Jendas - 1 year ago 161
C++ Question

# Eigen - Sort matrix diagonal

What I have is diagonal matrix of type

`Eigen::MatrixXi`
. I need elements on the diagonal to be sorted in ascending order. For example like this:

``````2 0 0      1 0 0
0 7 0  >>> 0 2 0
0 0 1      0 0 7
``````

I thought I would simply do:

``````std::sort(matrix.diagonal().begin(), matrix.diagonal().end());
``````

But apparently
`Eigen::Diagonal`
does not have begin nor end function. So the question is, is there any way of sorting elements on diagonal using internal std::sort or anything similarly elegant?

I went through official documentation but did not find anything useful.

There is no native support for sorting matrices as of today. There are two long-pending feature requests relevant to this functionality:

As suggested by @NicolasM in the comments, currently, the most elegant solution is to provide custom iterators yourself, e.g.:

``````namespace Eigen {
template<typename Scalar>
class iterator {
Scalar* ptr;
int stride;
public:
iterator(Scalar* ptr_, int stride_) : ptr(ptr_), stride(stride_) {}
Scalar& operator*() const { return *ptr;}
iterator& operator++() { ptr += stride; return *this;}
bool operator<(const iterator& other) { return ptr < other.ptr; }
// implement further operations, required for random access iterators ...
}

template<class Derived>
iterator begin(MatrixBase<Derived>&& mat)
{ return iterator(mat.data(), mat.innerStride()); }
template<class Derivde>
iterator end(MatrixBase<Derived>&& mat)
{ return iterator(mat.data() + mat.size()*mat.innerStride(), mat.innerStride()); }

} // namespace Eigen
``````
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