## Python

### Numpy

#### Choose a submatrix

We have a square matrix, take: `Y = np.arange(16).reshape(4,4)`

. Numpy has a nice option to take specific row column combination. `numpy.ix_`

. The example is as follows:

```
import numpy as np
Y = np.arange(16).reshape(4,4)
Y[np.ix_([0,3],[0,3])] # this will chose the columns and rows we need.
```

TBA