[Rd] quantiles of the hypergeometric distribution (PR#502)

[Torsten Hothorn]

> Hello!
> 
> I use R-version 1.0.0
> To get the 0.95 quantile of the hypergeometric
> distribution with the parameters m=45000,n=5000 and
> k=600 I use the R-command 
> 
> > qhyper(0.95,45000,5000,600).
> 
> 
> The value obtained is 600. However, the true value
> is 552. 

Well, 

had a short look. The problem arises in qhyper.c at line 56:

 term = exp(lfastchoose(NR, xr) + lfastchoose(NB, xb) - lfastchoose(N, n));

The expression inside the exp() function gives -1415.411856 and exp()
returns 0. So term and sum in 

 while(sum < p && xr < xend) {
        xr++;
        NB++;
        term *= (NR / xr) * (xb / NB);
        sum += term;
        xb--;
        NR--;
    }

are 0 all the time and xend = 600 is returned. 

R-Core: How to fix this?

Torsten

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