# std::adjacent_difference

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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;
    }

### 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

### See also

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

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