std::inclusive_scan(3)

✨ std::inclusive_scan ✨

okay bestie, so this is from the C++ Standard Library and it's literally about doing a running total but make it fashion 💅

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🎀 NAME 🎀

std::inclusive_scan — bestie it's std::inclusive_scan, that's the whole name, periodt

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🦋 Synopsis 🦋

sooo this lives in the header <numeric> and there are six whole overloads, like she's an overachiever fr fr 👑

template< class InputIt, class OutputIt >
OutputIt inclusive_scan( InputIt first, InputIt last,
                         OutputIt d_first );

(1) — since C++17, constexpr since C++20

template< class ExecutionPolicy,
          class ForwardIt1, class ForwardIt2 >
ForwardIt2 inclusive_scan( ExecutionPolicy&& policy,
                           ForwardIt1 first, ForwardIt1 last,
                           ForwardIt2 d_first );

(2) — since C++17

template< class InputIt, class OutputIt, class BinaryOp >
OutputIt inclusive_scan( InputIt first, InputIt last,
                         OutputIt d_first, BinaryOp op );

(3) — since C++17, constexpr since C++20

template< class ExecutionPolicy,
          class ForwardIt1, class ForwardIt2, class BinaryOp >
ForwardIt2 inclusive_scan( ExecutionPolicy&& policy,
                           ForwardIt1 first, ForwardIt1 last,
                           ForwardIt2 d_first, BinaryOp op );

(4) — since C++17

template< class InputIt, class OutputIt,
          class BinaryOp, class T >
OutputIt inclusive_scan( InputIt first, InputIt last,
                         OutputIt d_first, BinaryOp op, T init );

(5) — since C++17, constexpr since C++20

template< class ExecutionPolicy,
          class ForwardIt1, class ForwardIt2,
          class BinaryOp, class T >
ForwardIt2 inclusive_scan( ExecutionPolicy&& policy,
                           ForwardIt1 first, ForwardIt1 last,
                           ForwardIt2 d_first, BinaryOp op, T init );

(6) — since C++17

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🌸 Description 🌸

okay so let me break this down for you bestie 🫶

Overload (1) is literally equivalent to inclusive_scan(first, last, d_first, std::plus<>().

Overload (3) computes the inclusive prefix sum using op. no cap, that's the whole vibe.

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

1. Creates a sequence which is formed by the elements of [first, iter] in order, where iter is the next *i*th iterator of first.
2. Computes the generalized noncommutative sum of the sequence over op.
3. Assigns the result to *dest, where dest is the next *i*th iterator of d_first.

Overload (5) is same as (3), but each sequence created is formed by init followed by the elements of [first, iter] in order. she adds an init, she's extra like that 💖

Overloads (2), (4), (6) are same as (1), (3), (5), but executed according to policy. iconic behavior honestly ✨

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)

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🧚 the generalized noncommutative sum tea 🧚

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. simple as that bestie 🍓
• 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.

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💀 the warning section (pay attention bestie) 💀

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). 😭 so like, don't come for her when the answer is different every time
• For overloads (1-4), if binary_op(*first, *first) is not convertible to the value type of decltype(first), the program is ill-formed.
• For overloads (5,6), 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 — like, actual chaos, not the fun kind 🙈:
  • For overloads (1-4), the value type of decltype(first) is not MoveConstructible.
  • For overloads (5,6), T is not MoveConstructible.
  • binary_op modifies any element of [first, last).
  • binary_op invalidates any iterator or subrange of [first, last).

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✨ Parameters ✨

• first, last — the range of elements to sum 💖
• d_first — the beginning of the destination range; may be equal to first (she can multitask, we stan)
• policy — the execution policy to use. See execution policy for details. 👑
• 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 (if provided) 🦋

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🔥 Type requirements 🔥

• InputIt must meet the requirements of LegacyInputIterator.
• OutputIt must meet the requirements of LegacyOutputIterator.
• ForwardIt1, ForwardIt2 must meet the requirements of LegacyForwardIterator.

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💜 Return value 💜

Iterator to the element past the last element written. she returns where she left off, organized queen 💅

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🌈 Complexity 🌈

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

• (1,2) — (\scriptsize O(N))O(N) applications of std::plus<>().
• (3-6) — (\scriptsize O(N))O(N) applications of op.

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🫶 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. rude but okay 💀

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🌟 Example 🌟

run this code bestie, it's giving main character energy ✨

#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

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🧚 See also 🧚

• adjacent_difference — computes the differences between adjacent elements in a range (function template)
• accumulate — sums up or folds a range of elements (function template)
• partial_sum — computes the partial sum of a range of elements (function template)
• transform_inclusive_scan (C++17) — applies an invocable, then calculates inclusive scan (function template)
• exclusive_scan (C++17) — similar to std::partial_sum, excludes the *i*th input element from the *i*th sum (function template)

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okay that's the whole page bestie, you're officially an inclusive_scan expert now, go slay 💅✨👑

source: http://cppreference.com — version 2024.06.10 — page std::inclusive_scan(3) 🎀 mwah, love u, bye 🫶💖