[racket-dev] Racket Questions?

From: Michael Wilber (mwilber at uccs.edu)
Date: Sat Sep 15 14:45:38 EDT 2012

For the record, I've always just defined my own modulo when I need it
for floats:

; A modulo operator for floats!
(define (float-modulo p q)
  (- p (* q (truncate (/ p q)))))

It doesn't properly handle negative numbers though.

David Van Horn <dvanhorn at ccs.neu.edu> writes:
> On 9/14/12 3:36 PM, Becca MacKenzie wrote:
>> Hello!
>> So a friend of mine just started learning Racket and was wondering if
>> there's a particular reason why the modulo function in racket only takes
>> in integers? He wrote his own mod function to take in other things but
>> he was just wondering what the reasoning is behind this.
> Hi Becca,
> Excellent question -- I hope you don't mind that I've forwarded it to 
> the Racket developers list for a more authoritative answer (and 
> potentially a change to Racket).
> I don't believe there's any principled reason not to extend `modulo' to 
> other kinds of numbers such as rationals and (exact) complex numbers.  I 
> worry that the idea of modulo may not be well defined for inexact 
> numbers, but I could be wrong (inexact numbers don't obey a lot of the 
> usual mathematical properties we're used to).  I see that in 
> Mathematica, "the arguments of Mod can be any numeric quantities, not 
> necessarily integers".  Here are some examples:
>     http://reference.wolfram.com/mathematica/ref/Mod.html#6881
> Recently, Racket's GCD and LCM were extended to work on non-integer 
> arguments, and I believe this is a similar case where the function could 
> (and should?) be extended to work for more kinds of numbers.  But I'm 
> interested to hear what the dev list has to say on the matter.
> David
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