Assign Value to Diagonal Entries of Matrix

I need to access and assign single slots of an m*n matrix inside a for loop. The code so far:

rowCount <- 9
similMatrix = matrix(nrow = rowCount - 1, ncol = rowCount)
show(similMatrix)
for(i in (rowCount - 1)){
  for (j in rowCount)
    if (i == j){
      similMatrix[i == j] <- 0;
    }
}
show(similMatrix)

so if i = j the NA value in the matrix needs to be replaced with 0.