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Hello R-devel, I want to write extensions for R in C (maybe C++ and Fortran later) and it works fine, but there is one problem, which I cannot solve (in my view). I want to handle a matrix from R in C. For arrays there is "as.double(...)", but nothing for a matrix. I searched a while, but didn't find something. Last I looked at the source code of e1071 and of the core itself and