Re: probable bug of "ispower" function in GP: FALSE, apologies!

[luis.gallardo@math.cnrs.fr]

nice, Karim!


luis
***


Karim Belabas <Karim.Belabas@math.u-bordeaux.fr> a écrit :

> * Vincent Lefevre [2023-01-12 12:40]:
> > On 2023-01-12 12:31:09 +0100, luis.gallardo@math.cnrs.fr wrote:
> > > I misunderstood the definition of "ispower"!
> > >
> > > I wanted a function that identify directly if some integer is a power of 2
> >
> > I suppose that you can use something like
> >
> >   ispower(64,,&n)
> >
> > and check that n is 2. But I suppose that for large numbers, this may
> > be inefficient because ispower() will not just check for powers of 2.
>
> Use hammingweight(N) == 1.
>
> Or v = valuation(N,2); N >> v == 1 if you need the exact power. (Will be
> a little slower.)
>
> Cheers,
>
>     K.B.
> --
> Pr Karim Belabas, U. Bordeaux, Vice-président en charge du Numérique
> Institut de Mathématiques de Bordeaux UMR 5251 - (+33) 05 40 00 29 77
> http://www.math.u-bordeaux.fr/~kbelabas/
> `