[mlpack-git] [mlpack/mlpack] The format of the sparse instances in the input file (#656)

[Ryan Curtin]

Hi Yawei,

mlpack unfortunately doesn't have any current support for loading sparse matrices from disk.  In addition, because of this, the command-line programs only load dense data.

So if you want to use sparse data specifically, I think the best way is to write a C++ program using `arma::sp_mat`.  But to make it harder... Armadillo does not have good documentation for their support for loading sparse matrices.  You can load a coordinate list of the form

```
1 2 10.3
3 1 5.2
3 2 1.3
```

and this represents a matrix with three nonzero elements.  You can load it using the function

```
arma::sp_mat m;
m.load("file.txt", arma::coord_ascii);
```

and then you can use that in mlpack methods.  I wish that this was documented in the Armadillo docs but currently it is not.

I hope this is helpful... let me know if I can clarify anything.

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