[Rd] Re: [R] chisq.test freezing on certain inputs (PR#5701)

[torsten at hothorn.de]

On Thu, 11 Dec 2003, Jeffrey Chang wrote:

> Hello everybody,
>
> I'm running R 1.8.1 on both Linux and OS X compiled with gcc 3.2.2 and
> 3.3, respectively.  The following call seems to freeze the interpreter
> on both systems:
>  > chisq.test(matrix(c(233, 580104, 3776, 5786104), 2, 2),
> simulate.p.value=TRUE)
>
> By freeze, I mean, the function call never returns (running > 10 hours
> so far), the process is unresponsive to SIGINT (but I call kill it with
> TERM), and the process still consumes cycles on the CPU.
>

This is due to calling `exp' with a very small value leading to
a zero return value in rcont2 (src/appl/rcont.c) line 70:

            x = exp(fact[iap - 1] + fact[ib] + fact[ic] +
                    fact[idp - 1] - fact[ie] - fact[nlmp - 1] -
                    fact[igp - 1] - fact[ihp - 1] - fact[iip - 1]);
            if (x >= dummy) {
                goto L160;
            }
            sumprb = x;
            y = x;


y is never checked for zero and later on

L150:
            if (lsm) {
                goto L155;
            }

            /* Decrement entry in row L, column M */
            j = nll * (ii + nll);
            if (j == 0) {
                goto L154;
            }
            --nll;
            y = y * j / (double) ((id - nll) * (ia - nll));
            sumprb += y;
            if (sumprb >= dummy) {
                goto L159;
            }
            if (! lsp) {
                goto L140;
            }
            goto L150;


y has no chance of becoming larger than zero and we are in the goto trap
until the end of time. A simple fix would be checking for zero but I don't
know how one would proceed in this case ...

Best,

Torsten