How to flatten 2D matrix and then copy it to a specific column in another matrix

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For example, I have a 2D Matrix with 3-channels:

const auto img = imread("path.jpg", IMREAD_COLOR);
const auto ROWS = img.rows;
const auto COLS = img.cols;

And then i split it to vector to get 2D Matrix with 1-channel:

std::vector<Mat> channels{};
split(img, channels);

I want to store the channels as a Matrix of size [ROWS*COLS, 3] so i tried to do this. I thought it is supposed to flatten each Matrix in the channels above to Matrix of size [ROWS*COLS, 1], then copy it to the a specific column of in the result, but it was wrong:

Mat result(ROWS*COLS, 3, CV_32F);
auto i = 0;
for(const auto& channel : channels) {
    channel.reshape(0, ROWS*COLS).copyTo(result.col(i));
    ++i;
}

I used a naive way which give the correct result:

for (const auto& channel : channels) {
     for (auto y = 0; y < rows; ++y)
        for (auto x = 0; x < cols; ++x) {
            result.at<float>(y + x * rows, i) = channel.at<uchar>(y, x);
        }
    ++i;
}

What did i do wrong with the first solution?