NAME¶

std::exclusive_scan - std::exclusive_scan

Synopsis¶

Defined in header <numeric>
template< class InputIt, class OutputIt, class T
>
(since C++17)
OutputIt exclusive_scan( InputIt first, InputIt (until C++20)
last,

OutputIt d_first, T init );
template< class InputIt, class OutputIt, class T
>

constexpr OutputIt exclusive_scan( InputIt (since C++20)
first, InputIt last,

OutputIt d_first, T init );
template< class ExecutionPolicy, class
ForwardIt1, class ForwardIt2, class T >

ForwardIt2 exclusive_scan( ExecutionPolicy&& (2) (since C++17)
policy, ForwardIt1 first, ForwardIt1 last,

ForwardIt2 d_first, T init );
template< class InputIt, class OutputIt,
(1)
class T, class BinaryOperation >
OutputIt exclusive_scan( InputIt first, InputIt (since C++17)
last, (until C++20)

OutputIt d_first, T init, BinaryOperation
binary_op );
template< class InputIt, class OutputIt,

class T, class BinaryOperation >
constexpr OutputIt exclusive_scan( InputIt (since C++20)
first, InputIt last, (3)

OutputIt d_first, T init, BinaryOperation
binary_op );
template< class ExecutionPolicy, class
ForwardIt1, class ForwardIt2,

class T, class BinaryOperation >
ForwardIt2 exclusive_scan( ExecutionPolicy&& (4) (since C++17)
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. These overloads do not participate
in overload resolution unless
std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>
(until C++20)
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>
(since C++20) 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 FunctionObject that will be applied in to the result of
binary_op - 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
adjacent_difference range
(function template)
accumulate sums up a range of elements
(function template)
partial_sum computes the partial sum of a range of elements
(function template)
transform_exclusive_scan applies an invocable, then calculates exclusive scan
(C++17) (function template)
inclusive_scan similar to std::partial_sum, includes the ith input element
(C++17) in the ith sum
(function template)