Hi everybody, first post here!

While doing the Decision Trees optional labs, I stumbled into a “Division by zero” error while solving exercise 3.

I quickly discovered that the problem was one of the split subset being empty and making entropy function crash.

In fact, in my code I pre-calculate p1 like this:

```
p1 = count/len(y)
```

where `count`

is the number of ones in y. If length of y is zero, this line (obviously) crashes.

I solved the problem (and passed the test) by just adding:

```
if (len(y) == 0):
return 0
```

But I would like to know if I am doing something weird or stupid - and maybe the patch is just fixing a mistake made elsewhere.

Thanks!