Fastest way to "vectorize" a function with differing argument lengths

Question

Take the following simple function:

fun <- function(a, b, c, d, e) {
   
   stopifnot("Input you provide must be equal length." = length(a) == length(b) && length(b) == length(c) && length(c) == length(d) && length(d) == length(e))

   result <- (a + b / c + d) / sqrt(e)
   
   result2 <- a/result

   return(data.frame(result = result, result2 = result2, a = a, b = b, c = c, d = d, e = e))
}

Now, if I want to map over a look-up table of all combinations of input values, I could do the following, e.g., using purrr functionals:

library(purrr)

df <- expand.grid(a = 1:1000, b = c(1, 2, 3, 4, 5), c = 7, d = 3, e = 5)

out <- pmap_df(d, fun)

However, even for the relatively simple case of one larger and a smaller vector (in my application case, this would be the most common case though), this is pretty slow.

Unit: seconds                                                                             
      min       lq     mean   median       uq      max neval
 2.235245 2.235245 2.235245 2.235245 2.235245 2.235245     1

How to speed this up, especially for the simple case sketched above? Of course, as df gets larger and larger, things will slow down.

Answer 1

I cannot say my solution is fastest, but it is faster indeed. You can try the code below

do.call(fun, df)

and the benchmarking

df <- expand.grid(a = 1:1000, b = c(1, 2, 3, 4, 5), c = 7, d = 3, e = 5)


f_Rob <- function() pmap_df(df, function(a, b, c, d, e) fun(a = a, b = b, c = c, d = d, e = e))
f_TIC <- function() do.call(fun, df)

microbenchmark(
  f_Rob(),
  f_TIC(),
  unit = "relative",
  check = "equivalent",
  times = 10
)

and you will see

Unit: relative
    expr      min       lq     mean   median       uq      max neval
 f_Rob() 1074.886 1049.034 441.6319 854.2739 620.4029 92.29739    10
 f_TIC()    1.000    1.000   1.0000   1.0000   1.0000  1.00000    10