std::adjacent_difference

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

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.
op	-	binary operation function object that will be applied. The signature of the function should be equivalent to the following: `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; }

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

#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

partial_sum	computes the partial sum of a range of elements (function template)
accumulate	sums up a range of elements (function template)