====Flipping Your Model with Goalseek====

Although it is possible to figure out how to solve equations for different variables, it can become **incredibly** complex to do when there's a lot of variables or calculations involved.

Based on our [[creating a model in python|example]] - where we determine how many seconds of water we have left in the tank - we want to determine **how full** the tank needs to be to guarantee we have at least 5 minutes of water stored.

We //could// work out the equation to solve this, but in this case, we're a little lazy. We are going to use [[goalseek|goal-seeking]].

Goal-seeking basically asks the system "What value would I need for //A// so that the value of //B// is //C//?"

In this case, we're going to ask "what **level** would we need in the tank to guarantee **60** **seconds of water**?".

===Adding Constants First===

Because that time threshold has the potential to change in the future, we'll add a new [[constant|constant]] to hold it. 

<code>
MinTiming = mod.AddConstant("Tank Minimum Storage Time",60)
</code>

===Adding the Goalseek===

**GoalSeek** is a member of the Model class.

It takes three parameters - the //point you want to discover// (ie. the Level), the //point you want to watch// (the time) and the //value we're watching for// (the minimum timing, our 60s constant we just created).

<code>
mod.AddOutput("Min Level", lambda: mod.GoalSeek(LevelPerc,RemainingTime,MinTiming.num()),[LevelPerc,RemainingTime,MinTiming])
</code>

You can see the documentation on the GoalSeek function [[GoalSeek Function|here]].

===The Results===

{{ :model:modeloutput.png?400|}}

When run, the new model gives us a //Min Level// output of '5', indicating that the tank needs to be at least 5% full in order to ensure there's a minute of water remaining.

If we adjust the constant to 120s instead of 60, it immediately updates to 10% rather than 5.

===Full Code===

<code>
import ardimodel

host = ardimodel.ModelHost()

def TankModel(mod):
    Outflow = mod.AddInput("Outgoing Flow",10)
    LevelPerc = mod.AddInput("Tank Level Perc",50)
    
    Capacity = mod.AddConstant("Tank Max Volume",12000)
    MinTiming = mod.AddConstant("Tank Minimum Storage Time",60)
    
    Volume = mod.AddOutput("Tank Volume", lambda: (LevelPerc.num() / 100) * Capacity.num(), [LevelPerc,Capacity])
    RemainingTime = mod.AddOutput("Time-To-Empty", lambda: 86300 if Outflow.num() == 0 else Volume.num() / Outflow.num(), [Outflow,Volume])
    mod.AddOutput("Min Level", lambda: mod.GoalSeek(LevelPerc,RemainingTime,MinTiming.num()),[LevelPerc,RemainingTime,MinTiming])

# Add Models Here
host.Add(ardimodel.Create("Tank",TankModel))

host.ardiurl = "localhost"
host.ardisite="default"
host.Run()
</code>

===See More===

That takes you through most of the basics - you're now ready to build your own models and experiment.

There are also a few [[tips_and_tricks|tips and tricks]] you can try if you're making complex models.