[R] "Spreading risk" in a matrix

[ACroske]

I have a binary matrix that represents a map of invasive species risk (1 =
infested; 0 = uninfested). It started as a matrix of probabilities
(developed in ArcMap) that I converted to binary with this R code:
binary.matrix<-matrix(rbinom(length(prob.matrix),prob=prob.matrix,size=1),nrow=nrow(prob.matrix))

Now, I would like to "spread" the risk for year 2 of the invasion. I'd like
to do this by location, so, for every cell that is currently a 1, next year
it's immediate neighbors (above, below, to the right and left) would also
become ones. Here is a function I came up with to attempt this (clearly, my
function writing is rough, to say the least):

matrix.function<-function(N){
  
binary.matrix<-matrix(rbinom(length(prob.matrix),prob=prob.matrix,size=1),nrow=nrow(prob.matrix))
   res<-numeric(N)
       for (i in 1:N){
       res[i]<-binary.matrix
            if (res[i]==1){
            res[,i+1]=1
            res[,i-1]=1
            }
       }
}

When I run this code, I get many messages saying "In res[i]<-binary.matrix :
number of items to replace is not a multiple of replacement length"

I know that if I actually got this to work, it would be a string of numbers
(because of the "numeric"), but I'm just working with my limited
function-writing knowledge, and I assumed it would be possible to convert
the results to a matrix later on.

I'm aware that this function is probably a disaster, but that's why I'm
here!
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