[R] factorials

[Damien Joly]

I'm a total newbie at using R, and so there probably
is a better way to do this.  However, I couldn't find
one, and so maybe this will help someone.

I was calculating log-likelihoods using a multinomial
model, and found that for large n, prod(n:1) wouldn't
work to calculate factorials (e.g., prod(200:1) =
Inf).  The below function calculates the natural log
of a factorial (e.g. factorial(100) returns ln(100!)
or in other words 100! = exp(factorial(100)).  

factorial<-function(p) {P<-array(c(p:1),dim=c(p,1))
P<-log(P)
return(sum(P))}

Damien 

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