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std::adjacent_difference(3) C++ Standard Libary std::adjacent_difference(3) NAME std::adjacent_difference - std::adjacent_difference Synopsis Defined in header <numeric> template< class InputIt, class OutputIt > OutputIt adjacent_difference( InputIt first, InputIt (1) (const- expr since C++20) last, OutputIt d_first ); template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 > ForwardIt2 adjacent_difference( ExecutionPolicy&& policy, (2) (since C++17) ForwardIt1 first, ForwardIt1 last, ForwardIt2 d_first ); template< class InputIt, class OutputIt, class BinaryOp > OutputIt adjacent_difference( InputIt first, InputIt (3) (const- expr since C++20) last, OutputIt d_first, BinaryOp op ); template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class BinaryOp > ForwardIt2 adjacent_difference( ExecutionPolicy&& policy, (4) (since C++17) ForwardIt1 first, ForwardIt1 last, ForwardIt2 d_first, BinaryOp op ); 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++11) val - std::move(acc) (since C++11). c) Assigns the result to *++d_first. d) Copy (until C++11) Move (since C++11) 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 i th iterator of first, and prev is the next i - 1 th iterator of first. b) Assigns the result to *dest, where dest is the next i th iterator of d_first. 3) Same as (1), but computes op(val, acc) (until C++11) op(val, std::move(acc)) (since C++11) 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++11) binary_op(val, std::move(acc)) (since C++11) 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 fol- lowing conditions is satisfied, the behavior is undefined: * For overloads (1,3), T is not MoveAssignable. (since C++11) * For overloads (2,4), [first, last) and [d_first, d_last) over- laps. * 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]. 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 ob- ject 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. - OutputIt must meet the requirements of LegacyOutputIterator. - ForwardIt1, ForwardIt2 must meet the requirements of LegacyFor- wardIterator. Return value Iterator to the element past the last element written, or d_first if [first, last) is empty. Complexity 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 func- tion op. 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::termi- nate 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 adjacent_difference (1) 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++11 acc = std::move(val); } return ++d_first; } adjacent_difference (3) 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++11 acc = std::move(val); } return ++d_first; } Notes 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 iter- ated elements: * its type is the value type of InputIt * the value written to d_first (which is the return value of oper- ator- 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::multi- plies<long>{}); Example // Run this code #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 Defect reports The following behavior-changing defect reports were applied retroac- tively to previously published C++ standards. DR Applied to Behavior as published Correct behavior LWG 242 C++98 op could not have side effects it cannot modify the ranges involved the type requirements needed for the result LWG 539 C++98 evaluations and assignments to be valid added were missing LWG 2055 C++11 acc was not moved while being accumulated it is moved (P0616R0) for overloads (2,4), the result of each invocation as- sign the results LWG 3058 C++17 of operator- or op was assigned to a to the output temporary range directly object, and that object is assigned to the output range See also partial_sum computes the partial sum of a range of elements (function template) accumulate sums up or folds a range of elements (function template) http://cppreference.com 2024.06.10 std::adjacent_difference(3)
NAME | Synopsis | Parameters | Type requirements | Return value | Complexity | Exceptions | Possible implementation | Notes | Example | Output: | See also
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