[R] problem with coding for 'optim' in R

[Michael Rennie]

Hi, Roger,

At 01:37 PM 7/14/03 -0700, Roger D. Peng wrote:
>It's important to remember that in R functions return whatever happens to 
>be the last element of the function block, unless there is an explicit 
>'return' statement.  Your function 'f' in the second example is written 
>incorrectly and will not work in 'optim'.  The last element in the 
>function block is:
>
>write.table (bioday, file = "perch.csv", append = FALSE, sep=",", na = NA, 
>col.names = TRUE)
>
>>which I assume is *not* the value you want the function return.  Your 
>>function 'f' is returning whatever 'write.table' returns, which is 
>>nothing useful.  My guess is that you want your function 'f' to return 
>>the value 'f' defined in the function as
>>
>>f <- (((Wt-Wtmod)^2 + (Hgt-Hgtmod)^2)/2)^2
>>
>>So this statement should be the last line of your function.

This is valuable information.  Thanks very much.  I'll move things about 
and see what happens.


>>Also, your function 'f' (still from the second output) doesn't use the 
>>value 'q' at all, so I can't see how the optimizer can optimize a 
>>function that ignores its parameters.

 From what I've read and the examples I've encountered, the 'optim' 
function expects the first entry in

optim(x, f, etc....)

To be the starting point for your variable that you specify earlier in the 
loop under

f<- function (x)

If I am wrong on this, then this could be giving me problems as well.

>>-roger

Michael Rennie
M.Sc. Candidate
University of Toronto at Mississauga
3359 Mississauga Rd. N.
Mississauga, ON  L5L 1C6
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