std::adjacent_difference

Defined in header `<numeric>`
template< class InputIt, class OutputIt > OutputIt adjacent_difference( InputIt first, InputIt last, OutputIt d_first );	(1)
template< class ExecutionPolicy, class InputIt, class OutputIt > OutputIt adjacent_difference( ExecutionPolicy&& policy, InputIt first, InputIt last, OutputIt d_first );	(2)	(since C++17)
template< class InputIt, class OutputIt, class BinaryOperation > OutputIt adjacent_difference( InputIt first, InputIt last, OutputIt d_first, BinaryOperation op );	(3)
template< class ExecutionPolicy, class InputIt, class OutputIt, class BinaryOperation > OutputIt adjacent_difference( ExecutionPolicy&& policy, InputIt first, InputIt last, OutputIt 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. Unmodified copy of first is written to d_first.

1) Differences are calculated using operator-.

3) Differences are calculated using the given binary function op.

2,4) Same as (1,3), but executed according to policy. These overloads do not participate in overload resolution unless std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true

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)

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 `InputIterator`.
- `OutputIt` must meet the requirements of `OutputIterator`.

[edit] Return value

It to the element past the last element written.

[edit] Notes

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

[edit] Complexity

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

[edit] 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,

if policy is std::parallel_vector_execution_policy, std::terminate is called
if policy is std::sequential_execution_policy or std::parallel_execution_policy, the algorithm exits with an std::exception_list containing all uncaught exceptions. If there was only one uncaught exception, the algorithm may rethrow it without wrapping in std::exception_list. It is unspecified how much work the algorithm will perform before returning after the first exception was encountered.
if policy is some other type, the behavior is implementation-defined

If the algorithm fails to allocate memory (either for itself or to construct an std::exception_list when handling a user exception), std::bad_alloc is thrown.

[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)
accumulate	sums up a range of elements (function template)
std::experimental::parallel::adjacent_difference (parallelism TS)	parallelized version of `std::adjacent_difference` (function template)

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