std::adjacent_difference
| Defined in header <numeric>
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template< class InputIt, class OutputIt > OutputIt adjacent_difference( InputIt first, InputIt last, |
(1) | (constexpr since C++20) |
| template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 > |
(2) | (since C++17) |
template< class InputIt, class OutputIt, class BinaryOp > OutputIt adjacent_difference( InputIt first, InputIt last, |
(3) | (constexpr since C++20) |
| template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class BinaryOp > |
(4) | (since C++17) |
Let T be the value type of decltype(first).
[first, last) is empty, does nothing.- Creates an accumulator acc of type
T, and initializes it with *first. - Assigns acc to *d_first.
- For each iterator iter in
[++first,last)in order, performs the following operations in order:
T, and initializes it with *iter.[first, last) is empty, does nothing.- Assigns *first to *d_first.
- For each integer i in
[1,std::distance(first, last)), performs the following operations in order:
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):
-
Tis 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:
|
(since C++20) |
- 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].
- For overloads (2,4),
Contents |
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 &. |
| Type requirements | ||
-InputIt must meet the requirements of LegacyInputIterator.
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-OutputIt must meet the requirements of LegacyOutputIterator.
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-ForwardIt1, ForwardIt2 must meet the requirements of LegacyForwardIterator.
| ||
Return value
Iterator to the element past the last element written, or d_first if [first, last) is empty.
Complexity
Given N as std::distance(first, last):
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
ExecutionPolicyis one of the standard policies, std::terminate is called. For any otherExecutionPolicy, 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++20 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++20 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 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>{});
Example
#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 retroactively 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 |
| LWG 539 | C++98 | the type requirements needed for the result evaluations and assignments to be valid were missing |
added |
| LWG 3058 | C++17 | for overloads (2,4), the result of each invocation of operator- or op was assigned to a temporary object, and that object is assigned to the output range |
assign the results to the output range directly |
See also
| computes the partial sum of a range of elements (function template) | |
| sums up or folds a range of elements (function template) |
