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std::partial_sum(3) C++ Standard Libary std::partial_sum(3) NAME std::partial_sum - std::partial_sum Synopsis Defined in header <numeric> template< class InputIt, class OutputIt > OutputIt partial_sum( InputIt first, InputIt last, (1) (const- expr since C++20) OutputIt d_first ); template< class InputIt, class OutputIt, class BinaryOp > (2) (const- expr since C++20) OutputIt partial_sum( InputIt first, InputIt last, OutputIt d_first, BinaryOp op ); 1) If [first, last) is empty, does nothing. Otherwise, performs the following operations in order: 1. Creates an accumulator acc, whose type is the value type of In- putIt, and initializes it with *first. 2. Assigns acc to *d_first. 3. For each integer i in [1, std::distance(first, last)), performs the following operations in order: a) Computes acc + *iter (until C++11) std::move(acc) + *iter (since C++11), where iter is the next i th iterator of first. b) Assigns the result to acc. c) Assigns acc^[1] to *dest, where dest is the next i th iterator of d_first. 2) Same as (1), but computes op(acc, *iter) (until C++11) op(std::move(acc), *iter) (since C++11) instead. Given binary_op as the actual binary operation: * If any of the following conditions is satisfied, the program is ill-formed: * The value type of InputIt is not constructible from *first. * acc is not writable to d_first. * The result of binary_op(acc, *iter) (until C++11) binary_op(std::move(acc), *iter) (since C++11) is not implicitly convertible to the value type of InputIt. * Given d_last as the iterator to be returned, if any of the fol- lowing conditions is satisfied, the behavior is undefined: * 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]. 1. The actual value to be assigned is the result of the assignment in the previous step. We assume the assignment result is acc here. Parameters first, last - the range of elements to sum d_first - the beginning of the destination range; may be equal to first 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 type Type1 must be such that an object of type std::iterator_traits<InputIt>::value_type can be im- plicitly converted to Type1. The type Type2 must be such that an object of type InputIt can be dereferenced and then implicitly converted to Type2. The type Ret must be such that an object of type InputIt 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. 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) Exactly \(\scriptsize N-1\)N-1 applications of operator+. 2) Exactly \(\scriptsize N-1\)N-1 applications of the binary func- tion op. Possible implementation partial_sum (1) template<class InputIt, class OutputIt> constexpr // since C++20 OutputIt partial_sum(InputIt first, InputIt last, OutputIt d_first) { if (first == last) return d_first; typename std::iterator_traits<InputIt>::value_type sum = *first; *d_first = sum; while (++first != last) { sum = std::move(sum) + *first; // std::move since C++11 *++d_first = sum; } return ++d_first; // or, since C++14: // return std::partial_sum(first, last, d_first, std::plus<>()); } partial_sum (2) template<class InputIt, class OutputIt, class BinaryOp> constexpr // since C++20 OutputIt partial_sum(InputIt first, InputIt last, OutputIt d_first, BinaryOp op) { if (first == last) return d_first; typename std::iterator_traits<InputIt>::value_type acc = *first; *d_first = acc; while (++first != last) { acc = op(std::move(acc), *first); // std::move since C++11 *++d_first = acc; } return ++d_first; } Notes acc was introduced because of the resolution of LWG issue 539. The reason of using acc rather than directly summing up the results (i.e. *(d_first + 2) = (*first + *(first + 1)) + *(first + 2);) 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 store and provide the val- ues for each step of the computation: * its type is the value type of InputIt * it is written to d_first * its value is passed to operator+ or op * it stores the return value of operator+ or op enum not_int { x = 1, y = 2 }; char i_array[4] = {100, 100, 100, 100}; not_int e_array[4] = {x, x, y, y}; int o_array[4]; // OK: uses operator+(char, char) and assigns char values to int array std::partial_sum(i_array, i_array + 4, o_array); // Error: cannot assign not_int values to int array std::partial_sum(e_array, e_array + 4, o_array); // OK: performs conversions when needed // 1. creates acc of type char (the value type) // 2. the char arguments are used for long multiplication (char -> long) // 3. the long product is assigned to acc (long -> char) // 4. acc is assigned to an element of o_array (char -> int) // 5. go back to step 2 to process the remaining elements in the input range std::partial_sum(i_array, i_array + 4, o_array, std::multi- plies<long>{}); Example // Run this code #include <functional> #include <iostream> #include <iterator> #include <numeric> #include <vector> int main() { std::vector<int> v(10, 2); // v = {2, 2, 2, 2, 2, 2, 2, 2, 2, 2} std::cout << "The first " << v.size() << " even numbers are: "; // write the result to the cout stream std::partial_sum(v.cbegin(), v.cend(), std::ostream_iterator<int>(std::cout, " ")); std::cout << '\n'; // write the result back to the vector v std::partial_sum(v.cbegin(), v.cend(), v.begin(), std::multiplies<int>()); std::cout << "The first " << v.size() << " powers of 2 are: "; for (int n : v) std::cout << n << ' '; std::cout << '\n'; } Output: The first 10 even numbers are: 2 4 6 8 10 12 14 16 18 20 The first 10 powers of 2 are: 2 4 8 16 32 64 128 256 512 1024 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 LWG 539 C++98 result added evaluations and assignments to be valid were missing LWG 2055 C++11 acc was not moved while being accumulated it is moved (P0616R0) See also adjacent_difference computes the differences between adjacent ele- ments in a range (function template) accumulate sums up or folds a range of elements (function template) inclusive_scan similar to std::partial_sum, includes the i^th input element in (C++17) the i^th sum (function template) exclusive_scan similar to std::partial_sum, excludes the i^th input element (C++17) from the i^th sum (function template) http://cppreference.com 2024.06.10 std::partial_sum(3)
NAME | Synopsis | Parameters | Type requirements | Return value | Complexity | Possible implementation | Notes | Example | Output: | See also
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