min_element

Header File: Kokkos_StdAlgorithms.hpp

namespace Kokkos{
namespace Experimental{

//
// overload set accepting iterators
//
template <class ExecutionSpace, class IteratorType>
auto min_element(const ExecutionSpace& exespace,                        (1)
                 IteratorType first, IteratorType last);

template <class ExecutionSpace, class IteratorType>
auto min_element(const std::string& label,                              (2)
                 const ExecutionSpace& exespace,
                 IteratorType first, IteratorType last);

template <class ExecutionSpace, class IteratorType, class ComparatorType>
auto min_element(const ExecutionSpace& exespace,                        (3)
                 IteratorType first, IteratorType last,
                 ComparatorType comp);

template <class ExecutionSpace, class IteratorType, class ComparatorType>
auto min_element(const std::string& label,                              (4)
                 const ExecutionSpace& exespace,
                 IteratorType first, IteratorType last,
                 ComparatorType comp);

//
// overload set accepting views
//
template <class ExecutionSpace, class DataType, class... Properties>
auto min_element(const ExecutionSpace& exespace,                        (5)
                 const ::Kokkos::View<DataType, Properties...>& view);

template <class ExecutionSpace, class DataType, class... Properties>
auto min_element(const std::string& label,                              (6)
                 const ExecutionSpace& exespace,
                 const ::Kokkos::View<DataType, Properties...>& view);

template <class ExecutionSpace, class DataType, class ComparatorType, class... Properties>
auto min_element(const ExecutionSpace& exespace,                        (7)
                 const ::Kokkos::View<DataType, Properties...>& view,
                 ComparatorType comp);

template <class ExecutionSpace, class DataType, class ComparatorType, class... Properties>
auto min_element(const std::string& label,                              (8)
                 const ExecutionSpace& exespace,
                 const ::Kokkos::View<DataType, Properties...>& view,
                 ComparatorType comp);

} //end namespace Experimental
} //end namespace Kokkos

Description

• (1,2,5,6): Finds the smallest element in the range [first, last) (1,2) or in view (5,6) using operator< to compare two elements

• (3,4,7,8): Finds the smallest element in the range [first, last) (3,4) or in view (7,8) using the binary functor comp to compare two elements

Parameters and Requirements

• exespace:

  • execution space instance

• label:

  • used to name the implementation kernels for debugging purposes

  • for 1,3 the default string is: “Kokkos::min_element_iterator_api_default”

  • for 5,7 the default string is: “Kokkos::min_element_view_api_default”

• first, last:

  • range of elements to examine

  • must be random access iterators

  • must represent a valid range, i.e., last >= first (checked in debug mode)

  • must be accessible from exespace

• view:

  • Kokkos view to examine

  • must be rank-1, and have LayoutLeft, LayoutRight, or LayoutStride

  • must be accessible from exespace

• comp:

  • binary functor returning true if the first argument is less than the second argument; comp(a,b) must be valid to be called from the execution space passed, and convertible to bool for every pair of arguments a,b of type value_type, where value_type is the value type of IteratorType (for 1,2,3,4) or the value type of view (for 5,6,7,8) and must not modify a,b.

  • must conform to:

  struct Comparator
  {
     KOKKOS_INLINE_FUNCTION
     bool operator()(const value_type & a, const value_type & b) const {
       return /* true if a is less than b, based on your logic of "less than" */;
     }
  };
  

Return

Iterator to the smallest element. The following special cases apply:

• if several elements are equivalent to the smallest element, it returns the iterator to the first such element.

• if the range [first, last) is empty it returns last.

• if view is empty, it returns Kokkos::Experimental::end(view).

Example

namespace KE = Kokkos::Experimental;
Kokkos::View<double*> a("a", 13);
// fill a somehow

auto res = KE::min_element(Kokkos::DefaultExecutionSpace(), KE::begin(a), KE::end(a));

// passing the view directly
auto res = KE::min_element(Kokkos::DefaultExecutionSpace(), a);


// using a custom comparator
template <class ValueType1, class ValueType2 = ValueType1>
struct CustomLessThanComparator {
  KOKKOS_INLINE_FUNCTION
  bool operator()(const ValueType1& a,
                  const ValueType2& b) const {
   // here we use < but one can put any custom logic to return true if a is less than b
    return a < b;
  }

  KOKKOS_INLINE_FUNCTION
  CustomLessThanComparator() {}
};

// passing the view directly
auto res = KE::min_element(Kokkos::DefaultExecutionSpace(), a, CustomLessThanComparator<double>());