Matrix manipulation on MATLAB to C++ or OpenCV - c++

Question

I'm trying to do this operation in C++, but I cant wrap my head around it.
I tried looking in http://mathworks.com/help/matlab/ref/image.html but I still don't get it.
im is a matrix in Matlab. Width is 640
im(:,Width+(1:2),:) = im(:,1:2,:);
Is there anything similar to this operation in OpenCV Matrix or C++

Answer 1

Solution 1
You can use colRange function:
mat.colRange(0, 2).copyTo(mat.colRange(w, 2 + w));
Example:
//initilizes data
float data[2][4] = { { 1, 2, 3, 4}, { 5, 6, 7, 8 } };
Mat mat(2, 4, CV_32FC1, &data);
int w = 2; //w is equivelant to Width in your script, in this case I chose it to be 2
std::cout << "mat before: \n" << mat << std::endl;
mat.colRange(0, 2).copyTo(mat.colRange(w, 2 + w));
std::cout << "mat after: \n" << mat << std::endl;
Result:
mat before:
[1, 2, 3, 4;
5, 6, 7, 8]
mat after:
[1, 2, 1, 2;
5, 6, 5, 6]
Solution 2
Alternatively, use cv::Rect object, as follows:
cv::Mat roi = mat(cv::Rect(w, 0, 2, mat.rows));
mat(cv::Rect(0, 0, 2, mat.rows)).copyTo(roi);
There are several way to initialize a Rect, in my case I chose the following c-tor:
cv::Rect(int x, int y, int width, int height);
The result are the same as an in Solution 1.

Answer 2

Maybe you could also use Eigen that can serve the need. It has Block operations
Adapting the example provided under link you would need smth like:
Eigen::MatrixXf m(4, 4);
m << 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16;
cout <<"original matrix \n" << m << endl;
m.block<2, 2>(1, 1) = m.block<2, 2>(2, 2);
cout <<"modified matrix \n" << m << endl;
Output:
original matrix
1 2 3 4
5 6 7 8
9 10 11 12
13 14 15 16
modified matrix
1 2 3 4
5 11 12 8
9 15 16 12
13 14 15 16