Re: forprime

[Karim Belabas <Karim.Belabas@math.u-bordeaux1.fr>]

> 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.

   ? forcomposite(p = 4, 10, print(p))
   4
   6
   8 
   9
   10
   
   Omitting  b  means  we  will  run through all composites  >= a,  starting an
infinite loop;  it is expected that the user will break out of the loop himself
at some point, using break or return.

   Note that the value of n cannot be modified within seq:
   ? forcomposite(n = 2, 10, n = [])

    ***   at top-level: forcomposite(n=2,10,n=[])
    ***                                      ^---
    ***   index read-only: was changed to [].
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

Cheers,

    K.B.
-- 
Karim Belabas, IMB (UMR 5251)  Tel: (+33) (0)5 40 00 26 17
Universite Bordeaux 1          Fax: (+33) (0)5 40 00 69 50
351, cours de la Liberation    http://www.math.u-bordeaux1.fr/~belabas/
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