Reduce vs Accumulate Use Cases

std::reduce() and std::accumulate() serve similar purposes, but std::reduce() is often preferred in scenarios where performance and multi-threading are important.

Parallel Processing

std::reduce() is designed for parallel execution, making it suitable for tasks that can benefit from multi-core processors. For example, processing large datasets such as summing elements of a big array:

1#include <iostream>
2#include <numeric>
3#include <vector>
4
5int main() {
6  std::vector<int> largeNumbers(1000000, 1);
7
8  int result = std::reduce(
9    largeNumbers.begin(), largeNumbers.end(), 0);
10  std::cout << "Result: " << result;
11}

1Result: 1000000

In this case, std::reduce() can utilize multiple threads to speed up the reduction process, whereas std::accumulate() would process elements sequentially.

Non-Deterministic Operations

If the operation used is commutative and associative, std::reduce() can be more efficient because it does not need to maintain a strict left-to-right order. This flexibility allows for optimization in how elements are combined:

1#include <iostream>
2#include <numeric>
3#include <vector>
4
5int main() {
6  std::vector<int> numbers{1, 2, 3, 4, 5};
7
8  int result = std::reduce(
9    numbers.begin(), numbers.end(), 0);
10  std::cout << "Result: " << result;
11}

1Result: 15

For large datasets, this can lead to better performance compared to std::accumulate().

Complex Aggregations

In cases where you need to aggregate data in a more complex manner, such as combining objects with custom logic, std::reduce() can be more appropriate. Here's an example with a custom struct:

1#include <iostream>
2#include <numeric>
3#include <vector>
4
5struct Transaction {
6  int amount;
7  int count;
8};
9
10Transaction combine(
11  const Transaction& a, const Transaction& b) {
12  return {a.amount + b.amount, a.count + b.count};
13}
14
15int main() {
16  std::vector<Transaction> transactions{
17    {100, 1}, {200, 1}, {300, 1}};
18
19  Transaction result = std::reduce(
20    transactions.begin(), transactions.end(),
21    Transaction{0, 0}, combine
22  );
23  std::cout << "Total Amount: " << result.amount;
24  std::cout << "\nTotal Count: " << result.count;
25}

1Total Amount: 600
2Total Count: 3

Summary

Use std::reduce() when:

• You need to process large datasets efficiently.
• The operation is commutative and associative.
• You want to leverage multi-threading for better performance.
• You need to perform complex aggregations with custom logic.

std::accumulate() is simpler and guarantees deterministic results, but std::reduce() can offer significant performance benefits in the right scenarios.