How to change the outerstride in Eigen matrices without using map function - matrix

Question

I want to change the outerstride of Eigen matrix at compile time without using map function.
I tried to change this using OuterStrideAtCompileTime variable,but it doesn't work.Is there any way to do that ?
One more thing printing mat.Outerstride() every time gives number of rows of input matrix.How to print OuterStride of eigen matrix?
Thanks in advance.
I was defining an eigen matrix with map function like
MatrixXf mat;
float arr[16] = {1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16};
mat = Map<Matrix<float,Dynamic,Dynamic,Eigen::RowMajor>, 0, OuterStride<Dynamic> > (arr,4,4,OuterStride<Dynamic>(5));
It's working fine, whenever I tried to change the outer stride by using
mat.OuterStrideAtCompileTime = 7;
It's not working.

Answer 1

Outer stride is a parameter related to the data storage. A more commonly used name is leading dimension. You could find some explaination here.
http://www.ibm.com/support/knowledgecenter/SSFHY8_5.3.0/com.ibm.cluster.essl.v5r3.essl100.doc/am5gr_leaddi.htm
Basically for an existing matrix, it can not be changed. The only way to change it without changing the elements of the matrix is to copy the matrix to a new memory space using a different outer stride setting. This usually happens when you copy a matrix into another as a sub-matrix.
For a column major matrix the minimum possible outer stride equals to the number of rows, as the number you have printed out.
When using Eigen, you don't need worry about it as Eigen usually takes care of it for you expect for Eigen::Map.
You code actually doesn't work. Setting out stride to 5 is already out of range as the existing matrix(4x4) stored in arr is of stride 4 and stride 5 x 4 columns = 20 > 16.
#include <iostream>
#include <Eigen/Eigen>
int main(void) {
using namespace Eigen;
MatrixXf mat;
float arr[16] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16 };
mat = Map<Matrix<float, Dynamic, Dynamic, Eigen::RowMajor>, 0,
OuterStride<Dynamic> >(arr, 4, 4, OuterStride<Dynamic>(5));
std::cout << "mat with stride 5:\n" << mat << std::endl;
mat = Map<Matrix<float, Dynamic, Dynamic, Eigen::RowMajor>, 0,
OuterStride<Dynamic> >(arr, 4, 4, OuterStride<Dynamic>(4));
std::cout << "mat with stride 4:\n" << mat << std::endl;
return 0;
}
Please compare the output.
mat with stride 5:
1 2 3 4
6 7 8 9
11 12 13 14
16 0 0 5.01639e-14
mat with stride 4:
1 2 3 4
5 6 7 8
9 10 11 12
13 14 15 16
If you extend the array to 20 elements
#include <iostream>
#include <Eigen/Eigen>
int main(void) {
using namespace Eigen;
MatrixXf mat;
float arr[20] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 };
Map<Matrix<float, Dynamic, Dynamic, Eigen::RowMajor>, 0, OuterStride<Dynamic> > map1(arr, 4, 4, OuterStride<Dynamic>(5));
mat = map1;
std::cout << "map1 outer stride: " << map1.outerStride() << std::endl << map1 << std::endl;
std::cout << "mat outer stride: " << mat.outerStride() << std::endl << mat << std::endl;
Map<Matrix<float, Dynamic, Dynamic, Eigen::RowMajor>, 0, OuterStride<Dynamic> > map2(arr, 4, 4, OuterStride<Dynamic>(4));
mat = map2;
std::cout << "map2 outer stride: " << map2.outerStride() << std::endl << map2 << std::endl;
std::cout << "mat outer stride: " << mat.outerStride() << std::endl << mat << std::endl;
return 0;
}
The output will be
map1 outer stride: 5
1 2 3 4
6 7 8 9
11 12 13 14
16 17 18 19
mat outer stride: 4
1 2 3 4
6 7 8 9
11 12 13 14
16 17 18 19
map2 outer stride: 4
1 2 3 4
5 6 7 8
9 10 11 12
13 14 15 16
mat outer stride: 4
1 2 3 4
5 6 7 8
9 10 11 12
13 14 15 16
You could also see the outer stride changes when copying map1 to mat.
Hope this give you a better view of what out stride is.
In fact in your original code, you are using Map in a wrong way - you shouldn't have copied Map() to Matrix mat.
That's why when you print the stride of mat, it is always 4.
What you need to do is to eliminate the unnecessary data copy and print the stride of map1/map2.