[R-sig-Geo] overlay() in raster package

[Wearn, Oliver]

Dear all,

I'm trying to use the overlay() function in the raster package, but can't get 'if' clauses to work. Can anyone help?

I have two rasters, the first of which I would like to modify, depending on the value in the second (a forest cover raster). Specifically, if the value in the first (called ENN.500) is NA, I would like to change the value to zero, but only if the value in the forest cover raster is 1 (otherwise the value should just be left).

So, I tried:

test <- overlay(x = ENN.500,y = forest.cover,fun = function(x,y) {
  if (is.na(x))  {
    if (y==1)  {
      return(0)
    }
    else {
      return(x)
    }
  }
  else {
    return(x)
  }
})

...but I'm guessing overlay() doesn't work like this? I'm getting the following errors:

Error: object 'applymethod' not found
In addition: Warning message:
In if (is.na(x)) { :
  the condition has length > 1 and only the first element will be used

I also tried:

test <- overlay(x = ENN.500,y = forest.cover,fun = function(x,y) {
  if (x[is.na(x)])  {
    if (y[y==1])  {
      return(0)
    }
    else {
      return(x)
    }
  }
  else {
    return(x)
  }
})

...which returned the following error:

Error in .overlayList(rasters, fun = fun, filename = filename, datatype = datatype,  : 
  cannot use this formula

The alternative would be to use arithmetic operators: 

ENN.500[is.na(ENN.500] <- 0
forest.cover[forest.cover==0] <- 1
result <- ENN.500*forest.cover

...but it says in ?overlay that this is slow for very large rasters (which I will be using), so I'm guessing I should try not to use this method?

Thanks, in advance, for any help whatsoever.
Oliver