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std::exclusive_scan - cppreference.com

Equivalent to

.

3) Computes the exclusive prefix sum using op.

For each integer

in

, performs the following operations in order:

1. 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.
2. Computes the generalized noncommutative sum of the sequence over op.
3. 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:

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.

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].

Iterator to the element past the last element written.

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

applications of

.

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

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.

#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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