summation of elements of a 2D vector into a 1D vector. C++

Help me please.
Tell me how to sum each vector in a two-dimensional vector and write the result into a one-dimensional vector. On C++.
Input:

std::vector<std::vector<double>> a = { { 1, 2 }, { 3, 4 }, { 5, 6 } };

Required Output:

b = { 9, 12 }

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I found how to sum all the elements in a 2D vector, but I don’t know how to sum the vectors in an existing 2D vector and write the result to a 1D vector

#include <iostream>
#include <vector>

int main() 
{
    std::vector<std::vector<double>> a = { { 1, 2 }, { 3, 4 }, { 5, 6 } };
    std::vector<double> b;
    std::vector<int> v;

    int sum = 0;
    for (int i = 0; i < a.size(); i++)
        for (int j = 0; j < a[i].size(); j++)
        {
            sum += a[i][j];
        }
    std::cout << sum;
    return 0;
}

>Solution :

From your example I assume that in the sums vector that you require, element in index i, is the sum of all elements in index i in the vectors in a. This requires that all elements in a have the same size.

You can do it in the following way:
Traverse the elements in a. For each element: traverse the element (which is a vector itself) and sum element i into the ith bin of the output vector.

Something like:

#include <iostream>
#include <vector>
#include <assert.h>

int main()
{
    std::vector<std::vector<double>> a = { { 1, 2 }, { 3, 4 }, { 5, 6 } };
    size_t num_of_sums = a[0].size();   // we assume all elements in a have the same size
    std::vector<double> sums(num_of_sums);
    for (auto const & v : a)
    {
        assert(v.size() == num_of_sums);
        for (int j = 0; j < num_of_sums; j++)
        {
            sums[j] += v[j];
        }
    }
    for (auto d : sums)
    {
        std::cout << d << ", ";
    }
    std::cout << std::endl;
    return 0;
}

Output:

9, 12,

Note that my sums vector contains doubles, like your input data (unlike your sum which is an int and seems improper to sum double values).