Passing arguments to iterated function through apply

Question

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? `

Answer 1

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’.

Answer 2

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.

Answer 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=""))