Re: Why no mod function?

Actually

x mod y == integer MOD(x,y)$Lisp

is the function.

-Alasdair

On 7/11/07, Alasdair McAndrew <[email protected]> wrote:
>
> Well, positiveRemainder should be renamed "mod".  Also, addmod, submod,
> mulmod assume that their first two parameters are positive, which is an
> unreasonable restriction.  For example:
>
> (1) -> addmod(-13,0,10)
>
>    (1)  - 13
>
> (2) -> mulmod(-13,1,10)
>
>    (2)  - 3
>
> (3) -> submod(-13,0,10)
>
>    (3)  - 3
>
> (4) -> submod(0,13,10)
>
>    (4)  -3
>
> In fact, there should be two mod commands: x mod n, which produces a
> result between 0 and n-1, and x bmod n (for "balanced mod") which produces a
> result between -(n-1)/2 and (n-1)/2 if n is odd, and between -(n-2)/2 and
> n/2 if n is even.
>
> Looking through si.spad, it could easily be extended by
>
> x mod y == MOD(x,y)$Lisp::INT
>
> or some such.
>
> cheers,
> Alasdair
>
> On 7/11/07, Bill Page <[email protected]> wrote:
> >
> > On 7/10/07, Alasdair McAndrew wrote:
> > > Yes, that's pretty much what I did.  But surely such an important
> > function
> > > as mod should be available without having to write it yourself?
> > > ...
> >
> > Digging a little deeper I found in the domain 'Integer':
> >
> > (1) -> positiveRemainder(-13,10)
> >
> >    (1)  7
> >                                                 Type: PositiveInteger
> >
> > I presume this is what you were looking for?
> >
> > Also take a look at
> >
> > (2) -> addmod(13,1,10)
> >
> >    (2)  4
> >                                                  Type: PositiveInteger
> > (3) -> submod(13,1,10)
> >
> >    (3)  12
> >                                                  Type: PositiveInteger
> > (4) -> mulmod(13,2,10)
> >
> >    (4)  6
> >                                                  Type: PositiveInteger
> >
> > Regards,
> > Bill Page.
> >
>
>

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