Sparse matrixes passed to ranger

[alexilliamson]

Ranger was recently updated to handle sparse matrixes: imbs-hl/ranger#135

But caret converts data to a data.frame when before passing to ranger https://github.com/topepo/caret/blob/master/models/files/ranger.R

There was discussion of sparse matrices in #474, resulting in changes to the caret side of xgboost and glmnet. Would it be possible to make similar changes for ranger?

Thanks for the amazing work!

[topepo]

This is a good idea.

Unfortunately, caret is at the lifecycle point of having a fairly complete interface, so this isn't a good fit.

The development work is focused on squashing bugs and any changed required by the underlying model packages.