Re: floor(NaN) problem fixed in massdist.c (PR#291)

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Subject: Re: floor(NaN) problem fixed in massdist.c (PR#291)
maechler@stat.math.ethz.ch
Date: Sat 09 Oct 1999 - 02:54:09 EST


Message-Id: <199910081654.SAA16942@pubhealth.ku.dk>

Dear Naoki,
Thank you very much,

for your detailed bug report and your proposed patch.

>> This will fix the "problem 2 (crash in fft)" in Bug ID #277
>>
>> On Linux/Alpha, make check failed because R could not handle the following
>> example in base-Ex.R
>>
>> ##___ Examples ___:
>>
>> # The Old Faithful geyser data
>> data(faithful)
>> :
>> :
>> ## Missing values:
>> x <- xx <- faithful$eruptions
>> x[i.out <- sample(length(x), 10)] <- NA
>> doR <- density(x, bw=0.15, na.rm = TRUE)
>> doN <- density(x, bw=0.15, na.rm = FALSE)
>> ### it fails at this statement ###
>> lines(doR, col="blue")
>> ...

>> The following statement in density() was not getting the right output:
>>
>> y <- .C("massdist", x = as.double(x), nx = N, xlo = as.double(lo),
>> xhi = as.double(up), y = double(2 * n), ny = as.integer(n),
>> NAOK = has.na, PACKAGE = "base")$y
>>
>> y[1] and y[2] should be 0 and 0 but both of them became NaN after this
>> statement.
>>
>> So I looked at src/appl/masdist.c with a debugger. The bug is caused
>> by floor(). On Alpha/Linux, floor(NaN) returns 0.

Thank you so much for finding this!

However, this is clearly a bug of (your version of) gcc, respectively libm
on Alpha/Linux.
In general, we do rely on IEEE Arithmetic working correctly.

>> x[i] in the
>> following code is the list passed from the above statement in
>> density(). The array x contains a couple of NaN. When x[i] is NaN,
>> xpos become NaN. Then ix = floor(xpos) returns an unexpected result.
>> On linux/i386, ix become -2147483648 when xpos = NaN(0x80000000007a2).
>> However ix become 0 on Linux/Alpha when xpos=NaN. When xpos = NaN, it
>> shouldn't go into the following "if" statement. But it goes inside on
>> Alpha/Linux (because ixmin is 0), and set y[0] and y[1] to NaN.
>>
>> Following is extracted from massdist.c
>>
>> for(i=0 ; i<*nx ; i++) {
>> xpos = (x[i] - *xlow) / xdelta;
>> ix = floor(xpos);
>> fx = xpos - ix;
>> if(ixmin <= ix && ix <= ixmax) {
>> y[ix] += (1.0 - fx);
>> y[ix + 1] += fx;
>> }
>> else if(ix == -1) {
>> y[0] += fx;
>> }
>> else if(ix == ixmax + 1) {
>> y[ixmax + 1] += (1.0 - fx);
>> }
>> }
>>
>>
>> So I think the statement
>>
>> if(ixmin <= ix && ix <= ixmax) {
>>
>> should be changed to
>>
>> if((! isnan(ix)) && ixmin <= ix && ix <= ixmax) {
>>
>> or
>> ........

Instead of isnan () , it should rather be ISNAN() [an R defined macro]
which is true for both NaN and NA.
(note that we also try to have a working ISNAN() when there's no isnan()
available from C
  (note that ANSI or POSIX C unfortunately do NOT require IEEE arithmetic!)

I'll have a look at the code and your proposed patch.

Thanks again!

-- 
Martin Maechler <maechler@stat.math.ethz.ch>	http://stat.ethz.ch/~maechler/
Seminar fuer Statistik, ETH-Zentrum  LEO D10	Leonhardstr. 27
ETH (Federal Inst. Technology)	8092 Zurich	SWITZERLAND
phone: x-41-1-632-3408		fax: ...-1228			<><

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