# [R] "Spreading risk" in a matrix

From: ACroske <Audy3272_at_yahoo.com>
Date: Fri, 18 Jul 2008 11:24:01 -0700 (PDT)

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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Received on Fri 18 Jul 2008 - 18:35:27 GMT

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