35

I have a function like this dummy-one:

FUN <- function(x, parameter){
  if (parameter == 1){
      z <- DO SOMETHING WITH "x"}
  if (parameter ==2){
      z <- DO OTHER STUFF WITH "x"}
return(z)
}

Now, I would like to use the function on a dataset using apply. The problem is, that apply(data,1,FUN(parameter=1))

wont work, as FUN doesn't know what "x" is. Is there a way to tell apply to call FUN with "x" as the current row/col? `

Produnis
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3 Answers3

37

You want apply(data,1,FUN,parameter=1). Note the ... in the function definition:

> args(apply)
function (X, MARGIN, FUN, ...) 
NULL

and the corresponding entry in the documentation:

...: optional arguments to ‘FUN’.

Joshua Ulrich
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17

You can make an anonymous function within the call to apply so that FUN will know what "x" is:

apply(data, 1, function(x) FUN(x, parameter = 1))

See ?apply for examples at the bottom that use this method.

Chase
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    @Gavin - I guess I don't appreciate the difference in clarity or performance stand point. The R docs are pretty clear that the `...` are for additional parameters to `FUN` so point duly noted. However, it's just a small leap in complexity to needing that anonymous function capability and I'm guessing the OP didn't know about that "trick" previously. – Chase Jan 21 '11 at 16:39
  • Indeed, hence a comment and not a down-vote. Nothing wrong with what you wrote at all. Wasn't meant as criticism. – Gavin Simpson Jan 21 '11 at 17:06
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    Actually, I sounded like and arse and that wasn't the intention. I'm going to delete that comment. The substantive bit (so the other comments mean something) was; you could do the same a bit simpler with `apply(data, 1, FUN, parameter = 1)` – Gavin Simpson Jan 21 '11 at 17:29
  • Hah, no worries and no offense taken. I think it's good to take a step back sometimes and make sure your fundamental operating assumptions about a mundane task are sound. I honestly had forgotten all about the `...` so it was good to reconsider that for a bit. – Chase Jan 21 '11 at 17:44
3

Here's a practical example of passing arguments using the ... object and *apply. It's slick, and this seemed like an easy example to explain the use. An important point to remember is when you define an argument as ... all calls to that function must have named arguments. (so R understands what you're trying to put where). For example, I could have called times <- fperform(longfunction, 10, noise = 5000) but leaving off noise = would have given me an error because it's being passed through ... My personal style is to name all of the arguments if a ... is used just to be safe.

You can see that the argument noise is being defined in the call to fperform(FUN = longfunction, ntimes = 10, noise = 5000) but isn't being used for another 2 levels with the call to diff <- rbind(c(x, runtime(FUN, ...))) and ultimately fun <- FUN(...)

# Made this to take up time
longfunction <- function(noise = 2500, ...) {
  lapply(seq(noise), function(x) {
    z <- noise * runif(x)
  })
}

# Takes a function and clocks the runtime
runtime <- function(FUN, display = TRUE, ...) {
  before <- Sys.time()
  fun <- FUN(...)
  after <- Sys.time()
  if (isTRUE(display)) {
    print(after-before)
  }
  else {
    after-before
  }
}

# Vectorizes runtime() to allow for multiple tests
fperform <- function(FUN, ntimes = 10, ...) {   
  out <- sapply(seq(ntimes), function(x) {
    diff <- rbind(c(x, runtime(FUN, ...)))
  })
}

times <- fperform(FUN = longfunction, ntimes = 10, noise = 5000)

avgtime <- mean(times[2,])
print(paste("Average Time difference of ", avgtime, " secs", sep=""))
Rob
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