min_element

Header: <Kokkos_StdAlgorithms.hpp>

Description

Finds the smallest element in a range or in a rank-1 View using either operator< to compare two elements or a user-provided comparison operator.

Interface

Warning

This is currently inside the Kokkos::Experimental namespace.

Overload set accepting execution space

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);

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);

Overload set accepting a team handle

Added in version 4.2.

template <class TeamHandleType, class IteratorType>
KOKKOS_FUNCTION
auto min_element(const TeamHandleType& teamHandle,                      (9)
                 IteratorType first, IteratorType last);

template <class TeamHandleType, class DataType, class... Properties>
KOKKOS_FUNCTION
auto min_element(const TeamHandleType& teamHandle,                      (10)
                 const ::Kokkos::View<DataType, Properties...>& view);

template <class TeamHandleType, class IteratorType, class ComparatorType>
KOKKOS_FUNCTION
auto min_element(const TeamHandleType& teamHandle,                      (11)
                 IteratorType first, IteratorType last,
                 ComparatorType comp);

template <class TeamHandleType, class DataType, class ComparatorType,
          class... Properties>
KOKKOS_FUNCTION
auto min_element(const TeamHandleType& teamHandle,                      (12)
                 const ::Kokkos::View<DataType, Properties...>& view,
                 ComparatorType comp);

Parameters and Requirements

• exespace: execution space instance

• teamHandle: team handle instance given inside a parallel region when using a TeamPolicy

• label: string forwarded to internal parallel kernels for debugging purposes

  • 1 and 3: The default string is “Kokkos::min_element_iterator_api_default”.

  • 5 and 7: The default string is “Kokkos::min_element_view_api_default”.

  • NOTE: overloads accepting a team handle do not use a label internally

• first, last: range of elements to examine

  • must be random access iterators, e.g., returned from Kokkos::Experimental::(c)begin/(c)end

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

  • must be accessible from exespace or from the execution space associated with the team handle

• view: Kokkos view to examine

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

  • must be accessible from exespace or from the execution space associated with the team handle

• 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 Value

Returns 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>());