numpy.where method in c++

[dariodem]

Hi people,

I want to implement the following python line in c++

c = numpy.where(a < b)[0]

The way I found to implement it is:

// Example program
#include <iostream>
#include <valarray>

// Returns a vector with the indexes whose vector values are true
std::vector<unsigned int>	trueIndexes( const std::vector<bool> &vector )
{
	std::vector<unsigned int>	indices;

	auto it = std::find_if(std::begin(vector), std::end(vector), [](bool element){return element;});
	while (it != std::end(vector)) {
	   indices.emplace_back(std::distance(std::begin(vector), it));
	   it = std::find_if(std::next(it), std::end(vector), [](bool element){return element;});
	}
	indices.shrink_to_fit();

	return	indices;
} // end function trueIndexes

int main()
{
    std::valarray<float>    a = {0,9,1,8,2,7,3,6,4,5}, b = {0,1,2,3,4,5,6,7,8,9};
    std::vector<unsigned int>  c;
    std::valarray<bool>  aux(10);
    
    aux[a<b] = true;
    std::vector<bool>   aux1(std::begin(aux),std::end(aux));
    c = trueIndexes(aux1);
    
    for(const auto& s: a)
        std::cout << s << ' ';
    std::cout << "\n";
    for(const auto& s: b)
        std::cout << s << ' ';
    std::cout << "\n";
    for(const auto& s: c)
        std::cout << s << ' ';
    std::cout << "\n";
}

I do not like it very much.
I am not really convinced this is the best way of doing it.

Any suggestion?
Thanks