The Reduce and Accumulate Algorithms

# Deterministic Results with Reduce

## How do I ensure deterministic results with non-commutative operators using std::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 parallelÂ execution.

To ensure deterministic results, consider theseÂ strategies:

## 1. Use std::accumulate()

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 consistentÂ order.

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

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

int result = std::accumulate(
numbers.begin(), numbers.end(), 0,
[](int a, int b) {
return a - b;
});

std::cout << "Result: " << result;
}
Result: -15

## 2. Serial Execution Policy

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().

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

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

int result = std::reduce(std::execution::seq,
numbers.begin(), numbers.end(), 0,
[](int a, int b) {
return a - b;
});

std::cout << "Result: " << result;
}
Result: -15

## 3. Custom Reduction Logic

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

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

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

void accumulateRange(
const std::vector<int>& numbers,
int start, int end, int& result
) {
result = std::accumulate(
numbers.begin() + start,
numbers.begin() + end,
0,
[](int a, int b) {
return a - b;
}
);
}

int main() {
std::vector<int> numbers{1, 2, 3, 4, 5};
int result1 = 0, result2 = 0;

accumulateRange,
std::ref(numbers),
0,
numbers.size() / 2,
std::ref(result1)
);

accumulateRange,
std::ref(numbers),
numbers.size() / 2,
numbers.size(),
std::ref(result2)
);

t1.join();
t2.join();

std::cout << "Result 1: " << result1;
std::cout << "\nResult 2: " << result2;

int finalResult = result1 - result2;
std::cout << "\nFinal Result: " << finalResult;
}
Result 1: -3
Result 2: -12
Final Result: 9

## Summary

• 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 reductionÂ algorithms.

This Question is from the Lesson:

### The Reduce and Accumulate Algorithms

A detailed guide to generating a single object from collections using the std::reduce() and std::accumulate() algorithms

Answers to questions are automatically generated and may not have been reviewed.

This Question is from the Lesson:

### The Reduce and Accumulate Algorithms

A detailed guide to generating a single object from collections using the std::reduce() and std::accumulate() algorithms

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