# Repeating a user-defined function using replicate() or sapply()

I have defined a custom function, like this:

my.fun = function() { for (i in 1:1000) { ... for (j in 1:20) { ... } } return(output) }

which returns an output matrix, output, composed by 1000 rows and 20 columns.

What I need to do is to repeat the function say 5 times and to store the five output results into a brand new matrix, say final, but *without using another for-loop* (this for making the code clearer, and also because in a second moment I would like to try to parallelize these additional 5 repetitions).

Hence final should be a matrix with 5000 rows and 20 columns (the rationale behind these 5 repetitions is that within the two for-loops I use, among other functions, sample).

I tried to use final <- replicate(5, my.fun()), which correctly computes the five replications, but then I have to "manually" put the elements into a brand new 5000 x 20 matrix.. is there a more elgant way to do so? (maybe using sapply()?). Many thanks

## Answers

As is stands you probably have an array with three dimensions. If you wanted to have a list you would have added simplify=FALSE. Try this:

do.call( rbind, replicate(5, my.fun(), simplify=FALSE ) )

Or you can use aperm in the case where "final" is still an array:

fun <- function() matrix(1:10, 2,5) final <- replicate( 2, fun() ) > final , , 1 [,1] [,2] [,3] [,4] [,5] [1,] 1 3 5 7 9 [2,] 2 4 6 8 10 , , 2 [,1] [,2] [,3] [,4] [,5] [1,] 1 3 5 7 9 [2,] 2 4 6 8 10 > t( matrix(aperm(final, c(2,1,3)), 5,4) ) [,1] [,2] [,3] [,4] [,5] [1,] 1 3 5 7 9 [2,] 2 4 6 8 10 [3,] 1 3 5 7 9 [4,] 2 4 6 8 10

There may be more economical matrix operations. I just haven't discovered one yet.

If you replace replicate with rlply from the plyr package, you can use do.call with rbind:

library(plyr) do.call(rbind, rlply(5, my.fun()))

If you'd rather not rely on the plyr package, you can always do:

do.call(rbind, lapply(1:5, function(i) my.fun()))

Depends on which package you use for parallel computing, but here's how I would do it (hide it in a loop using sapply, just like replicate).

library(snowfall) sfInit(parallel = TRUE, cpus = 4, type = "SOCK") # sfExport() #export appropriate objects that will be needed inside a function, if applicable # sfLibrary() #call to any special library out <- sfSapply(1:5, fun = my.fun, simplify = FALSE) sfStop()

Try this:

final <- replicate(5, my.fun(), simplify = "matrix")

You will get the result of 'final' in the form of matrix.