Deterministic Results with Reduce

std::reduce() can produce non-deterministic results when used with non-commutative or non-associative operators because it allows elements to be combined in any order, especially in parallel execution.

To ensure deterministic results, consider these strategies:

If you need deterministic results, and your operation is non-commutative, consider using std::accumulate() instead of std::reduce(). std::accumulate() processes elements sequentially from left to right, ensuring a consistent order.

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::accumulate(
9    numbers.begin(), numbers.end(), 0,
10    [](int a, int b) {
11      return a - b;  
12    });
13
14  std::cout << "Result: " << result;
15}

1Result: -15

When using std::reduce(), you can specify the std::execution::seq execution policy to enforce serial execution. This ensures elements are combined in a specific order, similar to std::accumulate().

1#include <execution>
2#include <iostream>
3#include <numeric>
4#include <vector>
5
6int main() {
7  std::vector<int> numbers{1, 2, 3, 4, 5};
8
9  int result = std::reduce(std::execution::seq,
10    numbers.begin(), numbers.end(), 0,
11    [](int a, int b) {
12      return a - b;  
13    });
14
15  std::cout << "Result: " << result;
16}

1Result: -15

For more complex scenarios, you might need to implement a custom reduction logic that manually controls the order of operations.

This can be done by splitting the data into chunks, processing them individually, and then combining the results in a controlled manner.

1#include <iostream>
2#include <numeric>
3#include <vector>
4#include <thread>
5
6void accumulateRange(
7  const std::vector<int>& numbers,
8  int start, int end, int& result
9) {
10  result = std::accumulate(
11    numbers.begin() + start,
12    numbers.begin() + end,
13    0,
14    [](int a, int b) {
15      return a - b; 
16    }
17  );
18}
19
20int main() {
21  std::vector<int> numbers{1, 2, 3, 4, 5};
22  int result1 = 0, result2 = 0;
23
24  std::thread t1(
25    accumulateRange,
26    std::ref(numbers),
27    0,
28    numbers.size() / 2,
29    std::ref(result1)
30  );
31
32  std::thread t2(
33    accumulateRange,
34    std::ref(numbers),
35    numbers.size() / 2,
36    numbers.size(),
37    std::ref(result2)
38  );
39
40  t1.join();
41  t2.join();
42
43  std::cout << "Result 1: " << result1;
44  std::cout << "\nResult 2: " << result2;
45
46  int finalResult = result1 - result2; 
47  std::cout << "\nFinal Result: " << finalResult;
48}

1Result 1: -3
2Result 2: -12
3Final Result: 9

• Use the std::accumulate() Algorithm: For non-commutative operations where deterministic results are required.
• Serial Execution Policy: Apply std::execution::seq with std::reduce() to enforce a specific order.
• Custom Logic: Implement manual reduction logic to control the order of operations.

These strategies help ensure deterministic results even with non-commutative operations in reduction algorithms.