NAME

std::adjacent_difference - std::adjacent_difference

Synopsis

Defined in header <numeric>
template< class InputIt, class OutputIt >

OutputIt adjacent_difference( InputIt first, (until C++20)
InputIt last,

OutputIt d_first );
template< class InputIt, class OutputIt >

constexpr OutputIt adjacent_difference( InputIt (since C++20)
first, InputIt last,

OutputIt d_first );
template< class ExecutionPolicy, class
ForwardIt1, class ForwardIt2 >

ForwardIt2 adjacent_difference( (2) (since C++17)
ExecutionPolicy&& policy, ForwardIt1 first,
ForwardIt1 last,

ForwardIt2 d_first );
template< class InputIt, class OutputIt, class
BinaryOperation > (1)

OutputIt adjacent_difference( InputIt first, (until C++20)
InputIt last,

OutputIt d_first, BinaryOperation op );
template< class InputIt, class OutputIt, class
BinaryOperation >

constexpr OutputIt adjacent_difference( InputIt (since C++20)
first, InputIt last, (3)

OutputIt d_first, BinaryOperation op );
template< class ExecutionPolicy, class
ForwardIt1, class ForwardIt2, class
BinaryOperation >

ForwardIt2 adjacent_difference( (4) (since C++17)
ExecutionPolicy&& policy, ForwardIt1 first,
ForwardIt1 last,

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. These
overloads do not participate in overload resolution unless
std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>
(until C++20)
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>
(since C++20) 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 (since C++11)
modify any elements of the ranges involved.

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:

Ret fun(const Type1 &a, const Type2 &b);
op -
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>
constexpr // since C++20
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>
constexpr // since C++20
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>

auto print = [](auto comment, auto const& sequence) {
std::cout << comment;
for (const auto& n : sequence)
std::cout << n << ' ';
std::cout << '\n';
};

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

std::vector v {4, 6, 9, 13, 18, 19, 19, 15, 10};
print("Initially, v = ", v);
std::adjacent_difference(v.begin(), v.end(), v.begin());
print("Modified v = ", v);

// Fibonacci

std::array<int, 10> a {1};
adjacent_difference(begin(a), std::prev(end(a)), std::next(begin(a)), std::plus<> {});
print("Fibonacci, a = ", a);
}

Output:

Initially, v = 4 6 9 13 18 19 19 15 10
Modified v = 4 2 3 4 5 1 0 -4 -5
Fibonacci, a = 1 1 2 3 5 8 13 21 34 55