std::exclusive_scan - cppreference.com (original) (raw)

Defined in header
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)

Equivalent to exclusive_scan(first, last, d_first, init, std::plus<>().
Computes the exclusive prefix sum using op.

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

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.
Computes the generalized noncommutative sum of the sequence over op.
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:

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:

Selects any two adjacent elements elem1 and elem2 from the sequence.
Calculates binary_op(elem1, elem2) and replaces the two elements in the sequence with the result.
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].

[edit] 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.

[edit] Return value

Iterator to the element past the last element written.

[edit] Complexity

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

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

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

[edit] 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.

[edit] Example

#include #include #include #include #include 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(std::cout, " "), 0); std::cout << "\nInclusive sum: "; std::inclusive_scan(data.begin(), data.end(), std::ostream_iterator(std::cout, " ")); std::cout << "\n\nExclusive product: "; std::exclusive_scan(data.begin(), data.end(), std::ostream_iterator(std::cout, " "), 1, std::multiplies<>{}); std::cout << "\nInclusive product: "; std::inclusive_scan(data.begin(), data.end(), std::ostream_iterator(std::cout, " "), std::multiplies<>{}); }

Output: