map_reduce() [1/2]

template<concepts::product_type T, concepts::monoid M, typename Function>

auto kumi::map_reduce ( Function f, M && m, T && t )

inlinenodiscardconstexpr

Performs a tree-like reduction of all elements of a product type. The given map function is applied before excution the reduction to each element of the input.

Note

For associative operations, this produces the same result as a left or right fold preceeded by map, but have different intermediate evaluation order.

Parameters

f	Mapping function to apply
m	Monoid callable function to apply
t	Product type to reduce

Returns

The result of reducing t by recursively combining elements in a tree structure

Helper type

namespace kumi::result
{
  template<monoid M, product_type T> struct map_reduce;
 
  template<monoid M, product_type T>
  using map_reduce_t = typename map_reduce<M,T>::type;
}

Computes the return type of a call to kumi::map_reduce

Examples:

#include <kumi/kumi.hpp>
#include <iostream>
 
int main()
{
  auto t1 = kumi::tuple{2.,5,short{3},'\4'};
  auto t2 = kumi::tuple{2,1,short{55},' '};
 
  auto times_two = [&](auto e){ return 2*e; };
  std::cout << kumi::map_reduce(times_two, kumi::function::multiplies, t1) << "\n";
 
  auto is_pair = [&](auto e){ return e%2==0; };
  std::cout << kumi::map_reduce(is_pair, kumi::function::plus, t2)        << "\n";
}

#include <kumi/kumi.hpp>
#include <iostream>
 
int main()
{
  using namespace kumi::literals;
  auto r1 = kumi::record{"a"_id = 2.,"b"_id = 5,"c"_id = short{3},"d"_id = '\4'};
  auto r2 = kumi::record{"a"_id = 2 ,"b"_id = 1,"c"_id = short{55},"d"_id = ' '};
 
  auto times_two = [&](auto e){ return 2*e; };
  std::cout << kumi::map_reduce(times_two, kumi::function::multiplies, r1) << "\n";
 
  auto is_pair = [&](auto e){ return e%2==0; };
  std::cout << kumi::map_reduce(is_pair, kumi::function::plus, r2)        << "\n";
}