Re: forprime

[Bill Allombert <Bill.Allombert@math.u-bordeaux1.fr>]

On Sun, Sep 16, 2012 at 09:12:14AM +0200, Karim Belabas wrote:
> > On Sat, Sep 15, 2012 at 3:36 AM,  <michel.marcus@free.fr> wrote:
> > > forprime loops over prime numbers.
> > >
> > > is there a  function that would loop over composite numbers ?
> * Charles Greathouse [2012-09-15 09:41]:
> > I typically write
> > 
> > p=3; forprime(q=5, lim, for(n=p+1, q-1, /* your code here */); p=q)
> 
> It's not easy to do this properly in GP and the result is not that readable
> [ N.B. the above loops through composites only up to precprime(lim) ]
> 
> I just committed a function forcomposite() to 'master', following the (new)
> forprime() model:
> 
> %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
> (09:05) gp > ??forcomposite 
> forcomposite(n = a,{b},seq):
> 
>    Evaluates seq, where the formal variable n ranges over the composite numbers
> between  the  non-negative  real numbers a to b,  including a and b if they are
> composite. Nothing is done if a > b.

What is the usecase for such function ? I never needed it myself.

At worse you can do 
for(a=1,1000,if(!isprime(a),print(a)))

I feel concerned because I will have to write dedicated code for GP2C to handle
it...

Cheers,
Bill.