Faster way to copy vector elements to different structure of vector C++

Question

I have a one dimension vector of integer that stores table data. Which I want to transform into a multidimensional vector (e.g. a vector of int vectors).

This is what I tried:

std::vector<int> lin_table = {1,2,3,4, 1,2,3,4, 1,2,3,4, 1,2,3,4, 1,2,3,4}
std::vector<std::vector<int>> multi_table;
int num_cols = 4;
for (int column = 0; column < num_cols; column++)
{
   std::vector<int> temp_column;
   for(int element = column; element < lin_table.size(); element += num_cols)
   {
       temp_column.push_back(lin_table.at(element));
   }
   multi_table.push_back(temp_column);
}

This is working fine, but I would like to know if there is any faster way to do it?

Answer 1

I have a one dimension vector of integer that stores table data. Which I want to transform into a multidimensional vector (e.g. a vector of int vectors).

Since you insist on performance, as already stated many times here before, it is better to create a class that wraps around a std::vector which emulates what a two-dimensional vector would do.

#include <functional>
#include <vector>

template <typename T>
class Vec2DWrapper {
    std::reference_wrapper<std::vector<T>> vec_;
    size_t rows_;

public:
    using value_type = T;
    Vec2DWrapper(std::vector<T>& vec, size_t const rows)
     : vec_(std::ref(vec)), rows_(rows) {
    }
    T& operator()(size_t const x, size_t const y) {
        return vec_.get()[x * rows_ + y];
    }
    std::vector<T>& get_vector() const { return vec_.get(); }

};

Now, you can use it like this:

#include <iostream>

// ...

int main() {
    std::vector<int> lin_table { 1,2,3,4, 1,2,3,4, 1,2,3,4, 1,2,3,4, 1,2,3,4 };

    // Bind the 'lin_table' vector to the class
    Vec2DWrapper<int> vec2d(lin_table, 4);

    for (size_t i = 0; i < 4; i++) {
        for (size_t j = 0; j < 5; j++)
            std::cout << vec2d(j, i) << " ";
        std::cout << std::endl;
    }
}