std::transf...clusive_scan(3)

💖 NAME

Okay so bestie, this is the manual page for std::transform_exclusive_scan, straight out of the C++ Standard Library (manual section 3). It's giving "fancy algorithm with a long name" energy, and we're gonna get through it together. 💅

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✨ Synopsis

Defined in header <numeric>:

template< class InputIt, class OutputIt, class T,
          class BinaryOp, class UnaryOp >
OutputIt transform_exclusive_scan( InputIt first, InputIt last,
                                   OutputIt d_first, T init,
                                   BinaryOp binary_op, UnaryOp unary_op );

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

template< class ExecutionPolicy,
          class ForwardIt1, class ForwardIt2, class T,
          class BinaryOp, class UnaryOp >
ForwardIt2 transform_exclusive_scan( ExecutionPolicy&& policy,
                                     ForwardIt1 first, ForwardIt1 last,
                                     ForwardIt2 d_first, T init,
                                     BinaryOp binary_op, UnaryOp unary_op );

(2) (since C++17)

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🧠 Synopsis, but make it make sense

Okay so overload (1) computes the exclusive prefix sum using op. Yes, the page literally says op — that's the page's whole vibe, don't ask me where op is hiding 💀. For each integer i in [0, std::distance(first, last)), it performs these 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.

Overload (2) is the same as (1), but it's 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), or 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 like this:

• If the sequence only has one element, the sum is the value of the element.
• Otherwise, it 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 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).

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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.
• 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.

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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. Slay. ✨

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🔥 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.

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🌈 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.

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🌸 Notes

unary_op is never applied to init. Periodt. 💅

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🍓 Example

Run this code, bestie:

// 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

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💜 See also

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

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Source/version: http://cppreference.com; page dated 2024.06.10. That's the whole manual page, bestie — go forth and scan exclusively. Mwah 💋💖