 · Answer 2 · answered Apr 26 '18 at 02:36

3

It has to do with the [ function, not lazy evaluation. You'll get an error if you do the following:

f3 <- function(a,b) {a+b}; f3(a = 1)

Note that since b is not defined, R is interpreting it as if it didn't exist. Try doing:

a <- c(1,1) 
a[]

It seems that the subsetting function ( `[` ) actually takes ... as a parameter. I.e., specifying indices to subset are optional.

answered Apr 26 '18 at 02:36

David Klotz

2,316
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Thanks! How about a – kevin Apr 26 '18 at 02:40
1

Again, compare what happens when you do this: `f4 – David Klotz Apr 26 '18 at 02:54
1

@DavidKlotz - but `b` does exist in the environment. `debug(f2); f2(a=1)` then `exists("b")` returns `TRUE`. So it exists but is not evaluated, right? – thelatemail Apr 26 '18 at 03:14
1

It's defined in the function call, but it's still missing: `f2 – David Klotz Apr 26 '18 at 03:34
I see, thanks both! Sometimes I might forget to include the second variable b when calling function f2. Is it the best way to always use f – kevin Apr 26 '18 at 03:45
1

Everything correct here except the part about `...`. If `...` was involved, `\`[\`(a, i = 1, foo = 1)` would work. – Roland Apr 26 '18 at 06:23
1

The best way is to explicitly check for missingness: `f – Hugh Apr 26 '18 at 11:59
Good point, @Roland. So what's really going on is that the source code for the subset function (quoted by @Hugh) explicitly deals with the case of a missing or empty index. – David Klotz Apr 26 '18 at 13:07

R does not report error when an argument of a function is not provided but used for subsetting a vector

2 Answers2