Iterate over columns in C++ matrix - c++

Question

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?

Answer 1

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];

Answer 2

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;
}

Answer 3

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";
}