C++ apply standard library algorithms to functions

Question

Is there a way to apply non-modifying standard library algorithms to discrete functions instead of containers?

For example, consider the following function

int sqr(int i)
{
    return i*i;
}

How can I use std::find or std::lower_bound to search for the value 49, i.e. the algorithm should return 7? The easiest way would be to put the returns into a vector and apply the algorithm to the vector -- but this is obviously inefficient.

Answer 1

Hypothetically, you could use something like boost::iterator::counting_iterator. E.g., the following finds that 4 is the number whose square is 16:

#include <algorithm>                                                                                                                                                                                         
#include <iostream>

#include <boost/iterator/counting_iterator.hpp>


using namespace std;


int main(int, char**)
{

    auto f = std::find_if(
        boost::make_counting_iterator<int>(0),
        boost::make_counting_iterator<int>(20),
        [](int i){return i * i == 16;});
    cout << std::distance(
        boost::make_counting_iterator<int>(0),
        f) << endl;

    return 0;
}

I think that this approach is problematic in many ways. In the above, in particular, note that it searches for such numbers up to 20.