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std::adjacent_difference

From cppreference.com
< cpp‎ | algorithm
 
 
Algorithm library
Execution policies (C++17)
Non-modifying sequence operations
(C++11)(C++11)(C++11)
(C++17)
Modifying sequence operations
Operations on uninitialized storage
Partitioning operations
Sorting operations
(C++11)
Binary search operations
Set operations (on sorted ranges)
Heap operations
(C++11)
Minimum/maximum operations
(C++11)
(C++17)

Permutations
Numeric operations
(C++11)
adjacent_difference
C library
 
Defined in header <numeric>
template< class InputIt, class OutputIt >

OutputIt adjacent_difference( InputIt first, InputIt last,

                              OutputIt d_first );
(1)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 >

ForwardIt2 adjacent_difference( ExecutionPolicy&& policy, ForwardIt1 first, ForwardIt1 last,

                                ForwardIt2 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 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. 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 (overload (1)) or op(val, acc) (overload (3)), assigns the result to *(d_first + (i - first)), and move assigns from val to acc.
The input ranges may overlap, but cannot be identical: the behavior is undefined if first==d_first
2,4) First, creates an object whose type is ForwardIt1's value type, initializes it with *first, and assigns the result to *d_first. Then for every d in [1, last - first - 1], creates an object val whose type is ForwardIt1's value type, initializes it with *(first + d) - *(first + d - 1) (overload (2)) or op(*(first + d), *(first + d - 1)) (overload (4)), and assigns the result 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 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)

Contents

[edit] 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 InputIterator. InputIt's value type must be MoveAssignable and constructible from the type of *first
-
OutputIt must meet the requirements of OutputIterator. both acc (the accumulated value) and the result of val-acc or op(val, acc) must be writable to OutputIt
-
ForwardIt1, ForwardIt2 must meet the requirements of ForwardIterator. ForwardIt1's value_type must be CopyConstructible, constructible from the expression *first - *first or op(*first, *first), and assignable to ForwardIt2's value_type

[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 and ExecutionPolicy is one of the three 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.

[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.

#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

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