How would one efficiently negate the subdiagonal entries of a 2d numpy array?

### >Solution :

You may use numpy.tril_indices to compute the indices of the subdiagonal entries, considering a diagonal offset of `k = -1`

, and negate them with a mask, for instance:

```
import numpy as np
a = np.arange(16).reshape(4, 4)
>>> array([[ 0, 1, 2, 3],
[ 4, 5, 6, 7],
[ 8, 9, 10, 11],
[12, 13, 14, 15]])
idx_low_tri = np.tril_indices(a.shape[0], k=-1)
a[idx_low_tri] = -a[idx_low_tri]
>>> array([[ 0, 1, 2, 3],
[ -4, 5, 6, 7],
[ -8, -9, 10, 11],
[-12, -13, -14, 15]])
```

Hope this helps !