# Steering – changing CC3DML parameters on-the-fly.¶

(You may skip this paragraph in the first reading)

In the CC3D terminology, steering means changing simulation parameters on the fly as simulation is running. In fact the whole point of merging Python scripting with CC3D core code is to enable steering. What about parameters defined in CC3DML? Can they be modified as the simulation runs? The short answer is yes but not all of them. There are two ways of doing it – one way is to use player CC3DML panel and change parameters in the GUI. The second way is to use Python. Python code that alters CC3DML parameters during simulation runtime is quite clumsy-looking. It is not difficult to understand but is quite verbose and, to be honest it is not too much fun to write. Probably the best way to learn how to modify CC3DML from Python is to look at several examples. Let us start with the simplest one (Demos/SimulationSettings/Steering):

Here we will modify two CC3DML parameters one will be Temperature from the CC3DML Potts Section and the other one will be Contact energy between Condensing and NonCondensing.

Let’s look at CC3DMLcode first:

<Potts>
<Dimensions x="100" y="100" z="1"/>
<Steps>10000</Steps>
<Temperature>10</Temperature>
<NeighborOrder>2</NeighborOrder>
</Potts>


Potts XML element has 4 child elements – Dimensions, Steps, Temperature and NeighborOrder.

To modify Temperature parameter we use the following code:

class PottsSteering(SteppablePy):
def __init__(self, _simulator, _frequency=1):
SteppablePy.__init__(self, _frequency)
self.simulator = _simulator

def step(self, _mcs):
# get Potts section of XML file
pottsXMLData = self.simulator.getCC3DModuleData("Potts")
# check if we were able to successfully get the section from simulator
if pottsXMLData:
# get Temperature XML element
temperatureElement = pottsXMLData.getFirstElement("Temperature")
# check if the attempt was succesful
if temperatureElement:
# get value of the temperature and convert it to a floating point number
temperature = float(temperatureElement.getText())
# increase temperature by 1.0
temperature += 1.0
# update XML Temperature element by converting floating point
temperatureElement.updateElementValue(str(temperature))
# finally call simulator.updateCC3DModule(pottsXMLData) to update model
# parameters - this is actual steering
self.simulator.updateCC3DModule(pottsXMLData)


Each XML element that we fetch using Python (e.g. pottsXMLData, temperatureElement) has set of member functions that allow us to get to elements nested one level deeper (child elements) or functions which allow us to read and modify element values and element attributes. For the temperatureElement we are modifying its value.

Important: XML stores text. All numbers or other data types stored in the XML are converted to their text representations. Consequently, depending on your needs and particular applications, you may need to convert text to numbers and vice versa when interacting with XML through Python interface.

In the code presented above we first fetch Potts element (pottsXMLData) using special function from the simulator object. If the fetching operation was successful we try to fetch first Temperature child element of the Potts element. If this operation was successful we convert value of the temperature element to floating point number:

temperature = float(temperatureElement.getText())


we increase value of this number by one and then update value of the temperatureElement:

temperatureElement.updateElementValue(str(temperature))


Notice that we had to convert floating point number temperature back to string using str(temperature) call.

The steppable ends with call to simulator member function that informs CC3D that CC3DML was changed and simulation needs to get new parameters:

self.simulator.updateCC3DModule(pottsXMLData)


As you can see the code was not hard to write but is quite long and clumsy. We could simplify it a bit but getting rid of comments and if statements that check if fetching operation was successful. In such a case the code would look like that:

class PottsSteering(SteppablePy):
def __init__(self, _simulator, _frequency=1):
SteppablePy.__init__(self, _frequency)
self.simulator = _simulator

def step(self, _mcs):
pottsXMLData = self.simulator.getCC3DModuleData("Potts")
temperatureElement = pottsXMLData.getFirstElement("Temperature")
temperature = float(temperatureElement.getText())
temperature += 1.0
temperatureElement.updateElementValue(str(temperature))
self.simulator.updateCC3DModule(pottsXMLData)


This is not so bad but still it is a lot of work to change one number. But do we have a choice here? In fact we do. All we have to do is to change cell membrane fluctuation amplitude using the following code:

newTemperature=51
for cell in self.cellList:
cell.fluctAmpl= newTemperature


In practice you don’t use need to modify CC3DML from Python level too often. CC3D has modules e.g. AdhesionFlex, FocalPointPlasticity, VolumeLocalFlex that require initialization of their parameters in Python but also offer much simpler programing interfaces making coding much less cumbersome. Please make sure that before writing complicated CC3DML steering code you familiarize yourself with modules that are designed to be flexible and do not rely on CC3DML-type of steering.

