foreach doesn't change value of raster cell in R - r

Question

I'm trying to simulate herding behavior in R.
Here's the code
library(raster)
library(sp)
library(foreach)
K=100
sig=0.2
G=0.3
x <- raster(ncol=2000,nrow=2000)
values(x) <- sign(rnorm(4000000,mean=0,sd=0.3))
y <- raster(ncol=2000,nrow=2000)
values(y) <- sign(rnorm(4000000,mean=0,sd=0.3))
#plot(x)
ei <- rnorm(4000000)
j=0
while(j < 30) {
for(i in 1:4000000){
ad <- adjacent(x,cell=c(i))[,2]
y[i] <- sign(K*sum(x[ad])+sig*ei[i]+G)
}
x <- y
plot(x)
j = j+1
}
The classic loop approach is too slow.
If I use a foreach loop instead of a classic for loop it doesn't change the values of y in every iteration.
I can't fix it at all.
Can someone please help about this?
Thank you

Answer 1

You have a dynamic model in which the output of each (time) step is input for the next step. It is not possible to do that in parallel. But that does not mean you cannot make the model run faster.
Looping over raster cells in R is always going to be slow, so we need to avoid that. Normally a problem like this could be solved with focal (see code a the bottom) --- but in this case it is difficult because you effectively use two rasters (x and ei) --- I will look at implementing multi-layer focal operations in the terra package.
Here is an approach with getFocalValues. It is much faster (and I use Sys.sleep to slow it down a bit).
library(raster)
set.seed(0)
x <- raster(ncol=200, nrow=200)
values(x) <- sign(rnorm(ncell(x),mean=0,sd=0.3))
y <- raster(x)
values(y) <- sign(rnorm(ncell(x),mean=0,sd=0.3))
ei <- rnorm(ncell(x))
K=100
sig=0.2
G=0.3
for (j in 1:29) {
# with large rasters, you may need to do the below in chunks
v <- getValuesFocal(x, 1, nrow(x), c(3,3))
# only keep the rook neighbors
v <- v[, c(2,4,6,8)]
v <- rowSums(v, na.rm=TRUE)
values(x) <- sign(K*v+sig*ei+G)
plot(x)
Sys.sleep(0.1)
}
This how you could use focal in similar cases
w <- matrix(c(0,1,0,1,0,1,0,1,0), 3, 3)
y <- focal(x, w, fun=function(i)sign(K*sum(i)+sig+G))
Also see the cellular automata examples in ?focal