Re: Help to convert a function

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

* Quique Becerra [2005-10-02 17:30]:
> Hi people
> Can any1 help me to convert a function to PARI ?
> It's made in c++, and need help to make
> this function work in PARI
> 
> ---------------------------------------
> int quick_exp (int a, int b)
> {
>   int z, x, resul;
>   z = b;
>   x = a;
>   resul = 1;
>   while(z > 0)
>   {
>      if(z % 1 == 1)

I assume you mean z % 2 here. Otherwise, you may as well return 1 directely.

>        resul = resul * x;
>      x = x*x;
>      z = z/z;

I assume you mean z /= 2 here.

>   }
>   return(resul);
> }
> ---------------------------------------

In GP:

quick_exp(a, b) = a^b

This is not exactly equivalent to the above since it does not take
overflow into account. If you really intend overflow to take place, then

quick_exp(a, b, bits_in_long = 32) = lift(Mod(a, 2^bits_in_long)^b)

is suitable.

If by "converting to PARI" you mean "converting to a C function using the
PARI library" then (assuming pari-2.2.10 is used):

long quick_exp(long a, long b)
{
  pari_sp av = avma;
  long c = itos( gpow(stoi(a), stoi(b), 0) );
  avma = av; return c;
}

or possibly (this one does overflow)

ulong quick_exp(ulong a, ulong b) { return upowuu(a, b); }

For a direct translation, see leftright_pow().

    Karim.
-- 
Karim Belabas                  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-bordeaux.fr/~belabas/
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