Re: [boost] Compile-time power of a run-time base

From: JOAQUIN LOPEZ MU?Z (joaquin_at_[hidden])
Date: 2008-02-02 07:57:23

----- Mensaje original -----
De: Bruno Lalande <bruno.lalande_at_[hidden]>
Fecha: Sábado, Febrero 2, 2008 12:18 pm
Asunto: [boost] [Boost] Compile-time power of a run-time base
Para: boost_at_[hidden]

> Hello,
> 
> I have written for the need of a coworker a function template that 
> computesthe compile-time power of a base that is known only at 
> runtime. The original
> need was just to avoid having to write things like
> "value*value*value*value*value".
> 
> Before doing this, I've been searching for such a thing in the Boost
> documentation and mailing lists, but could only find discussions 
> about an
> entierely compile-time version (compile-time exponent of compile-
> time base)
> intended for use in metaprograms. Is there any reason why the
> "semi-compile-time" version I describe here hasn't been discussed? 
> Eventhough it can be viewed as a mere syntactic sugar, it can be 
> very practical
> when it comes to write by hand a power with a big exponent.
> 
> 
> In case of interest, here is my code :
> 
> 
> template <int N>
> struct positive_power
> {
>        template <typename T>
>        static float result(T base)
>        { return base*positive_power<N-1>::result(base); }
> };

I think this is suboptimal: you need only log(N) multiplications
to get positive_power<N>::result(base):

template <int N>
struct positive_power
{
  template <typename T>
  static float result(T base)
  { 
    float aux=positive_power<N/2>::result(base);
    return (N%2)?base*aux*aux:aux*aux;
  }
};

HTH,

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo