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# std::adjacent_difference

From cppreference.com
< cpp‎ | algorithm

Defined in header `<numeric>` |  |
template< class InputIt, class OutputIt > OutputIt adjacent_difference( InputIt first, InputIt last, OutputIt d_first ); | (1) |
template< class InputIt, class OutputIt, class BinaryOperation > OutputIt adjacent_difference( InputIt first, InputIt last, OutputIt d_first, BinaryOperation op ); | (2) |

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`. Unmodified copy of `first` is written to `d_first`. The first version uses `operator-` to calculate the differences, the second version uses the given binary function `op`.

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)

### [edit] Parameters

first, last | - | the range of elements
d_first | - | the beginning of the destination range
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 `InputIterator`.
- `OutputIt` must meet the requirements of `OutputIterator`.

### [edit] Return value

It to the element past the last element written.

### [edit] Complexity

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

### [edit] 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 - acc;
            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, acc);
            acc = std::move(val);
        }
        return ++d_first;
    }

### [edit] Example

The following code converts a sequence of even numbers to repetitions of the number 2 and converts a sequence of ones to a sequence of Fibonacci numbers.

Run this code

    ​#include <numeric>
    #include <vector>
    #include <iostream>
    #include <functional>

    int main()
    {
        std::vector<int> 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';

        v = {1, 1, 1, 1, 1, 1, 1, 1, 1, 1};
        std::adjacent_difference(v.begin(), v.end() - 1, v.begin() + 1, std::plus<int>());

        for (auto n : v) {
            std::cout << n << ' ';
        }
        std::cout << '\n';
    }​

Output:

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

### [edit] See also

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

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