Re: [eigen] Map with negative outer stride

[Gael Guennebaud <gael.guennebaud@xxxxxxxxx>]

On Tue, Feb 3, 2015 at 10:34 PM, Christoph Hertzberg <chtz@xxxxxxxxxxxxxxxxxxxxxxxx> wrote:

On 02.02.2015 16:48, Chen-Pang He wrote:

#define EIGEN_NO_DEBUG  // Disable eigen_assert

#include <Eigen/Core>
#include <iostream>

int main()
{
     using namespace Eigen;

     int data[] = { 0, 0, 0, 0, 0, 1, 2, 3, 4, 5, 6 };

     std::cout << Map<Matrix<int, 6, 6>, 0, OuterStride<-1> >(data + 5, 6, 6)
               << std::endl;
}

Too lazy to actually test it, but the following should work without disabling assertions:
Map<Matrix<int, 6, 6>, 0, OuterStride<1> >(data, 6, 6).rowwise().reverse()

This still requires a temporary vector with leading zeros. If you want to avoid that, I guess you need to write a small loop or write a CustomNullaryOp:
http://eigen.tuxfamily.org/dox-devel/classEigen_1_1DenseBase.html#a5b84f7ce5cc30b011683e3c2c6d816a8

or combine it with a .triangularView<lower>().

gael

 

Christoph



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