std::partial_sum

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Defined in header `<numeric>`

template< class InputIterator, class OutputIterator > OutputIterator partial_sum( InputIterator first, InputIterator last, OutputIterator d_first );	(1)

template< class InputIterator, class OutputIterator, class BinaryOperation > OutputIterator partial_sum( InputIterator first, InputIterator last, OutputIterator d_first, BinaryOperation op );	(2)

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.

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);
...

The signature does not need to have const &.
The type Type1 must be such that an object of type iterator_traits<InputIterator>::value_type can be implicitly converted to Type1. The type Type2 must be such that an object of type InputIterator can be dereferenced and then implicitly converted to Type2. The type Ret must be such that an object of type iterator_traits<InputIterator>::value_type can be assigned a value of type Ret.

[edit] Return value

Iterator to the element past the last element written.

[edit] Complexity

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

[edit] Equivalent function

First version

template<class InputIterator, class OutputIterator>
OutputIterator partial_sum(InputIterator first, InputIterator last, 
                           OutputIterator d_first)
{
    return std::partial_sum(first, last, d_first, 
                            std::plus<InputIterator, InputIterator>());
}

Second version

template<class InputIterator, class OutputIterator, class BinaryOperator>
OutputIterator partial_sum(InputIterator first, InputIterator last, 
                           OutputIterator d_first, BinaryOperation op)
{
    if (first == last) return d_first;
 
    typename std::iterator_traits<InputIterator>::value_type sum = *first;
    *d_first = sum;
 
    while (++first != last) {
       sum = op(sum, *first);
       *++d_first = sum;
    }
    return ++d_first;
}

[edit] Example

#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};
 
    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

[edit] See also

adjacent_difference

computes the differences between adjacent elements in a range
(function template)

accumulate

sums up a range of elements
(function template)