[R] how to use try()

[r.ghezzo]

Hello, I have a program with this section:
..
for(i in 1:20){
   lo <- nls(y~y0+a/(1+(x/x0)^b),start=list(y0=0.1,a=a0,x0=x00,b=-8.1))
   beta[i] <- lo$m$getPars()[4]
}
..
If the fit works this is OK but if the fit fails, the whole program 
fails so:
..
for(i in 1:20){
   try(lo <- 
nls(y~y0+a/(1+(x/x0)^b),start=list(y0=0.1,a=a0,x0=x00,b=-8.1)))
   beta[i] <- lo$m$getPars()[4]
}
..
but the try catches the error in nls and beta[i] gets assigned beta[i-1] 
from the previous loop. This is bad but no so bad as it can be checked,
Now in some cases the error is in i=1 and the program stops!!
is there a way to set lo$m$getPars() to zero before the call?
I tried to understand the use of tryCatch() but frankly it is above me. 
Sorry
Thanks for any help
Heberto Ghezzo
Meakins-Christie Labs