How to make a 0 stay as a 0 in a matrix - r

Question

I use the following code to generate a matrix
randomdiv <-
function(nchrom, ndivs, size) {
sz <- matrix(nrow = nchrom, ncol = ndivs)
for (j in 1:nchrom) {
n <- size
for (i in 1:ndivs)
{
old_subs <- rbinom (1, n, 0.5)
num_chrom <- rep(1 / nchrom, nchrom)
new_subs <- rmultinom(1, size * nchrom / 2, prob = c(num_chrom))
m <- old_subs + new_subs
sz[j,i] <- m[1,1]
n <- m
}
}
return (sz)
}
>randomdiv(3, 3, 10)
[,1] [,2] [,3]
[1,] 11 13 12
[2,] 6 8 5
[3,] 12 11 9
The only adjustment I need to make is that when a 0 is generated in the column by the rbinom function, I need that occurence to stay as a 0 for the remainder of the matrix, but anything >0 needs to go through the rest of the loop and have new_subs added to it.
I have tried;
randomdiv <- function(nchrom, ndivs, size) {sz <- matrix(nrow = nchrom, ncol = ndivs)
for (j in 1:nchrom) {
n <- size
for (i in 1:ndivs)
{
old_subs <- rbinom (1, n, 0.5)
num_chrom <- rep(1/nchrom, nchrom)
new_subs <- rmultinom(1, size*nchrom/2, prob = c(num_chrom))
m <- ifelse(old_subs>0, old_subs + new_subs, old_subs+0)
sz[j,i] <- m[1,1]
n <- m
}
}
return (replicate(ncell, sz, simplify = FALSE))
}
> randomdiv(3, 3, 10)
#Error in m[1, 1] : incorrect number of dimensions
I've tried a few different tactics with the ifelse function, but I think it only treats the columns as a whole, so if there is a 0 at all, nothing happens for the whole column, whereas I need each value in the columns to be treated individually.

Answer 1

You just need to use if() with an else and skip several lines of code if there's a 0:
randomdiv <-
function(nchrom, ndivs, size) {
sz <- matrix(nrow = nchrom, ncol = ndivs)
for (j in 1:nchrom) {
n <- size
for (i in 1:ndivs)
{
old_subs <- rbinom (1, n, 0.5)
if(old_subs>0){
num_chrom <- rep(1 / nchrom, nchrom)
new_subs <- rmultinom(1, size * nchrom / 2, prob = c(num_chrom))
m <- old_subs + new_subs
sz[j,i] <- m[1,1]
} else sz[j,i] <- old_subs
n <- m
}
}
return (sz)
}
randomdiv(3, 3, 2)
# [,1] [,2] [,3]
# [1,] 2 2 0
# [2,] 1 2 4
# [3,] 1 1 0