I have written a function to calculate entropy of a vector where each element represents number of elements of a class.

```
function x = Entropy(a)
t = sum(a);
t = repmat(t, [1, size(a, 2)]);
x = sum(-a./t .* log2(a./t));
end
```

e.g: `a = [4 0]`

, then `entropy = -(0/4)*log2(0/4) - (4/4)*log2(4/4)`

But for above function, the entropy is `NaN`

when the split is pure because of `log2(0)`

, as in above example. The entropy of pure split should be zero.

How should I solve the problem with least effect on performance as data is very large? Thanks