Now let us take a look at the code that alters contact energy, but first quick glance at the CC3DML that we will be modifying:

<Plugin Name="Contact">
<Energy Type1="NonCondensing" Type2="Condensing">11</Energy>
<Energy Type1="Condensing"    Type2="Condensing">2</Energy>
…
<NeighborOrder>2</NeighborOrder>
</Plugin>


Our task here is to first fetch Plugin XML Element and then fetch Energy Element for type pair Condensing and NonCondensing. Here is the code that does it:

class ContactSteering(SteppablePy):
def __init__(self, _simulator, _frequency=10):
SteppablePy.__init__(self, _frequency)
self.simulator = _simulator

def step(self, mcs):
# get <Plugin Name="Contact"> section of XML file
contactXMLData = self.simulator.getCC3DModuleData("Plugin", "Contact")
# check if we were able to successfully get the section from simulator
if contactXMLData:
# get <Energy Type1="NonCondensing" Type2="Condensing"> element
energyNonCondensingCondensingElement = contactXMLData.getFirstElement \
("Energy", d2mss({"Type1": "NonCondensing", "Type2": "Condensing"}))
# check if the attempt was succesful
if energyNonCondensingCondensingElement:
# get value of <Energy Type1="NonCondensing" Type2="Condensing"> element
# and convert it into float
val = float(energyNonCondensingCondensingElement.getText())
# increase the value by 1.0
val += 1.0
# update <Energy Type1="NonCondensing" Type2="Condensing"> element
# remembering abuot converting the value bask to string
energyNonCondensingCondensingElement.updateElementValue(str(val))
# finally call simulator.updateCC3DModule(contactXMLData) to tell simulator
# to update model parameters - this is actual steering
self.simulator.updateCC3DModule(contactXMLData)


We first fetch Plugin element using simulator object member function:

When this operation succeeds we attempt to fetch Python object representation for the

<Energy Type1="NonCondensing" Type2="Condensing">11</Energy> element:

energyNonCondensingCondensingElement=contactXMLData.getFirstElement("Energy",
d2mss({"Type1":"NonCondensing","Type2":"Condensing"})
)


Notice that when we call getFirstElement member function of the contactXMLData we pass the name of the element but also a partial list of element attributes. Here we use d2mss function what converts Python dictionary {"Type1":"NonCondensing","Type2":"Condensing"} to C++ map<string,string>. With so much information passed to getFirstElement function only one element fits the description and this is the one that we are looking for. The reminder of the steppable looks almost identical as in the example where we changed temperature.

The next example demonstrates how to update attribute of the XML element. You can find full code in Demos/Models/bacterium_macrophage_2D_steering. Again let us look at the CC3DML that we will be modifying:

<Plugin Name="Chemotaxis">
<ChemicalField Source="FlexibleDiffusionSolverFE" Name="ATTR" >
<ChemotaxisByType Type="Macrophage" Lambda="20"/>
</ChemicalField>
</Plugin>


We would like to periodically decrease lambda chemotaxis by 3 units. This is how we do it in Python:

class ChemotaxisSteering(SteppablePy):
def __init__(self, _simulator, _frequency=100):
SteppablePy.__init__(self, _frequency)
self.simulator = _simulator

def step(self, mcs):
if mcs > 100 and not mcs % 100:
# get <Plugin Name="Chemotaxis"> section of XML file
chemotaxisXMLData = self.simulator.getCC3DModuleData("Plugin", "Chemotaxis")
# check if we were able to successfully get the section from simulator
if chemotaxisXMLData:
# get <ChemicalField Source="FlexibleDiffusionSolverFE" Name="ATTR" >
chemicalField = chemotaxisXMLData.getFirstElement("ChemicalField",
d2mss({"Source": "FlexibleDiffusionSolverFE",
"Name": "ATTR"}))
# check if the attempt was succesful
if chemicalField:
# get <ChemotaxisByType Type="Macrophage" Lambda="xxx"/>
chemotaxisByTypeMacrophageElement = chemicalField.getFirstElement("ChemotaxisByType",
d2mss({"Type": "Macrophage"}))
if chemotaxisByTypeMacrophageElement:
# get value of Lambda from <ChemotaxisByType> element
# notice that no conversion fro strin to float is necessary as
# getAttributeAsDouble returns floating point value

lambdaVal = chemotaxisByTypeMacrophageElement.getAttributeAsDouble("Lambda")
print "lambdaVal=", lambdaVal
# decrease lambda by 0.2
lambdaVal -= 3
# update attribute value of Lambda converting float to string
chemotaxisByTypeMacrophageElement.updateElementAttributes(d2mss({"Lambda": str(lambdaVal)}))
self.simulator.updateCC3DModule(chemotaxisXMLData)


As you can see the structure of the code is very similar to the previous 2 examples. First we fetch Plugin element describing Chemotaxis properties:

chemotaxisXMLData = self.simulator.getCC3DModuleData("Plugin","Chemotaxis")


Next, we fetch ChemicalField element:

chemicalField = chemotaxisXMLData.getFirstElement("ChemicalField",
d2mss({"Source": "FlexibleDiffusionSolverFE", "Name": "ATTR"}))


and using ChemicalField element we get ChemotaxisByType element:

chemotaxisByTypeMacrophageElement = chemicalField.getFirstElement("ChemotaxisByType",
d2mss({"Type": "Macrophage"}))


Using chemotaxisByTypeMacrophageElement we fetch its attribute lambda convert it to floating point number decrease by 3 units and assing new value of lambda:

lambdaVal = chemotaxisByTypeMacrophageElement.getAttributeAsDouble("Lambda")
lambdaVal -= 3
chemotaxisByTypeMacrophageElement.updateElementAttributes(d2mss({"Lambda": str(lambdaVal)}))


The rest of the code is analogous to the previous examples. This completes the discussion of CC3DML steering.

