On 30-11-2012, at 23:55, Mac Gaulin wrote:
Hello,
I am trying to understand a small quirk I came across in R. The
following code results in an error:
k <- c(2, 1, 1, 5, 5)
f <- c(1, 1, 1, 3, 2)
loglikelihood <- function(theta,k,f){
if( theta<1 && theta>0 )
return(-1*sum(log(choose(k,f))+f*log(theta)+(k-f)*log(1-theta)))
return(NA)
}
nlm(loglikelihood ,0.5, k, f )
Running this code results in:
Error in nlm(logliklihood, 0.5, k, f) :
invalid function value in 'nlm' optimizer
if you do a <- NA str(a) you'll see that a is a logical. And that is an invalid value for the function to return. When you do things such as NA+0 or -NA you get a numeric value NA and that is a non-finite value which is why you get the warning.. Instead of NA you could return something like .Machine$double.xmax . Or use constrOptim as advised. Berend
However if the line return(NA) is changed to return(-NA) or even return(1*NA) or return(1/0), the code works. Is this expected behavior? The nlm help file says not NA or not NaN are acceptable values but I don't understand what that means. Thank you for your time, Mac Gaulin
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