Re: Inconsistency: sometimes an integer, sometimes a float

["Pascal J. Bourguignon" <pjb@informatimago.com>]

Doug Lewan <dougl@shubertticketing.com> writes:

> Cool bug! How in the world did you find it?

It is not a bug, it is a design of the language.  Yes, emacs lisp is
ill-designed.  If you want a better designed lisp, consider Common Lisp.

Namely, emacs numeric datatypes are limited to fixnums and floats.
see: most-positive-fixnum

On 32-bit systems, fixnums are limited to 29 bits.

most-positive-fixnum       --> 536870911
(format "%o  "  536870911) --> "3777777777"
(integer-length 536870911) --> 29


On 64-bit systems, fixnums are limited to 61 bits.

most-positive-fixnum --> 2305843009213693951

(/     2305843009213693951   1000000) --> 2305843009213
(/ (1+ 2305843009213693951)  1000000) --> -2305843009213
(/     2305843009213693952   1000000) --> 2305843009213.694 

In emacs lisp, / is very wrong, since (/ 2 3) --> 0
For floating point numbers it is ok: (/ 2.0 3.0) --> 0.6666666666666666
but of course, people often have very wrong expectations about floating
point numbers, I can't fathom why.


Notice that 1+ (like +, -, *, etc) is also very wrong, on emacs lisp
fixnums, since (minusp (1+ most-positive-fixnum)).

On the other hand, reading 536870912 on a 32-bit emacs, or
2305843009213693952 on a 64-bit emacs, reads a floating point number
(since there is no such fixnum). 


This is clearly ludicrously insane!




If you want to compute, better use Common Lisp:

cl-user> (/ most-positive-fixnum 1000000)
46116860184273879/40000
cl-user> (/ (1+ most-positive-fixnum)  1000000)
18014398509481984/15625
cl-user> (/ (float (1+ most-positive-fixnum))  1000000)
1.1529215E+12
cl-user> (/ (complex most-positive-fixnum most-positive-double-float)  1000000)
#C(1.152921504606847D+12 1.797693134862316D+302)



-- 
__Pascal Bourguignon__
http://www.informatimago.com/
"Le mercure monte ?  C'est le moment d'acheter !"