# Setup Files: A first example

### Basics

The format of the setup files that Schnek can read is a structured format containing blocks of parameters. Each type of block corresponds to a class in the simulation code and each definition of a block in the setup file will create an object of the corresponding class. To create a simulation block, simply create a class that extends Block.

class SimulationBlock : public Block
{};


In order to read an input file and create an object of the new class SimulationBlock you need to register the class with a BlockClasses object. In your main function you can specify the following.

  BlockClasses blocks;
blocks("sim").setClass<SimulationBlock>();


The first line creates a BlockClasses object. The second line registers a new block name called "sim". You can choose whatever name you want here. The third line links the block name "sim" with the SimulationBlock class. This line is needed by the parser to create the correct objects from the setup file. The last two lines of the code above can be abbreviated.

  blocks.addBlock("sim").setClass<SimulationBlock>();


Now that we have registered the simulation blocks, we can use the parser to read a setup file.

  std::ifstream in("example.setup");

Parser P("my_simulation", "sim", blocks);
pBlock application = P.parse(in);


The first line simply opens a file called example.setup. The second line creates a Parser object by specifying three parameters.

"my_simulation" is simply the name of the simulation that you want to run. You can set this to anything you like.

"sim" is the block name of the root block. This means that the contents of the file are interpreted as belonging to the "sim" block.

blocks is the BlockClasses object that specifies the available block names and the types that are associated with them.

The last line reads the setup file from the input stream and returns a boost::shared_ptr to a Block object. This will be of the type specified by the root block. You can safely cast it to the class SimulationBlock class which has been linked to the "sim" root block.

  SimulationBlock &mysim = dynamic_cast<SimulationBlock&>(*application);


Until now, we have only created a SimulationBlock object from some setup file. We haven’t even specified what the setup file should look like. That’s because we are not reading in any values from the file. But our simulation block will contain parameters that you will want to specify in the setup file. It turns out that it is actually quite easy to register parameters and read values into them.

Let’s extend the definition of our SimulationBlock class.

class SimulationBlock : public Block
{
private:
int size;
double dx;
std::string info;

protected:
void initParameters(BlockParameters &parameters)
{
}
};


We have defined three parameters called size, dx, and info. The three parameters are of type int, double, and std::string. These are the three types currently supported by Schnek.

In addition to declaring the class parameters we have implemented a method called initParameters. This method is a virtual method defined by the Block class. It is called before the setup file is read and is used to register all the parameters. Calling addParameter on the BlockParameters object thath is passed into the method registers the parameters.

The addParameter method takes two arguments. The first argument is the name of the parameter as it would appear in the setup file. The second argument is the memory address of the variable that the value should be assigned to.

Once we have registered all the parameters, all we have to do is to make sure that the values are evaluated from the setup file. This is achieved by a call to evaluateParameters on the root simulation block.

  mysim.evaluateParameters();


### The Setup File

Now that we have written the basic code for reading setup files, let’s take a look at the format of the setup file itself. Some possible contents of example.setup are as follows.

size = 123;
dx = 3.056;
info = "A test setup";


The format somewhat resemble C-style syntax. Lines are terminated with semicolons and whitespaces are ignored. In the setup file you can directly assign values to the parameters registered in the root block of your simulation. But the format of the setup file is a lot more versatile than this. Let’s look at the following example.

float pihalf = 3.14159/2.0;
int counter = 7;

size = counter^2;
dx = 3.056*pihalf;
info = "data"+counter+".out";


You can define your own variables in the setup file and use them to calculate the parameters. The basic operations addition, subtraction, multiplication, division and exponentiation are supplied out of the box. The input file above will result in the parameters of the root SimulationBlock object to be set to the following values.

size: 49
dx: 4.80035
info: data7.out


Let’s go one step further and register all the mathematical function in the standard cmath library. To do this, we have to modify our code by adding a single line before parsing the setup file.

  Parser P("my_simulation", "sim", blocks);
registerCMath(P.getFunctionRegistry());
pBlock application = P.parse(in);


Now we can change the setup file as follows.

float pihalf = 3.14159/2;
int counter = 7;

size = counter^2;
dx = sin(pihalf*0.5)*exp(-1.5^2);
info = "data"+counter+".out";


Note how we have used the sin and exp functions. The resulting values assigned to the SimulationBlock members are as follows.

size: 49
dx: 0.0745285
info: data7.out


The code for this tutorial can be found here, the input files are
example_setup_first01.setup
example_setup_first02.setup
example_setup_first03.setup