Stdtransform Exclusive Scan In C++

A hypothetical C++ function called std::transformexclusivescan combines the features of std::transform and std::exclusive_scan. This hypothetical function would compute an exclusive scan (prefix sum) across the transformed elements after applying a unary transformation function to each element in a sequence. An initial value for the scan, an output iterator pointing to the beginning of the destination range where the result will be stored, input iterators defining the range of elements to be transformed, a binary operation used to combine the elements during the scan, and a unary operation applied to each element of the input sequence might all be accepted by the function signature.

The function would iterate over the input sequence, transforming each element with the unary operation before storing the results in the output range. After that, it would combine elements using the binary operation, beginning with the original value and doing an exclusive scan over the transformed elements. Such a function would increase the expressive power and Flexibility of the C++ Standard Library for numerical and data processing tasks by offering a quick and easy method to convert and compute prefix sums over sequences in a single operation.

Syntax:

It has the following syntax:

Example


template<typename InputIt, typename OutputIt, typename UnaryOp, typename BinaryOp>
OutputIt std::transform_exclusive_scan(InputIt first, InputIt last, OutputIt result,
typename std::iterator_traits<InputIt>::value_type init, BinaryOp binary_op, 
UnaryOp unary_op);

Parameters:

✅ first, last: Iterators for input that specify the set of elements that need to be scanned and transformed.
✅ result: An iterator for output that stores the exclusive scan's results.
✅ binary_op: The exclusive scan uses a binary operation.
✅ unary_op: Before scanning, each element is applied to a unary operation.

Operation:

✅ The function would store the results in the output range beginning at the result and perform the unary operation unary_op to every element in the input range [first, last].
✅ After that, it would use the binary operation binary_op to perform an exclusive scan over the transformed elements, beginning with the initial value init.
✅ At a suitable position in the output range, the result of each scan step would be stored.

Return Value:

✅ An iterator to the element past the last element written in the output range may be returned by the function.

Advantages:

Several advantages of the std::transformexclusivescan in C++ are as follows:

✅ Convenience: The std::transformexclusivescan would provide an efficient way of performing an exclusive scan operation in addition to a transformation in a single function call. It would simplify and improve the code’s readability by removing the requirement for separate loops or function calls.
✅ Efficiency: The std::transformexclusivescan may optimize memory access patterns and reduce unnecessary intermediate storage by merging the transformation and scan into a single operation. It might result in better performance than separate transformation and scan operations.

Disadvantages:

Several disadvantages of the std::transformexclusivescan in C++ are as follows:

✅ Complexity: Combining the transformation and scan functions into a single function may increase the complexity of the function interface and implementation. In order to use the function efficiently, users would need to comprehend both the transformation and scan aspects, which might cause confusion or misuse.
✅ Limited Flexibility: The std::transformexclusivescan might not be able to handle all scenarios, but it would be helpful in most situations when an exclusive scan follows a transformation. In certain more complicated or specialized use scenarios, users might still have to turn to independent transformation and scanning operations.

Example:

Let us take an example to illustrate the std::transformexclusivescan function in C++.

Example


#include <functional>
#include <iostream>
#include <iterator>
#include <numeric>
#include <vector>
int main()
{
    std::vector elements{2, 4, 9, 6, 3, 6, 8, 2};
    auto times_5 = [](int x) { return x *5; };
    std::cout << "The 5 times exclusive sum: ";
    std::transform_exclusive_scan(elements.begin(), elements.end(),
                                  std::ostream_iterator<int>(std::cout, " "),
                                  0, std::plus<int>{}, times_5);
    std::cout << "\nThe 5 times inclusive sum: ";
    std::transform_inclusive_scan(elements.begin(), elements.end(),
                                  std::ostream_iterator<int>(std::cout, " "),
                                  std::plus<int>{}, times_5);
    std::cout << '\n';
}

Output:

Output


The 5 times exclusive sum: 0 10 30 75 105 120 150 190 
The 5 times inclusive sum: 10 30 75 105 120 150 190 200

Explanation:

This C++ code demonstrates how to use lambdas and other standard components when combined with the Standard Library’s std::transformexclusivescan and std::transforminclusivescan functions.

An integer sequence {2, 4, 9, 6, 3, 6, 8, 2} may be found in the elements vector.

In order to multiply an integer by five, use the lambda function times_five.

An exclusive scan (prefix sum) across the members of the elements vector is computed by std::transformexclusivescan. Starting with an initial value of 0, the times5 transformation is applied to each element, and then addition is used to accumulate the results. The std::transforminclusivescan computes an inclusive scan across the items of the elements vector, and the results are streamed to std::cout. It performs the times5 transformation on each element, the same as std::transformexclusivescan, and then adds up the results.

However, the present element is included in the result. The results are also streamed to std::cout. Times_5 converts each element of the original vector and accumulates correspondingly, displaying the outcomes of both exclusive and inclusive scans in the output.