resize matrix with copying the data opencv - opencv

Question

I am trying to create a matrix with only one column from another matrix and of course with copying the data.
void redim( Mat in , Mat &out) {
for (int l=0 ; l < in.rows*in.cols ; l++){
for(int j=0; j< in.rows ; j++){
for(int i=0 ; i < in.cols; i++){
out.at <float> (l,0)= in.at <float> (j,i);
}
}
}
}
int main(){
Mat It3;
It3 = (Mat_<double>(2,3) << 0,4,6,7,8,9);
Mat S= Mat :: zeros ( It3.rows* It3.cols , 1, CV_32FC1) ;
redim(It3,S);
waitKey();
}
But I got as result the matrix S=[0;0;0;0;0;0].

Answer 1

Your are mixing float and double as data type. Choose one!
Your for loops don't make any sense.
You can:
correct your for loop, as in toSingleColumn1
using a for loop, but using indices, as in toSingleColumn2
using cv::reshape, which does exactly this, as in toSingleColumn3
See the code below. b1, b2 and b3 will be equal:
#include <opencv2\opencv.hpp>
using namespace cv;
// Use two for loops, with coordinates
void toSingleColumn1(const Mat1f& src, Mat1f& dst)
{
int N = src.rows * src.cols;
dst = Mat1f(N, 1);
int i=0;
for (int r = 0; r < src.rows; ++r) {
for (int c = 0; c < src.cols; ++c) {
dst(i++, 0) = src(r, c);
}
}
}
// Use a single for loop, with indices
void toSingleColumn2(const Mat1f& src, Mat1f& dst)
{
int N = src.rows * src.cols;
dst = Mat1f(N, 1);
for (int i = 0; i < N; ++i) {
dst(i) = src(i);
}
}
// Use cv::reshape
void toSingleColumn3(const Mat& src, Mat& dst)
{
// The 'clone()' is needed to deep copy the data
dst = src.reshape(src.channels(), src.rows*src.cols).clone();
}
int main()
{
Mat1f a = (Mat1f(2,3) << 0.f, 4.f, 6.f, 7.f, 8.f, 9.f);
Mat1f b1, b2, b3;
toSingleColumn1(a, b1);
toSingleColumn2(a, b2);
toSingleColumn3(a, b3);
return 0;
}