std::exclusive_scan (3) - Linux Manuals

std::exclusive_scan: std::exclusive_scan

NAME

std::exclusive_scan - std::exclusive_scan

Synopsis

Defined in header <numeric>
template< class InputIt, class OutputIt, class T >
OutputIt exclusive_scan( InputIt first, InputIt last, (1) (since C++17)
OutputIt d_first, T init );
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class T >
ForwardIt2 exclusive_scan( ExecutionPolicy&& policy, ForwardIt1 first, ForwardIt1 last, (2) (since C++17)
ForwardIt2 d_first, T init);
template< class InputIt, class OutputIt,
class T, class BinaryOperation > (3) (since C++17)
OutputIt exclusive_scan( InputIt first, InputIt last,
OutputIt d_first, T init, BinaryOperation binary_op );
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2,
class T, class BinaryOperation > (4) (since C++17)
ForwardIt2 exclusive_scan( ExecutionPolicy&& policy, ForwardIt1 first, ForwardIt1 last,
ForwardIt2 d_first, T init, BinaryOperation binary_op );

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 i-th input element is not included in the i-th 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, a
1, ..., a
N) is defined as follows:

* if N=1, a
  1
* if N > 1, op(GNSUM(op, a
  1, ..., a
  K), GNSUM(op, a
  M, ..., a
  N)) 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. This overload only participates in overload resolution if std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true.
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.

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