C++: What is the difference between std::accumulate and std::reduce?

accumulate — sequential. reduce — can be parallel (C++17).

std::accumulate (C++98):

• Sequential.
• Left-to-right.
• O(n).

1std::vector<int> v = {1, 2, 3, 4, 5};
2
3// Sum
4int sum = std::accumulate(v.begin(), v.end(), 0);
5// 15
6
7// Product
8int product = std::accumulate(v.begin(), v.end(), 1,
9    std::multiplies<int>()
10);
11// 120

std::reduce (C++17):

• Can be parallel.
• Order not guaranteed.
• O(n) with parallelization.

1#include <execution>
2
3int sum = std::reduce(
4    std::execution::par,
5    v.begin(), v.end(), 0
6);

Key differences:

• accumulate — sequential, deterministic.
• reduce — parallelizable, non-deterministic order.