[R] lsoda with arbitrary zero thresholds

[Martin Henry H. Stevens]

using R 2.0.0
I am trying to do some population modeling with lsoda, where I set 
arbitrary zero population sizes when values get close to zero, but am 
having no luck.
As an example of what I have tried, I use code below from the help page 
on lsoda in which I include my modification bordered by ###

parms <- c(k1=0.04, k2=1e4, k3=3e7)
my.atol <- c(1e-6,  1e-10,  1e-6)
times <- seq(0,)

lsexamp <- function(t, y, p)
   { ### The next line is where I try to insert the threshold
ifelse(y < 0.4,  0, y)
###### all else is unchanged
     yd1 <- -p["k1"] * y[1] + p["k2"] * y[2]*y[3]
     yd3 <- p["k3"] * y[2]^2
     list(c(yd1,-yd1-yd3,yd3),c(massbalance=sum(y)))
   }
out <- lsoda(c(.5,0,.5),times,lsexamp, parms, rtol=1e-4, atol= my.atol) 
# Initial values differ from help page
matplot(out[,1],out[,2:5], type="l")
out[dim(out)[1],] # The intent of my could was to cause population 1 to 
fall to zero as soon as it reached < 0.4

Any thoughts would be appreciated. Thanks!
Hank Stevens


Dr. Martin Henry H. Stevens, Assistant Professor
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Botany Department
Miami University
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