raster - R: Overlay using functions containing if/else statements

I have two rasters, x and y, and I'd like to do calculations with them using the overlay function. For example,

MyFun <- function(x, y) x * y
res <- overlay(x, y, fun = MyFun)

This seems to work fine. Now, I want to add a conditional statement to my function like this:

MyFun <- function(x, y) {
    if(x > 0){ 
        x * y
    }else{
        abs(x) * y
    }
}

This gives a warning message like below:

Warning messages:

1: In if (x > 0) { :
  the condition has length > 1 and only the first element will be used

Clearly, R isn't happy with my code.

How should I write conditional statements in my functions so that they are compatible with overlay and rasters?

NB This is a toy example. My real task is more complicated, but the principle is the same.

Answer

You could use ifelse as an alternative to if and else blocks in a function. You can nest multiple statements in an ifelse and if you are trying to vectorize a problem, it is much cleaner. Note that an absolute abs statement on a zero value still returns zero so, I just used a very small number as a constant.

library(raster)
x <- raster(nrows=100, ncols=100)

  x[] <- runif(ncell(x))
y <- raster(nrows=100, ncols=100)
  y[] <- runif(ncell(y))

myFun <- function(x, y, p = 1.0e-08) { ifelse( x > 0, x * y, ifelse(x <= p * y)) }
( res <- overlay(stack(x, y), fun = myFun) )