C++ Vectors: Error in push_back member function call

I am trying to create the transpose of a matrix – by traversing into 2-D vectors and assigning the values of indices accordingly.

[[2, 4, 6], [6, 8, 9]] --> [[2,6], [4, 8], [6, 9]] // transpose matrix representation 

Here’s my code

vector<vector<int>> transpose(vector<vector<int>>& matrix) {
        vector<vector<int>> result = {};
        for (size_t i = 0; i < matrix.size(); i++)
        {
            for (size_t j = 0; j < matrix[0].size(); j++)
            {
                int element = matrix[i][j];
                result[j][i].push_back(element); // error occurs here
            }
        }
        return result;
}

While calling the function with proper main body – I am getting the error in calling the push_back member function.

The error message

transpose_matrix.cpp:11:30: error: request for member 'push_back' in '(& result.std::vector<std::vector<int> >::operator[](j))->std::vector<int>::operator[](i 
', which is of non-class type '__gnu_cxx::__alloc_traits<std::allocator<int>, int>::value_type' {aka 'int'}
   11 |                 result[j][i].push_back(element);
      |                              ^~~~~~~~~

If you’ve any alternative suggestion then it’ll be also helpful.

>Solution :

result[j][i] is not a vector, it’s an int. Also, you have to at least resize the vector to the number of rows in the transposed matrix (equal to number of columns in the input matrix).

#include <iostream>
#include <vector>

std::vector<std::vector<int>> transpose(std::vector<std::vector<int>>& matrix)
{
    std::vector<std::vector<int>> result;
    if (matrix.empty())
        return result;

    result.resize(matrix[0].size());    
    for (size_t i = 0; i < matrix.size(); i++)
    {
        for (size_t j = 0; j < matrix[0].size(); j++)
        {
            int element = matrix[i][j];
            result[j].push_back(element);
        }
    }
    return result;
}

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

    auto res = transpose(v);
    for (auto& row : res)
    {
        for (auto ele : row)
            std::cout << ele << " ";
        std::cout << std::endl;
    }
}