Compare two rasters in R with the cell value from one replacing the cell value from another

Question

I am trying to create a single raster from two rasters. The first raster is made up of all 0 values and the second is made up of the current date value (formatted in days from 001 to 365). The problem is using raster overlay I am struggling to get the result I am after. I can combine the to using the following without any issue:

zeroRaster <- overlay(changeDate, zeroRaster, fun=function(x,y){return(x+y)})

however when a specific cell changes multiple times the two dates are added together which is obviously incorrect (with the x+y function being to blame).

As I am struggling to explain I will try and visually represent what I am trying to achieve below:

Essentially I just want to replace any cell value in raster y with the new date value from raster x without performing any mathematical functions.

Answer 1

Here is what Mikkel suggested (use of max)

library(raster)
cell100 <- raster(nr=3, nc=3, vals=c(100,0,0,0,100,0,0,0,100))
cell101 <- raster(nr=3, nc=3, vals=c(0,0,101,0,101,0,101,0,0))
r <- max(cell100, cell101)

as.matrix(r)
#     [,1] [,2] [,3]
#[1,]  100    0  101
#[2,]    0  101    0
#[3,]  101    0  100

Another (more complex) approach could be

cell100 <- raster(nr=3, nc=3, vals=c(100,NA,NA,NA,100,NA,NA,NA,100))
cell101 <- raster(nr=3, nc=3, vals=c(NA,NA,101,NA,101,NA,101,NA,NA))
x <- cell101
x <- cover(x, cell100)
x <- reclassify(x, cbind(NA, 0))