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std::exclusive_scan - cppreference.com

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Defined in header <numeric>

template< class InputIt, class OutputIt, class T > OutputIt exclusive_scan( InputIt first, InputIt last, OutputIt d_first, T init );

 (1) (since C++17)
(constexpr 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)

template< class InputIt, class OutputIt, class T, class BinaryOp > OutputIt exclusive_scan( InputIt first, InputIt last, OutputIt d_first, T init, BinaryOp op );

 (3) (since C++17)
(constexpr since C++20)

template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class T, class BinaryOp > ForwardIt2 exclusive_scan( ExecutionPolicy&& policy, ForwardIt1 first, ForwardIt1 last, ForwardIt2 d_first, T init, BinaryOp op );

 (4) (since C++17)

1) Equivalent to exclusive_scan(first, last, d_first, init, std::plus<>().

3) Computes the exclusive prefix sum using op.

For each integer i in [0, std::distance(first, last)), performs the following operations in order:

  1. Creates a sequence which is formed by init followed by the elements of [first, iter) in order, where iter is the next ith iterator of first.
  2. Computes the generalized noncommutative sum of the sequence over op.
  3. Assigns the result to *dest, where dest is the next ith iterator of d_first.

2,4) Same as (1,3), but executed according to policy.

These overloads participate in overload resolution only if all following conditions are satisfied:

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)

The generalized noncommutative sum of a sequence of elements over a binary operation binary_op is defined as follows:

  • If the sequence only has one element, the sum is the value of the element.
  • Otherwise, performs the following operations in order:
  1. Selects any two adjacent elements elem1 and elem2 from the sequence.
  2. Calculates binary_op(elem1, elem2) and replaces the two elements in the sequence with the result.
  3. Repeats steps 1 and 2 until there is only one element in the sequence.


Given binary_op as the actual binary operation:

  • The result is non-deterministic if the binary_op is not associative (such as floating-point addition).
  • If any of the following values is not convertible to T, the program is ill-formed:
  • binary_op(init, *first)
  • binary_op(init, init)
  • binary_op(*first, *first)
  • If any of the following conditions is satisfied, the behavior is undefined:
  • T is not MoveConstructible.
  • binary_op modifies any element of [first, last).
  • binary_op invalidates any iterator or subrange of [first, last].

Parameters

first, last - the pair of iterators defining 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
init - the initial value
op - binary FunctionObject that will be applied in to the result of dereferencing the input iterators, the results of other 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.

Return value

Iterator to the element past the last element written.

Complexity

Given N as std::distance(first, last):

1,2) O(N) applications of std::plus<>().

3,4) O(N) applications of op.

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

#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