[Rd] Re: [R] Odd underflow(?) error

[William Faulk]

Martin Maechler wrote:
"TL" == Thomas Lumley <[EMAIL PROTECTED]>
   on Fri, 3 Dec 2004 15:22:07 -0800 (PST) writes:

    TL> On Fri, 3 Dec 2004, William Faulk wrote:
    >> I'm still trying to install R on my Irix machine.  Now I have a new problem 
    >> that crops up during the checks.  I've found the root cause, and it's that R 
    >> is returning zero for certain things for reasons I don't understand.
    >> 
    >> 2.225073859e-308, entered directly into R, responds "2.225074e-308".
    >> 2.225073858e-308 responds "0".
    >> 
    >> Their negative values respond similarly, so it would appear that somewhere in 
    >> there is the smallest absolute value that that installation of R will hold.

    TL> Yes.  .Machine$double.xmin tells you the smallest number representable to 
    TL> full precision, which is 2.225074e-308 (I think on all machines where R 
    TL> works)

    >> On another machine where the checks passed, both responses are correct, not 
    >> just the first one.  The underflow there is significantly lower, with much 
    >> less accuracy, as opposed to what seems to be good accuracy on what looks 
    >> like the broken one.  The values there are:
    >> 
    >> 2.4703282293e-324 gives 4.940656e-324
    >> 2.4703282292e-324 gives 0

    TL> Machines can differ in what they do with numbers smaller than 
    TL> .Machine$double.xmin. They can report zero, or they can add leading zeros 
    TL> and so lose precision.  Suppose you had a 4-digit base 10 machine with 2 
    TL> digits of exponent.  The smallest number representable to full accuracy 
    TL> would be
    TL> 1.000e-99
    TL> but by allowing the leading digits to be zero you could represent
    TL> 0.001e-99
    TL> ie, 1e-102, to one digit accuracy (these are called "denormalized" 
    TL> numbers).

    TL> My Mac laptop denormalizes, and agrees with your other computer, giving 
    TL> the smallest representable number as 4.940656e-324. It is 
    TL> .Machine$double.xmin/2^52.   This number has very few bits left to 
    TL> represent values, so for example
    >> (a/2^52)*1.3==(a/2^52)
    TL> [1] TRUE
    TL> where a is .Machine$double.xmin

(very nice explanation, thanks Thomas!)
    TL> Both your machines should be correct. I don't think we deliberately 
    TL> require denormalized numbers to work anywhere.

yes, indeed.
I can imagine that some of regression tests (aka "validation" !)
implicitly use some property -- but as Thomas said, that's not
deliberate (and a buglet in our tests).
William, could you move this topic from R-help to R-devel and
give more specifics about what is failing for your installation?
Sure.  Sorry for talking on the wrong list.
The first problem I encountered with the checks has to do with R not 
understanding dates prior to 1/1/1970, but I'll start another thread for 
that.

The problem I'm talking about here occurs in print-tests.R.  Here's the 
output from the make:

running code in 'print-tests.R' ... OK
comparing 'print-tests.Rout' to './print-tests.Rout.save' ...256c256
< [1] 9
---
[1] 11
260c260
< [1]  0.000000e+00 2.225074e-308 2.225074e-308 2.227299e-308 2.447581e-308
---
[1] 2.002566e-308 2.222849e-308 2.225074e-308 2.225074e-308 2.225074e-308 2.227299e-308 2.447581e-308
266c266
< [1]  0.000000e+00  0.000000e+00  0.000000e+00 2.447581e-308 1.566452e-306 
1.253162e-305
---
[1] 2.002566e-308 2.447581e-308 1.281643e-306 1.566452e-306 1.025314e-305 1.253162e-305
269,273c269,273
< [1,]  0e+00  0e+00  0.0e+00  0.00e+00  0.000e+00  0.0000e+00  0.00000e+00
< [2,]  0e+00  0e+00  0.0e+00  0.00e+00  0.000e+00  0.0000e+00  0.00000e+00
< [3,]  0e+00  0e+00  0.0e+00  0.00e+00  0.000e+00  0.0000e+00  0.00000e+00
< [4,]  0e+00  0e+00 2.4e-308 2.45e-308 2.448e-308 2.4476e-308 2.44758e-308
< [5,] 2e-306 2e-306 1.6e-306 1.57e-306 1.570e-306 1.5660e-306 1.56650e-306
---
[1,] 2e-308 2e-308 2.0e-308 2.00e-308 2.003e-308 2.0026e-308 2.00257e-308
[2,] 2e-308 2e-308 2.4e-308 2.45e-308 2.448e-308 2.4476e-308 2.44758e-308
[3,] 1e-306 1e-306 1.3e-306 1.28e-306 1.280e-306 1.2820e-306 1.28160e-306
[4,] 2e-306 2e-306 1.6e-306 1.57e-306 1.570e-306 1.5660e-306 1.56650e-306
[5,] 1e-305 1e-305 1.0e-305 1.03e-305 1.025e-305 1.0250e-305 1.02530e-305
355c355
< 4.141593+  1i 4.341593+ 10i      NaN+NaNi      Inf+  0i     -Inf+NaNi      
NaN+Infi
---
4.141593+  1i 4.341593+ 10i            NA      Inf+  0i     -Inf+NaNi      NaN+Infi
358c358
< [1,] 4.141593+ 1i NaN+NaNi -Inf+NaNi
---
[1,] 4.141593+ 1i     NA -Inf+NaNi
364c364
< [3,] NaN+      NaNi  NaN+     Infi
---
[3,]             NA  NaN+     Infi
 OK
Hmm.  You know, I just noticed that "OK" at the end.  And then a very 
small error message afterwards about the next test, which also seems to 
have to do with dates.

So, uh, nevermind.  I may bring up my other problems that I'm sure are 
actually problems.  Or at least I am right now....

Sorry.
-Bitt
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