[R] A function taken out of its original environment performs more slowly.

[Uwe Ligges]

On 18.08.2013 01:05, Xiao He wrote:
> Hi dear R-users,
>
> I encountered an interesting pattern. Take for example the function
> combn(), I copied and pasted the function definition and saved it as a new
> function named combn2() (see the end of this email). As it turned out,
> combn2() seems to be substantially slower than the original function
> combn() (see benchmark below),
>
>> system.time(combn(30, 5)); system.time(combn2(30, 5))

combn is bytecode compiled, combn2 not:

try

library("compiler")
combn2 <- cmpfun(combn2)

and you won't see such a difference anymore.

Best,
Uwe Ligges




>     user  system elapsed
>    0.304   0.003   0.308
>     user  system elapsed
>    1.591   0.007   1.602
>
>
> I wonder if there is any reason for this difference and if there is any way
> to reduce the performance difference. Thanks!
>
> combn2 <- function (x, m, FUN = NULL, simplify = TRUE, ...)
> {
>      stopifnot(length(m) == 1L)
>      if (m < 0)
>          stop("m < 0", domain = NA)
>      if (is.numeric(x) && length(x) == 1L && x > 0 && trunc(x) ==
>          x)
>          x <- seq_len(x)
>      n <- length(x)
>      if (n < m)
>          stop("n < m", domain = NA)
>      m <- as.integer(m)
>      e <- 0
>      h <- m
>      a <- seq_len(m)
>      nofun <- is.null(FUN)
>      if (!nofun && !is.function(FUN))
>          stop("'FUN' must be a function or NULL")
>      len.r <- length(r <- if (nofun) x[a] else FUN(x[a], ...))
>      count <- as.integer(round(choose(n, m)))
>      if (simplify) {
>          dim.use <- if (nofun)
>              c(m, count)
>          else {
>              d <- dim(r)
>              if (length(d) > 1L)
>                  c(d, count)
>              else if (len.r > 1L)
>                  c(len.r, count)
>              else c(d, count)
>          }
>      }
>      if (simplify) {
>          out <- matrix(r, nrow = len.r, ncol = count)
>      }
>      else {
>          out <- vector("list", count)
>          out[[1L]] <- r
>      }
>      if (m > 0) {
>          i <- 2L
>          nmmp1 <- n - m + 1L
>          while (a[1L] != nmmp1) {
>              if (e < n - h) {
>                  h <- 1L
>                  e <- a[m]
>                  j <- 1L
>              }
>              else {
>                  e <- a[m - h]
>                  h <- h + 1L
>                  j <- 1L:h
>              }
>              a[m - h + j] <- e + j
>              r <- if (nofun)
>                  x[a]
>              else FUN(x[a], ...)
>              if (simplify)
>                  out[, i] <- r
>              else out[[i]] <- r
>              i <- i + 1L
>          }
>      }
>      if (simplify)
>          array(out, dim.use)
>      else out
> }
>
> 	[[alternative HTML version deleted]]
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>