Question

If there are multiple boolean expressions as arguments to the which function, are they evaluated lazily?

For example:

which(test1 & test2)

If test1 returns false, then test2 is not evaluated as the compound expression will be false anyway.

Answer 1

With if there can be efficiency gains as a result of that behavior. It is documented to work that way, and I don’t think it is due to lazy evaluation. Even if you “force()-ed” that expression it would still only evaluate a series of &’s until it had a single FALSE. See this help page:

?Logic

@XuWang probably deserved the credit for emphasizing the difference between “&” and “&&”. The “&” operator works on vectors and returns vectors. The “&&” operator acts on scalars (actually vectors of length==1) and returns a vector of length== 1. When offered a vector or length >1 as either side of the arguments, it will work on only the information in the first value of each and emit a warning. It is only the “&&” version that does what is being called “lazy” evaluation. You can see that hte “&” operator is not acting in a “lazy fashion with a simepl test:

 fn1 <- function(x) print(x)
 fn2 <- function(x) print(x)
 x1 <- sample(c(TRUE, FALSE), 10, replace=TRUE)

 fn1(x1) & fn2(x1)  # the first two indicate evaluation of both sides regardless of first value
# [1] FALSE FALSE  TRUE FALSE  TRUE  TRUE FALSE FALSE FALSE FALSE
# [1] FALSE FALSE  TRUE FALSE  TRUE  TRUE FALSE FALSE FALSE FALSE
# [1] FALSE FALSE  TRUE FALSE  TRUE  TRUE FALSE FALSE FALSE FALSE