Lately at work I've been doing some data analysis in LibreOffice Calc that requires the binary entropy function. The function itself looks like *H*_{2}(*x*) = − *x*log_{2}(*x*) − (1 − *x*)log_{2}(1 − *x*), where 0log_{2}(0) is taken to be 0. It's this latter point that makes things a little tricky. LibreOffice Calc doesn't have this function built-in, sadly, and you have to explicitly guard for the case where *x* is 0 or 1, which is not easy to pull off inside a cell.

So I wrote a basic macro that implements it:

```
Function BINENT(x)
If x = 0 Or x = 1 Then
BINENT = 0
ElseIf x > 0 And x < 1 Then
BINENT = -(x*Log(x) + (1 - x)*Log(1 - x))/Log(2)
Else
BINENT = Null
End If
End Function
```

To be able to use `BINENT` in Calc, go to the “Tools” menu, “Macros”, “Organize Macros”, and select “LibreOffice Basic...”. You'll be shown a window with a library hierarchy on the left. Under the “My Macros” collection, select the “Standard” library, then click the “New” button on the right. This will open up the editor. Copy and paste the `BINENT` code above into the editor window, save it, then close the window.

Now you should be able to use `BINENT` in the cells of any spreadsheet just like any other single-parameter function. Of course, the same approach is used to add any function that isn't already built-in.

Comment to add? Send me a message: <brendon@quantumfurball.net>