Error in `[<-`(`*tmp*`, i, succ, value = 1) : subscript out of bounds - r

Question

I have to simulate an image with a white crack on a black background. So I defined a function that adds to a matrix with all elements equal to zero some consecutive points equal to one.
The function is the following:
crepa<-function(matrice) {
start<-sample(1:ncol(matrice),1)
matrice[1,start]<-1
for (i in 2:nrow(matrice)) {
alpha<-sample(c(-1,0,1),1)
succ<-start+alpha
if (succ==(ncol(matrice)+1)) succ==ncol(matrice)
if (succ==0) succ==1
matrice[i,succ]<-1
start<-succ
}
matrice<-as.matrix(matrice)
}
To control whether the function works well, I applied it over and over again to the following matrix:
m<-matrix(0,64,64)
imma<-crepa(m)
par(mar=rep(0,4))
image(t(imma), axes = FALSE, col = grey(seq(0, 1, length = 256)))
In most cases the result is correct. However, in few cases I run into this Error:
Error in [<-(*tmp*, i, succ, value = 1) : subscript out of bounds

Answer 1

These two lines:
if (succ==(ncol(matrice)+1)) succ==ncol(matrice)
if (succ==0) succ==1
Should be:
if (succ==(ncol(matrice)+1)) succ=ncol(matrice)
if (succ==0) succ=1
In case you still can't see it, you've used the equality test == when you should use assignment = or <-.
The error message told me it had to be the element going off the matrix, so I started printing out the values of succ and then noticed it wasn't being reset within the right range, and only then did I spot the mistake. I probably looked at the code ten times without noticing. I also figured that kind of error was more likely with a small matrix, and so tested with a 6x6 matrix which meant I could be more likely to see it than with a 64x64!