====Adding Logic to Equations===

In our [[creating a model in python|example]], we created a very simple model telling us how many seconds of water we have stored in our tank.

===Handling Divide-By-Zero===

If you're doing anything with rates and speeds, you'll likely come up across a divide-by-zero error at some point.

These can cascade and cause chaos with your model, so it's always a good idea to handle these. 

Luckily, Python allows you to use in-line conditions, so you can put logic into your functions.

We can change the code....

<code>
lambda: Volume.num() / Outflow.num(), [Outflow,Volume]
</code>

to something that returns 86400 (a full day in seconds) when there's no flow out of the tank.

<code>
lambda: 86400 if Outflow.num() == 0 else Volume.num() / Outflow.num()
</code>

<WRAP center round info 80%>
An 'easy' way of avoiding divide-by-zero is by wrapping the variable in **max(//value//,0.1)**, which ensures that the value will always greater than 0.

However, in some cases this can be dangerous - certain equipment (such as pumps) can be run in reverse, where they generate negative values. Using //max// would prevent these negative values from being used in your calculations.
</WRAP>

How about we [[connect our model to data]]?