std::partial_sum

 

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Defined in header <numeric>
	(1)
template< class InputIt, class OutputIt > OutputIt partial_sum( InputIt first, InputIt last, OutputIt d_first );			(until C++20)
template< class InputIt, class OutputIt > constexpr OutputIt partial_sum( InputIt first, InputIt last, OutputIt d_first );			(since C++20)
		(2)
template< class InputIt, class OutputIt, class BinaryOperation > OutputIt partial_sum( InputIt first, InputIt last, OutputIt d_first,                       BinaryOperation op );				(until C++20)
template< class InputIt, class OutputIt, class BinaryOperation > constexpr OutputIt partial_sum( InputIt first, InputIt last, OutputIt d_first,                                 BinaryOperation op );				(since C++20)

Computes the partial sums of the elements in the subranges of the range [first, last) and writes them to the range beginning at d_first. The first version uses operator+ to sum up the elements, the second version uses the given binary function op, both applying std::move to their operands on the left hand side (since C++20).

Equivalent operation:

*(d_first)   = *first;
*(d_first+1) = *first + *(first+1);
*(d_first+2) = *first + *(first+1) + *(first+2);
*(d_first+3) = *first + *(first+1) + *(first+2) + *(first+3);
...

op must not invalidate any iterators, including the end iterators, or modify any elements of the range involved.

Parameters

first, last	-	the range of elements to sum
d_first	-	the beginning of the destination range; may be equal to first
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 type Type1 must be such that an object of type iterator_traits<InputIt>::value_type can be implicitly 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 iterator_traits<InputIt>::value_type can be 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.

Complexity

Exactly (last - first) - 1 applications of the binary operation

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++20 *++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 BinaryOperation> constexpr // since C++20 OutputIt partial_sum(InputIt first, InputIt last, OutputIt d_first, BinaryOperation op) { if (first == last) return d_first;   typename std::iterator_traits<InputIt>::value_type sum = *first; *d_first = sum;   while (++first != last) { sum = op(std::move(sum), *first); // std::move since C++20 *++d_first = sum; }   return ++d_first; }

Example

Run this code

#include <numeric>
#include <vector>
#include <iostream>
#include <iterator>
#include <functional>
 
int main()
{
    std::vector<int> v = {2, 2, 2, 2, 2, 2, 2, 2, 2, 2}; // or std::vector<int>v(10, 2);
 
    std::cout << "The first 10 even numbers are: ";
    std::partial_sum(v.begin(), v.end(), 
                     std::ostream_iterator<int>(std::cout, " "));
    std::cout << '\n';
 
    std::partial_sum(v.begin(), v.end(), v.begin(), std::multiplies<int>());
 
    std::cout << "The first 10 powers of 2 are: ";
    for (auto 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 retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 242	C++98	op cannot have side effects	it cannot modify the ranges involved

See also

adjacent_difference	computes the differences between adjacent elements in a range (function template)
accumulate	sums up or folds a range of elements (function template)
inclusive_scan (C++17)	similar to std::partial_sum, includes the ith input element in the ith sum (function template)
exclusive_scan (C++17)	similar to std::partial_sum, excludes the ith input element from the ith sum (function template)