## Basic Terminology¶

When accessing variables defined in the XML element we are typically dealing with attributes and values. Let’s consider the following XML element

<Energy Type1=“Condensing” Type2=“NonCondensing”>20.0</Energy>


The difference between attribute and value of the element is that attributes are all the labels inside <> element marker to which we assign some value. For example Type1 and Type2 are attributes of the XML element Energy, whereas 20.0 is the value of this XML element. In what follows below we will be accessing and modifying both attributes and Values of the XML elements. Please pay close attention whwther the function we are using is of *XMLValue or *XMLAttributeValue type

Note Some XML elements might have only attributes e.g. :

<VolumeConstraint LambdaVolume=“20” TargetVolume="125" Type="Condensing"/>


and some might have only values:

<Steps>10000</Steps>


The above examples demonstrate how to steer CC3DML-based part of the simulation in a fairly verbose way i.e. the amount of code is quite significant. In 3.7.1 we have introduced more compact way to access and modify CC3DML elements: Let us look at the implementation of the ContactSteeringAndTemperature steppable using new style coding:

class ContactSteeringAndTemperature(SteppableBasePy):
def __init__(self, _simulator, _frequency=10):
SteppableBasePy.__init__(self, _simulator, _frequency)

def step(self, mcs):
temp = float(self.getXMLElementValue(['Potts'], ['Temperature']))
self.setXMLElementValue(temp + 1, ['Potts'], ['Temperature'])

val = float(
self.getXMLElementValue(
['Plugin', 'Name', 'Contact'], ['Energy', 'Type1', 'NonCondensing', 'Type2', 'Condensing']))

self.setXMLElementValue(
val + 1, ['Plugin', 'Name', 'Contact'], ['Energy', 'Type1', 'NonCondensing', 'Type2', 'Condensing']
)

self.updateXML()


Instead of using verbose code to access CC3DML elements we now specify access path to particular element . Access path is a sequence of Python lists. First element of each list is the name of the CC3DML element followed by a sequence of pairs (attribute,value) which fully specify the XML element:

[ElementName, AttrName1, AttrValue1, Attr2, AttrValue2, …, AttrNameN, AttrValueN]

In the CC3DML code below:

<CompuCell3D>
<Potts>
<Dimensions x="100" y="100" z="1"/>
<Anneal>10</Anneal>
<Steps>10000</Steps>
<Temperature>10</Temperature>
<Flip2DimRatio>1</Flip2DimRatio>
<NeighborOrder>2</NeighborOrder>
</Potts>

<Plugin Name="Volume">
<TargetVolume>25</TargetVolume>
<LambdaVolume>2.0</LambdaVolume>
</Plugin>

<Plugin Name="CellType">
<CellType TypeName="Medium" TypeId="0"/>
<CellType TypeName="Condensing" TypeId="1"/>
<CellType TypeName="NonCondensing" TypeId="2"/>
</Plugin>

<Plugin Name="Contact">
<Energy Type1="Medium" Type2="Medium">0</Energy>
<Energy Type1="NonCondensing" Type2="NonCondensing">16</Energy>
<Energy Type1="Condensing"    Type2="Condensing">2</Energy>
<Energy Type1="NonCondensing" Type2="Condensing">11</Energy>
<Energy Type1="NonCondensing" Type2="Medium">16</Energy>
<Energy Type1="Condensing"    Type2="Medium">16</Energy>
<NeighborOrder>2</NeighborOrder>
</Plugin>

...
</CompuCell3D>


to access Temperature element from the Potts section we construct our access path following one simple rule:

1. Recursively identify child element of the current element that leads you to the desired place in the CC3DML code. Notice: we skip root element <CompuCell3D> element.

In our example to access <Temperature> element we first locate <Potts> as a child of <CompuCell3D> element (remember in the access path we do not include <CompuCell3D> element) and

then <Temperature> appears to be child of the <Potts>. Hence our access path is a simple sequence of two Python lists, each list with one element:

['Potts'],['Temperature']


A bit more complex, but still much simpler than our original code, is the example where we locate one of the Contact plugin elements. <Plugin Name="Contact"> is a child of the <CompuCell3D> hence:

['Plugin','Name','Contact']


will be first Python list of ther access path. Notice that the first element of this list is the same as name of the child element (Plugin) and the two next elements constitute an XML attribute-value pair. In other words, XML’s Name="Contact" gets translated into 'Name',’Contact’ of the Python list.

Now we locate correct <Energy> element. Since there are many of these the correct identification of the one which is of interest for us will require specification of all its attributes: Type1="NonCondensing" Type2="Condensing". Consequently our access path from <Plugin> to <Energy> will look as follows:

['Energy','Type1','NonCondensing','Type2','Condensing']


And the full path is simply

['Plugin','Name','Contact'],['Energy','Type1','NonCondensing','Type2','Condensing']