Re: [eigen] Re: Raising double to integer powers

[Marc Glisse <marc.glisse@xxxxxxxx>]

On Mon, 10 May 2021, Ian Bell wrote:

> Of course, shortly after having sent this message I figured it out, but it
> doesn't actually result in an increase in my throughput sadly. For
> posterity:
>
> #include <Eigen/Dense>
> #include <iostream>
>
> using namespace Eigen;
>
> struct myUnaryFunctor {
>  const double m_base;
>  myUnaryFunctor(double base): m_base(base) {};
>  typedef double result_type;
>  result_type operator()(const int &e) const
>  {
>      return pow(m_base, e);
>  }
> };
>
> int main()
> {
>    auto e = Eigen::ArrayXi::LinSpaced(11, 0, 10).eval();
>    double base = 2.9;
>    std::cout << e.unaryExpr(myUnaryFunctor(base));
> }

Assuming pow is actually your own function and does the usual repeated 
squaring, unlike std::pow, this may do a lot of redundant computation (in 
particular base*base is computed many times). Do you know anything about 
the integers? In particular, are they always small? I assume the LinSpaced 
example doesn't look like the true data. Does your pow function already 
cache some results?

--
Marc Glisse