NAME¶

std::transform_exclusive_scan - std::transform_exclusive_scan

Synopsis¶

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

class BinaryOp, class UnaryOp >
OutputIt transform_exclusive_scan (1) (since C++17)
( InputIt first, InputIt last, OutputIt d_first, T (constexpr since C++20)
init,

BinaryOp binary_op, UnaryOp unary_op );
template< class ExecutionPolicy,

class ForwardIt1, class ForwardIt2, class T,
class BinaryOp, class UnaryOp >
ForwardIt2 transform_exclusive_scan (2) (since C++17)
( ExecutionPolicy&& policy,
ForwardIt1 first, ForwardIt1 last, ForwardIt2
d_first, T init,

BinaryOp binary_op, UnaryOp unary_op );

1) 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 values transformed
from the elements of [first, iter) in order by unary_op, where iter is the next
i
th iterator of first.
2. Computes the generalized noncommutative sum of the sequence over binary_op.
3. Assigns the result to *dest, where dest is the next i
th iterator of d_first.
2) Same as (1), but executed according to policy.
This overload participates 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)

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.

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, init)
* binary_op(init, unary_op(*first))
* binary_op(unary_op(*first), unary_op(*first))

If any of the following conditions is satisfied, the behavior is undefined:

* T is not MoveConstructible.
* unary_op or binary_op modifies any element of [first, last).
* unary_op or binary_op invalidates any iterator or subrange of [first, last].

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
unary_op - unary FunctionObject that will be applied to each element of the input
range. The return type must be acceptable as input to binary_op.
binary_op - binary FunctionObject that will be applied in to the result of
unary_op, 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.

Return value¶

Iterator to the element past the last element written.

Complexity¶

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

1,2) \(\scriptsize O(N)\)O(N) applications of unary_op and binary_op respectively.

Exceptions¶

The overload with a template parameter named ExecutionPolicy reports 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.

Notes¶

unary_op is never applied to init.

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};

auto times_10 = [](int x) { return x * 10; };

std::cout << "10 times exclusive sum: ";
std::transform_exclusive_scan(data.begin(), data.end(),
std::ostream_iterator<int>(std::cout, " "),
0, std::plus<int>{}, times_10);
std::cout << "\n10 times inclusive sum: ";
std::transform_inclusive_scan(data.begin(), data.end(),
std::ostream_iterator<int>(std::cout, " "),
std::plus<int>{}, times_10);
std::cout << '\n';
}

Output:¶

10 times exclusive sum: 0 30 40 80 90 140 230 250
10 times inclusive sum: 30 40 80 90 140 230 250 310

See also¶

partial_sum computes the partial sum of a range of elements
(function template)
exclusive_scan similar to std::partial_sum, excludes the i^th input
(C++17) element from the i^th sum
(function template)
transform_inclusive_scan applies an invocable, then calculates inclusive scan
(C++17) (function template)