Iterate over columns in C matrix-CodePudding

I want to iterate over single row and column in std::vector<std::vector<int>> matrix and get their sum.

I know that I can do this in nested loop, but here is my question. Can I use

int val_sum = 0;
std::for_each(matrix_[row].begin(),matrix_[row].end(),[&](int x) {  val_sum  = x;});

for columns and how to do that?

CodePudding user response：

The analogous way of your proposal is to iterate the matrix rows and accumulate the elements in the given column.

int val_sum = 0;
std::for_each(matrix.begin(),matrix.end(),[&](std::vector<int> &row) {  val_sum  = row[column];});

But I would still prefer to use the c 11 range-loop version

int val_sum = 0;
for ( const std::vector<int> &row : matrix )
   val_sum  = row[column];

CodePudding user response：

You can use nested std::accumulate:

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

int main() {
    std::vector<std::vector<int>> matrix = { {1, 2, 3}, {2, 3, 4}, {3, 4, 5} };
    const int sum = std::accumulate(matrix.cbegin(), matrix.cend(), 0, [](const auto acc, const auto row) {
        return acc   std::accumulate(row.cbegin(), row.cend(), 0);
    });
    std::cout << sum;
}

CodePudding user response：

You could nest two for_each. You simply have to notice that every element of the outer for_each will be another vector<int>, a row. [Demo]

#include <algorithm>  // for_each
#include <iostream>  // cout
#include <vector>

int main()
{
    std::vector<std::vector<int>> v{ {1, 2}, {3, 4}, {5, 6} };
    std::for_each(std::cbegin(v), std::cend(v), [](auto& row) {
        std::for_each(std::cbegin(row), std::cend(row), [](auto n) {
            std::cout << n << " ";
        });
        std::cout << "\n";
    });
}

If you want to sum all the elements of the matrix, you can use accumulate instead of the inner for_each loop. [Demo]

#include <algorithm>  // for_each
#include <iostream>  // cout
#include <numeric>  // accumulate
#include <vector>

int main()
{
    int result{};
    std::vector<std::vector<int>> v{ {1, 2}, {3, 4}, {5, 6} };
    std::for_each(std::cbegin(v), std::cend(v), [&result](auto& row) {
        result  = std::accumulate(std::cbegin(row), std::cend(row), 0);
    });
    std::cout << "sum = " << result << "\n";
}

Or even with two nested accumulate. [Demo]

#include <iostream>  // cout
#include <numeric>  // accumulate
#include <vector>

int main()
{
    std::vector<std::vector<int>> v{ {1, 2}, {3, 4}, {5, 6} };
    auto result = std::accumulate(std::cbegin(v), std::cend(v), 0, [](auto total, auto& row) {
        return total   std::accumulate(std::cbegin(row), std::cend(row), 0);
    });
    std::cout << "sum = " << result << "\n";
}

If (after all, I misunderstood you from a beginning, and) you want to add all the elements of a given column. [Demo]

#include <iostream>  // cout
#include <numeric>  // accumulate
#include <vector>

int sum_column(const std::vector<std::vector<int>>& v, size_t column)
{
    return std::accumulate(std::cbegin(v), std::cend(v), 0, [&column](auto total, auto& row) {
        return total   row[column];
    });
}

int main()
{
    std::vector<std::vector<int>> v{ {1, 2}, {3, 4}, {5, 6} };
    std::cout << "sum of column 1 = " << sum_column(v, 1) << "\n";
}