GNU bug report logs - #20185
24.4.50; problem with built-in function "ldexp" Emacs Ver: 24.4.50.1

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Bernard Hurley <bernard <at> marcade.biz>
Subject: bug#20185: closed (Re: 24.4.50; problem with built-in function
 "ldexp" Emacs Ver: 24.4.50.1)
Date: Tue, 24 Mar 2015 18:58:02 +0000

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Your bug report

#20185: 24.4.50; problem with built-in function "ldexp" Emacs Ver: 24.4.50.1

which was filed against the emacs package, has been closed.

The explanation is attached below, along with your original report.
If you require more details, please reply to 20185 <at> debbugs.gnu.org.

-- 
20185: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=20185
GNU Bug Tracking System
Contact help-debbugs <at> gnu.org with problems

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From: Paul Eggert <eggert <at> cs.ucla.edu>
To: Bernard Hurley <bernard <at> marcade.biz>
Cc: 20185-done <at> debbugs.gnu.org
Subject: Re: 24.4.50;
 problem with built-in function "ldexp" Emacs Ver: 24.4.50.1
Date: Tue, 24 Mar 2015 11:57:10 -0700

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Thanks for catching that bug.  While fixing it I noticed some related 
minor glitches with ldexp, so I installed the attached patch.

[0001-Fix-minor-ldexp-issues.patch (text/x-patch, attachment)]

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From: Bernard Hurley <bernard <at> marcade.biz>
To: bug-gnu-emacs <at> gnu.org
Subject: 24.4.50; problem with built-in function "ldexp" Emacs Ver: 24.4.50.1
Date: Mon, 23 Mar 2015 20:29:13 +0000

(24.4.50) 3.2 Floating-Point Basics we read:

---------- snip ------------
 -- Function: ldexp sig &optional exp
     This function returns a floating-point number corresponding to the
     significand SIG and exponent EXP.
----------snip -------------

However it is easy to verify that the second argument is not
optional. For example:

    (ldexp 5.0)

triggers the error:

    *** Eval error ***  Wrong type argument: integerp, nil

The code for ldexp in floatfn.c reads:

---------- snip ---------------
DEFUN ("ldexp", Fldexp, Sldexp, 1, 2, 0,
       doc: /* Construct number X from significand SGNFCAND and exponent EXP.
Returns the floating point value resulting from multiplying SGNFCAND
(the significand) by 2 raised to the power of EXP (the exponent).   */)
  (Lisp_Object sgnfcand, Lisp_Object exponent)
{
  CHECK_NUMBER (exponent);
  return make_float (ldexp (XFLOATINT (sgnfcand), XINT (exponent)));
}
---------- snip -------------

The declaration conforms to that in the manual but nothing seems to have
been done to take into account what happens when `exp' is missing.  It
seems to me that there are two possibilities:

EITHER: The declaration in floatfns.c is incorrect and should read:

    DEFUN ("ldexp", Fldexp, Sldexp, 1, 1, 0,
    ....

In which case it will need to be changed and the manual will need to be
changed accordingly.


OR: `exp' is supposed to have a default value (0 say), when the code in
floatfns.c should be changed accordingly.  In this case it might still
be a good idea to update the manual to explain that it has this value.

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