std::adjacent_difference (3) - Linux Manuals

std::adjacent_difference: std::adjacent_difference

NAME

std::adjacent_difference - std::adjacent_difference

Synopsis

Defined in header <numeric>
template< class InputIt, class OutputIt >
OutputIt adjacent_difference( InputIt first, InputIt last, (until C++20)
OutputIt d_first );
template< class InputIt, class OutputIt >
constexpr OutputIt adjacent_difference( InputIt first, InputIt last, (since C++20)
OutputIt d_first );
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 >
ForwardIt2 adjacent_difference( ExecutionPolicy&& policy, ForwardIt1 first, ForwardIt1 last, (2) (since C++17)
ForwardIt2 d_first ); (1)
template< class InputIt, class OutputIt, class BinaryOperation >
OutputIt adjacent_difference( InputIt first, InputIt last, (until C++20)
OutputIt d_first, BinaryOperation op );
template< class InputIt, class OutputIt, class BinaryOperation >
constexpr OutputIt adjacent_difference( InputIt first, InputIt last, (3) (since C++20)
OutputIt d_first, BinaryOperation op );
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class BinaryOperation >
ForwardIt2 adjacent_difference( ExecutionPolicy&& policy, ForwardIt1 first, ForwardIt1 last, (4) (since C++17)
ForwardIt2 d_first, BinaryOperation op );

Computes the differences between the second and the first of each adjacent pair of elements of the range [first, last) and writes them to the range beginning at d_first + 1. An unmodified copy of *first is written to *d_first.
1,3) First, creates an accumulator acc whose type is InputIt's value type, initializes it with *first, and assigns the result to *d_first. Then, for every iterator i in [first + 1, last) in order, creates an object val whose type is InputIt's value type, initializes it with *i, computes
val - acc
(until C++20)
val - std::move(acc)
(since C++20) (overload (1)) or
op(val, acc)
(until C++20)
op(val, std::move(acc))
(since C++20) (overload (3)), assigns the result to *(d_first + (i - first)), and move assigns from val to acc.
first may be equal to d_first.
2,4) Performs *d_first = *first;. For every d in [1, last - first - 1], assigns *(first + d) - *(first + d - 1) (overload (2)) or op(*(first + d), *(first + d - 1)) (overload (4)) to *(d_first + d). This is executed according to policy. This overload only participates in overload resolution if std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true.
The behavior is undefined if the input and output ranges overlap in any way.

Equivalent operation:

  *(d_first) = *first;
  *(d_first+1) = *(first+1) - *(first);
  *(d_first+2) = *(first+2) - *(first+1);
  *(d_first+3) = *(first+3) - *(first+2);
  ...

op must not have side effects. (until C++11)
op must not invalidate any iterators, including the end iterators, or modify any elements of the ranges involved. (since C++11)

Parameters

first, last - the range of elements
d_first - the beginning of the destination range
policy - the execution policy to use. See execution_policy for details.
              binary operation function object that will be applied.
              The signature of the function should be equivalent to the following:
op - Ret fun(const Type1 &a, const Type2 &b);
              The signature does not need to have const &.
              The types Type1 and Type2 must be such that an object of type iterator_traits<InputIt>::value_type can be implicitly converted to both of them. The type Ret must be such that an object of type OutputIt can be dereferenced and assigned a value of type Ret.

Type requirements

-
InputIt must meet the requirements of LegacyInputIterator. InputIt's value type must be MoveAssignable and constructible from the type of *first
-
OutputIt must meet the requirements of LegacyOutputIterator. both acc (the accumulated value) and the result of
val - acc or op(val, acc)
(until C++20)
val - std::move(acc) or op(val, std::move(acc))
(since C++20) must be writable to OutputIt
-
ForwardIt1, ForwardIt2 must meet the requirements of LegacyForwardIterator. The results of *first, *first - *first (for (2)) and op(*first, *first) (for (4)) must be writable to ForwardIt2.

Return value

Iterator to the element past the last element written.

Notes

If first == last, this function has no effect and will merely return d_first.

Complexity

Exactly (last - first) - 1 applications of the binary operation

Exceptions

The overloads with a template parameter named ExecutionPolicy report errors as follows:

* If execution of a function invoked as part of the algorithm throws an exception and ExecutionPolicy is one of the standard_policies, std::terminate is called. For any other ExecutionPolicy, the behavior is implementation-defined.
* If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Possible implementation

First version

  template<class InputIt, class OutputIt>
  OutputIt adjacent_difference(InputIt first, InputIt last,
                               OutputIt d_first)
  {
      if (first == last) return d_first;

      typedef typename std::iterator_traits<InputIt>::value_type value_t;
      value_t acc = *first;
      *d_first = acc;
      while (++first != last) {
          value_t val = *first;
          *++d_first = val - std::move(acc); // std::move since C++20
          acc = std::move(val);
      }
      return ++d_first;
  }

Second version

  template<class InputIt, class OutputIt, class BinaryOperation>
  OutputIt adjacent_difference(InputIt first, InputIt last,
                               OutputIt d_first, BinaryOperation op)
  {
      if (first == last) return d_first;

      typedef typename std::iterator_traits<InputIt>::value_type value_t;
      value_t acc = *first;
      *d_first = acc;
      while (++first != last) {
          value_t val = *first;
          *++d_first = op(val, std::move(acc)); // std::move since C++20
          acc = std::move(val);
      }
      return ++d_first;
  }

Example

// Run this code

  #include <numeric>
  #include <vector>
  #include <array>
  #include <iostream>
  #include <functional>
  #include <iterator>

  int main()
  {
      // Default implementation - the difference b/w two adjacent items

      std::vector v {2, 4, 6, 8, 10, 12, 14, 16, 18, 20};
      std::adjacent_difference(v.begin(), v.end(), v.begin());

      for (auto n : v)
          std::cout << n << ' ';

      std::cout << '\n';

      // Fibonacci

      std::array<int, 10> a {1};

      adjacent_difference(begin(a), std::prev(end(a)), std::next(begin(a)), std::plus<> {});

      copy(begin(a), end(a), std::ostream_iterator<int> {std::cout, " "});
  }

Output:

  2 2 2 2 2 2 2 2 2 2
  1 1 2 3 5 8 13 21 34 55