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std::adjacent_difference - cppreference.com

Let T be the value type of decltype(first).

1) If [first, last) is empty, does nothing.

Otherwise, performs the following operations in order:

1. Creates an accumulator acc of type T, and initializes it with *first.
2. Assigns acc to *d_first.
3. For each iterator iter in [++first, last) in order, performs the following operations in order:

a) Creates an object val of type T, and initializes it with *iter.

b) Computes val - acc(until C++20)val - std::move(acc)(since C++20).

c) Assigns the result to *++d_first.

d) Copy(until C++20)Move(since C++20) assigns from val to acc.

2) If [first, last) is empty, does nothing.

Otherwise, performs the following operations in order:

1. Assigns *first to *d_first.
2. For each integer i in [1, std::distance(first, last)), performs the following operations in order:

a) Computes curr - prev, where curr is the next ith iterator of first, and prev is the next i - 1th iterator of first.

b) Assigns the result to *dest, where dest is the next ith iterator of d_first.

3) Same as (1), but computes op(val, acc)(until C++20)op(val, std::move(acc))(since C++20) instead.

4) Same as (2), but computes op(curr, prev) instead.

Given binary_op as the actual binary operation:

• If any of the following conditions is satisfied, the program is ill-formed:

• For overloads (1,3):

• T is not constructible from *first.
• acc is not writable to d_first.
• The result of binary_op(val, acc)(until C++20)binary_op(val, std::move(acc))(since C++20) is not writable to d_first.

• For overloads (2,4):

• *first is not writable to d_first.
• The result of binary_op(*first, *first) is not writable to d_first.

• Given d_last as the iterator to be returned, if any of the following conditions is satisfied, the behavior is undefined:

• For overloads (2,4), [first, last) and [d_first, d_last) overlaps.
• binary_op modifies any element of [first, last) or [d_first, d_last).
• binary_op invalidates any iterator or subrange in [first, last] or [d_first, d_last].

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. ​

Iterator to the element past the last element written, or d_first if [first, last) is empty.

Given \(\scriptsize N\)N as std::distance(first, last):

1,2) Exactly \(\scriptsize N-1\)N-1 applications of operator-.

3,4) Exactly \(\scriptsize N-1\)N-1 applications of the binary function op.

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.

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;
}

template<class InputIt, class OutputIt, class BinaryOp>
constexpr // since C++20
OutputIt adjacent_difference(InputIt first, InputIt last, 
                             OutputIt d_first, BinaryOp 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;
}

acc was introduced because of the resolution of LWG issue 539. The reason of using acc rather than directly calculating the differences is because the semantic of the latter is confusing if the following types mismatch:

• the value type of InputIt
• the writable type(s) of OutputIt
• the types of the parameters of operator- or op
• the return type of operator- or op

acc serves as the intermediate object to cache values of the iterated elements:

• its type is the value type of InputIt
• the value written to d_first (which is the return value of operator- or op) is assigned to it
• its value is passed to operator- or op

char i_array[4] = {100, 100, 100, 100};
int  o_array[4];
 
// OK: performs conversions when needed
// 1. creates “acc” of type char (the value type)
// 2. “acc” is assigned to the first element of “o_array”
// 3. the char arguments are used for long multiplication (char -> long)
// 4. the long product is assigned to the output range (long -> int)
// 5. the next value of “i_array” is assigned to “acc”
// 6. go back to step 3 to process the remaining elements in the input range
std::adjacent_difference(i_array, i_array + 4, o_array, std::multiplies<long>{});

#include <array>
#include <functional>
#include <iostream>
#include <iterator>
#include <numeric>
#include <vector>
 
void println(auto comment, const auto& sequence)
{
    std::cout << comment;
    for (const auto& n : sequence)
        std::cout << n << ' ';
    std::cout << '\n';
};
 
int main()
{
    // Default implementation - the difference between two adjacent items
    std::vector v{4, 6, 9, 13, 18, 19, 19, 15, 10};
    println("Initially, v = ", v);
    std::adjacent_difference(v.begin(), v.end(), v.begin());
    println("Modified v = ", v);
 
    // Fibonacci
    std::array<int, 10> a {1};
    std::adjacent_difference(std::begin(a), std::prev(std::end(a)),
                             std::next(std::begin(a)), std::plus<>{});
    println("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

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

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