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std::transform_inclusive_scan

From cppreference.com
< cpp‎ | algorithm
 
 
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
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(until C++17)(C++11)
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C library
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transform_inclusive_scan
(C++17)

Operations on uninitialized memory
 
 
Defined in header <numeric>
template< class InputIt, class OutputIt,

          class BinaryOp, class UnaryOp >
OutputIt transform_inclusive_scan
    ( InputIt first, InputIt last, OutputIt d_first,

      BinaryOp binary_op, UnaryOp unary_op );
(1) (since C++17)
(constexpr since C++20)
template< class ExecutionPolicy,

          class ForwardIt1, class ForwardIt2,
          class BinaryOp, class UnaryOp >
ForwardIt2 transform_inclusive_scan
    ( ExecutionPolicy&& policy,
      ForwardIt1 first, ForwardIt1 last, ForwardIt2 d_first,

      BinaryOp binary_op, UnaryOp unary_op );
(2) (since C++17)
template< class InputIt, class OutputIt,

          class BinaryOp, class UnaryOp, class T >
OutputIt transform_inclusive_scan
    ( InputIt first, InputIt last, OutputIt d_first,

      BinaryOp binary_op, UnaryOp unary_op, T init );
(3) (since C++17)
(constexpr since C++20)
template< class ExecutionPolicy,

          class ForwardIt1, class ForwardIt2,
          class BinaryOp, class UnaryOp, class T >
ForwardIt2 transform_inclusive_scan
    ( ExecutionPolicy&& policy,
      ForwardIt1 first, ForwardIt1 last, ForwardIt2 d_first,

      BinaryOp binary_op, UnaryOp unary_op, T init );
(4) (since C++17)
1) Computes the inclusive prefix sum using op.
For each integer i in [0std::distance(first, last)), performs the following operations in order:
  1. Creates a sequence which is formed by the values transformed from the elements of [firstiter] 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.
3) Same as (1), but each sequence created is formed by init followed by the elements of [firstiter] in order.
2,4) Same as (1,3), but executed according to policy.
These overloads participate 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).

For overloads (1,2), if binary_op(unary_op(*first), unary_op(*first)) is not convertible to the value type of decltype(first), the program is ill-formed.

For overloads (3,4), 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:

  • For overloads (1,2), the value type of decltype(first) is not MoveConstructible.
  • For overloads (3,4), T is not MoveConstructible.
  • unary_op or binary_op modifies any element of [firstlast).
  • unary_op or binary_op invalidates any iterator or subrange of [firstlast].

Contents

[edit] 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 if provided
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 N as std::distance(first, last):

1-4) O(N) applications of unary_op and binary_op respectively.

[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] Notes

unary_op is never applied to init.

The parameter init appears last, differing from std::transform_exclusive_scan, because it is optional for this function.

[edit] Example

#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

[edit] See also

computes the partial sum of a range of elements
(function template) [edit]
applies a function to a range of elements, storing results in a destination range
(function template) [edit]
similar to std::partial_sum, includes the ith input element in the ith sum
(function template) [edit]
applies an invocable, then calculates exclusive scan
(function template) [edit]