Re: [eigen] How to raise double to array powers in Eigen

[Chen-Pang He <jdh8@xxxxxxxxxxxxxx>]

I suggest not find the max exponent in advance, but compute
higher powers ad hoc.

To achieve more encapsulation, we can implement this
functionality in a functor, and let ArrayBase::unaryExpr call it.
Examples of functors are in Eigen/src/Core/functors.

Cheers,
Chen-Pang

在 2014 八月 19 週二 16:30:23，Christoph Hertzberg 寫道:
> On 19.08.2014 16:13, Ian Bell wrote:
> > New idea:
> > 1. Cache the set of exponents in a std::map<int, double>
> 
> Just a warning: std::map is often very inefficient, as it requires 
> multiple branches and indirect memory accesses. If the key is int, and 
> the range is known and not too big, it is very often much more efficient 
> to use a plain C-array or a std::vector.
> 
> E.g.
> int lo = 3, hi=17;  // calculate powers from 3 to 16
> std::vector<double> powers(hi-lo);
> double power=std::pow(base, lo);
> for(int i=0; i<hi-lo; ++i) {
>    powers[i]=power;
>    power *= base;
> }
> 
> // Access pow(base, ex) like this:
>    double res = powers[ex-lo];
> 
> If the exponents are sorted, you can probably safe the extra table and 
> simply accumulate the power variable as you go through the result array.
> 
> 
> Christoph