bugAxiom Computer Algebra System - Bugs: bug #4565, log10 in GCL returns a bad value...


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bug #4565: log10 in GCL returns a bad value for log10(1000)

Submitter:  David Mentré <dmentre>
Submitted:  Thu 31 Jul 2003 05:25:43 PM UTC
Category:  interpreter Severity:  2 - Minor
Item Group:  code bug Status:  transferred
Privacy:  Public Assigned to:  None
Originator Name:  Joergen Weiss Open/Closed:  Open
Release:  Friday July 18, 2003 at 13:33:22 Reproducibility:  None
Fixed Release: 
Error message: 

Example of code trigerring the bug: 

)lisp (log10 1000)

Internal cause: 

Fri 05 Sep 2003 04:23:00 PM UTC, comment #3: 
[Added information or comment]

From Tim:
I've already patched the output system to round (log10 1000).
The issue is still open because I need to remember to search
the sources for any other use of log10. I doubt we use it elsewhere
but I might as well do a quality job, yaknow?

David Mentré <dmentre>
Group administrator
Thu 04 Sep 2003 07:54:19 PM UTC, comment #2: 
[Added information or comment]

From Camm Maguire:
Our lisp spec expert tells me that routines returning floating point
values like (log10...) are not required to return exact integers in
cases like these. 

What you want is something like this:

(defun logi (x y &aux i r)
        (declare (longfloat x y r))
        (declare (fixnum i))
        (multiple-value-bind (i r) (round (log x y))

David Mentré <dmentre>
Group administrator
Wed 03 Sep 2003 08:58:48 PM UTC, comment #1: 
[Added information or comment]

Remind to tell about (log10 1000) issue on GCL developer mailing list.

David Mentré <dmentre>
Group administrator
Thu 31 Jul 2003 05:25:43 PM UTC, original submission:  

In some versions of GCL the LOG10 function returns improperly rounded values.
The symptom is:
(24) -> [1000]
   (24)  [100]
The common lisp failure can be shown with:

(25) -> )lisp (log10 1000)
Value = 2.9999999999999996

This previous boot code was:

    u < MOST_-POSITIVE_-LONG_-FLOAT => 1+negative+FLOOR LOG10 u
and should be restored when the GCL bug is fixed.
    u < MOST_-POSITIVE_-LONG_-FLOAT => 1+negative+FLOOR ((LOG10 u) + 0.0000001)

David Mentré <dmentre>
Group administrator


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Follow 2 latest changes.

Date Changed by Updated Field Previous Value => Replaced by
2005-01-26 billpage1 StatusNone transferred
2003-09-04 dmentre Severity3 - Normal None

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