How to use std::acumulate for matrix

December 31, 2021

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


using matrix = std::vector<std::vector<int>>;

int main()
{
    matrix mtx{5, std::vector<int>(5)};

    int sum = 0;
    for (const auto i : mtx) // can be avoided ? 
    sum += std::accumulate(i.begin(), i.end(), 0, 
        [](int a, int b){a > 0 ? a + b : a;});

}

I want to use std::accumulate for std::vector<std::vector<int>> but I am curious if I can avoid the loop. Also I want to know if the last argument is ok.

>Solution :

Based on your lambda, looks like you want to just sum the positive entries:

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

using Number = int;
using Matrix = std::vector<std::vector<Number>>;

int main() {
  Matrix mtx{5, std::vector<Number>(5, 1)};

  Number sum_positives = std::accumulate(
      mtx.begin(), mtx.end(), Number(0), [](Number const acc, auto const &v) {
        return std::accumulate(
            v.begin(), v.end(), acc,
            [](Number const a, Number const b) { return b > 0 ? a + b : a; });
      });

  std::cout << sum_positives << std::endl;  // 25
  return 0;
}

Don’t forget that the lambda you pass to std::accumulate() must return a value.