[R] Efficient multinom probs

[Ingmar Visser]

Dear R-helpers,

I need to compute probabilties of multinomial observations, eg by doing the
following:

y=sample(1:3,15,1)
prob=matrix(runif(45),15)
prob=prob/rowSums(prob)
diag(prob[,y])

However, my question is whether this is the most efficient way to do this.
In the call prob[,y] a whole matrix is computed which seems a bit of a
waste. 

Is there maybe a vectorized version of dmultinom which does this?

Best, Ingmar