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std::exclusive_scan

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Defined in header <numeric>
(1)
template< class InputIt, class OutputIt, class T >

OutputIt exclusive_scan( InputIt first, InputIt last,

                         OutputIt d_first, T init );
 (since C++17)
(until C++20)
template< class InputIt, class OutputIt, class T >

constexpr OutputIt exclusive_scan( InputIt first, InputIt last,

                                   OutputIt d_first, T init );
 (since C++20)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class T >

ForwardIt2 exclusive_scan( ExecutionPolicy&& policy,
                           ForwardIt1 first, ForwardIt1 last,

                           ForwardIt2 d_first, T init );
 (2) (since C++17)
(3)
template< class InputIt, class OutputIt,

          class T, class BinaryOperation >
OutputIt exclusive_scan( InputIt first, InputIt last,
                         OutputIt d_first, T init,

                         BinaryOperation binary_op );
 (since C++17)
(until C++20)
template< class InputIt, class OutputIt,

          class T, class BinaryOperation >
constexpr OutputIt exclusive_scan( InputIt first, InputIt last,
                                   OutputIt d_first, T init,

                                   BinaryOperation binary_op );
 (since C++20)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2,

          class T, class BinaryOperation >
ForwardIt2 exclusive_scan( ExecutionPolicy&& policy,
                           ForwardIt1 first, ForwardIt1 last,
                           ForwardIt2 d_first, T init,

                           BinaryOperation binary_op );
 (4) (since C++17)

Computes an exclusive prefix sum operation using binary_op (or std::plus<>() for overloads (1,2)) for the range [first, last), using init as the initial value, and writes the results to the range beginning at d_first. "exclusive" means that the ith input element is not included in the ith sum.

Formally, assigns through each iterator i in [d_first, d_first + (last - first)) the value of the generalized noncommutative sum of init, *j... for every j in [first, first + (i - d_first)) over binary_op,

where generalized noncommutative sum GNSUM(op, a1, ..., aN) is defined as follows:

  • if N = 1, a1
  • if N > 1, op(GNSUM(op, a1, ..., aK), GNSUM(op, aM, ..., aN)) for any K where 1 < K + 1 = M ≤ N

In other words, the summation operations may be performed in arbitrary order, and the behavior is nondeterministic if binary_op is not associative.

Overloads (2,4) are executed according to policy. These overloads participate in overload resolution only if

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true.

(until C++20)

std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true.

(since C++20)

binary_op shall not invalidate iterators (including the end iterators) or subranges, nor modify elements in the ranges [first, last) or [d_first, d_first + (last - first)). Otherwise, the behavior is undefined.

Contents

Parameters

first, last - the range of elements to sum
d_first - the beginning of the destination range; may be equal to first
policy - the execution policy to use. See execution policy for details.
init - the initial value
binary_op - binary FunctionObject that will be applied in to the result of dereferencing the input iterators, the results of other binary_op, and init
Type requirements
-
InputIt must meet the requirements of LegacyInputIterator.
-
OutputIt must meet the requirements of LegacyOutputIterator.
-
ForwardIt1, ForwardIt2 must meet the requirements of LegacyForwardIterator.
-
T must meet the requirements of MoveConstructible. and binary_op(init, *first), binary_op(init, init), and binary_op(*first, *first) must be convertible to T.

Return value

Iterator to the element past the last element written.

Complexity

O(last - first) applications of the binary operation.

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::terminate is called. For any other ExecutionPolicy, the behavior is implementation-defined.
  • If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Example

Run this code
#include <functional>
#include <iostream>
#include <iterator>
#include <numeric>
#include <vector>
 
int main()
{
    std::vector data{3, 1, 4, 1, 5, 9, 2, 6};
 
    std::cout << "Exclusive sum: ";
    std::exclusive_scan(data.begin(), data.end(),
                        std::ostream_iterator<int>(std::cout, " "),
                        0);
 
    std::cout << "\nInclusive sum: ";
    std::inclusive_scan(data.begin(), data.end(),
                        std::ostream_iterator<int>(std::cout, " "));
 
    std::cout << "\n\nExclusive product: ";
    std::exclusive_scan(data.begin(), data.end(),
                        std::ostream_iterator<int>(std::cout, " "),
                        1, std::multiplies<>{});
 
    std::cout << "\nInclusive product: ";
    std::inclusive_scan(data.begin(), data.end(),
                        std::ostream_iterator<int>(std::cout, " "),
                        std::multiplies<>{});
}

Output: 

Exclusive sum: 0 3 4 8 9 14 23 25
Inclusive sum: 3 4 8 9 14 23 25 31
 
Exclusive product: 1 3 3 12 12 60 540 1080
Inclusive product: 3 3 12 12 60 540 1080 6480

See also

 computes the differences between adjacent elements in a range
(function template) [edit]
sums up or folds a range of elements
(function template) [edit]
computes the partial sum of a range of elements
(function template) [edit]
(C++17)
 applies an invocable, then calculates exclusive scan
(function template) [edit]
(C++17)
 similar to std::partial_sum, includes the ith input element in the ith sum
(function template) [edit]
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