Understanding std::transform_reduce in Modern C++

Understanding std::transform_reduce in Modern C++

std::transform_reduce is a powerful C++17 algorithm that combines the functionality of transform and reduce (or accumulate) in a single pass. It allows you to apply a transformation to elements and then reduce them using a binary operation, which can help write concise and efficient code.

Syntax

template<class InputIt1, class InputIt2, class T,
         class BinaryOp1, class BinaryOp2>
T transform_reduce(InputIt1 first1, InputIt1 last1,
                   InputIt2 first2, T init,
                   BinaryOp1 binary_op1,
                   BinaryOp2 binary_op2);

template<class InputIt, class T,
         class BinaryOp1, class UnaryOp>
T transform_reduce(InputIt first, InputIt last,
                   T init,
                   BinaryOp1 binary_op1,
                   UnaryOp unary_op);

In simple words:

• It applies a unary transformation to each element (optional).
• Then it reduces all results using a binary operation like sum, product, or custom combination.
• It can work in parallel with execution policies in C++17/20.

Example 1: Sum of squares

#include <iostream>
#include <vector>
#include <numeric>
#include <execution>

int main() {
    std::vector<int> numbers = {1, 2, 3, 4, 5};

    int sum_of_squares = std::transform_reduce(
        std::execution::seq,   // sequential execution
        numbers.begin(),
        numbers.end(),
        0,                     // initial value
        std::plus<>(),    // binary operation (sum)
        [](int x){ return x*x; } // unary transform (square)
    );

    std::cout << "Sum of squares: " << sum_of_squares << std::endl;
    return 0;
}

Example 2: Dot product of two vectors

#include <iostream>
#include <vector>
#include <numeric>

int main() {
    std::vector<int> a = {1, 2, 3};
    std::vector<int> b = {4, 5, 6};

    int dot_product = std::transform_reduce(
        a.begin(), a.end(),
        b.begin(),
        0  // initial value
    );

    std::cout << "Dot product: " << dot_product << std::endl;
    return 0;
}

Example 3: Parallel transform_reduce

#include <iostream>
#include <vector>
#include <numeric>
#include <execution>

int main() {
    std::vector<double> numbers(1'000'000, 1.5);

    double sum = std::transform_reduce(
        std::execution::par,  // parallel execution
        numbers.begin(),
        numbers.end(),
        0.0
    );

    std::cout << "Parallel sum: " << sum << std::endl;
    return 0;
}

Key Points

• transform_reduce avoids creating an intermediate container for the transformed values.
• It supports sequential and parallel execution policies.
• Two main forms: single range with unary transform, or two ranges for pairwise operations (like dot product).
• Initial value is required to correctly handle empty ranges.

Using std::transform_reduce can make your code cleaner and more efficient, especially for large datasets or parallel computation scenarios.

C/C++ Programming

• Understanding std::transform_reduce in Modern C++
• Implement a Lock Acquire and Release in C++
• Detecting Compile-time vs Runtime in C++: if consteval vs std::is_constant_evaluated()
• C++ Forward References: The Key to Perfect Forwarding
• Understanding dynamic_cast in C++: Safe Downcasting Explained
• C vs C++: Understanding the restrict Keyword and its Role in Optimization
• C++ Lvalue, Rvalue and Rvalue References
• C++ assert vs static_assert
• Why auto_ptr is Deprecated in C++?
• C++ What is the consteval? How is it different to const and constexpr?
• Tutorial on C++ std::move (Transfer Ownership)
• const vs constexpr in C++
• Tutorial on C++ Ranges
• Tutorial on C++ Smart Pointers
• Tutorial on C++ Future, Async and Promise
• The Memory Manager in C/C++: Heap vs Stack
• The String Memory Comparision Function memcmp() in C/C++
• Modern C++ Language Features
• Comparisions of push_back() and emplace_back() in C++ std::vector
• C++ Coding Reference: is_sorted_until() and is_sorted()
• C++ Coding Reference: iota() Setting Incrementing Values to Arrays or Vectors
• C++ Coding Reference: next_permutation() and prev_permutation()
• C++ Coding Reference: count() and count_if()
• C++ Code Reference: std::accumulate() and Examples
• C++ Coding Reference: sort() and stable_sort()
• The Next Permutation Algorithm in C++ std::next_permutation()

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