Static assertion fails for row selection with lambdas on submatrices

[Tim Zimmermann]

Consider the following code snippet:

#include "blaze/math/DynamicMatrix.h"
#include "blaze/math/Submatrix.h"
#include "blaze/math/Rows.h"

int main() {
    const size_t N = 10;
    blaze::DynamicMatrix<double> M(N, N, 0);
    auto submat = blaze::submatrix(M, 0, N / 2, N / 2, N / 2);

    // This line compiles just fine.
    //auto odd_rows = blaze::rows(submat, {1, 3}); 

    //This won't: static_assert(!::blaze::IsSubmatrix_v<T>, "Submatrix type detected")
    auto odd_rows = blaze::rows(submat, [](int i) { return 2 * i + 1; }, N / 2);
    return 0;
}

It seems like I can only get row selections of submatrices, if I specify all rows explicitly. Using a lambda, however, fails for submatrices but works for proper matrices (i.e. replace submat with M and everything works fine).

Is this the expected behavior? If so, can you document it in the wiki? Maybe I didn't look carefully enough but I couldn't find any information on this at first sight.

Thanks for the support, Tim

[Tim Zimmermann]

fixed some typos

[Klaus Iglberger]

Hi Tim!

Thanks a lot for taking the time to report this error. Also thank for the minimum example, which helped to quickly reproduce the issue. You are correct, this is a bug on our side. Of course it should also be possible to create a callable-based row selection on a submatrix. We will fix the issue as quickly as possible and apologize for the inconvenience.

We have noted two possible improvements for your code: First, please prefer size_t to int in order to evade mixing signed and unsigned integrals. Second, there will only be two odd rows in the resulting row selection (i.e. row 1 and 3).

auto odd_rows = blaze::rows(submat, [](size_t i) { return 2 * i + 1; }, 2);
                                         ^                              ^
                                         |                              |
//                     prefer 'size_t' to 'int'        There will only be two odd rows in the submatrix (i.e. '1' and '3')

Thanks again, we really appreciate the help,

Best regards,

Klaus!

[Klaus Iglberger]

Commit 2551cd9 provides the missing overloads for the rows() and columns() functions, which enable a callable-based selection on a submatrix. The fix is immediately available via cloning the Blaze repository and will be officially released in Blaze 3.6.