# Tutorial: Creating simulations¶

When you create an agent-based model, there are generally three things to consider: creating an initial population, agent behaviour during the simulation and reporting.

## Initial population¶

FastSTI allows you to generate the initial population or to read agents in from a file.

The before_events parameter tells FastSTI how to initialize the agents.

### Generating the population¶

The easiest way to generate a population is to use the _generate_and_pair event. E.g. in the simulation configuration file there would be a line like this:

before_events = _generate_and_pair; <other events to run before the simulation>


The _generate_and_pair event is a composite event which calls five other events: _generate_agents, _initial_mating, _shuffle_mating, _rkpm and _initial_rel events.

Together these events create the initial agents in the simulation, and place some of them into relationships. Click on each of these events to see how they work and how you can specify the number of agents, the ages of the agents, their sexes, sexual preferences and the distribution of the relationships in the set of initial agents.

Reading in an agent file perhaps gives greater flexibility than the built-in agent generating functionality of FastSTI.

The file would typically be created in a scripting language like R or Python or even be hand-crafted.

The _read_agents event is used to read in the agents. In the simulation configuration file there would be a line like this:

before_events = _read_agents; <other events to run before the simulation>


Read the section on agent input formats to find out how to set up the agent input CSV file.

## Agent behaviour¶

In FastSTI agent behaviour takes place in events. There are predefined events that provide for agents to age, form sexual relationships with other agents and break-up with them, become infected, start treatment <stage_ref>, change their infection status <stage_ref> (e.g. become sicker or become better), be born, and die.

Most of these events are usually executed using the during_events parameter. Although it’s conceivable you may also need to execute agent behaviour before and after a simulation using the before_events and after_events parameters respectively.

If these events don’t meet your needs the FastSTI framework allows you to code your own events in C.

## Reporting¶

Usually, you need statistics about the simulation before, during and after the simulation has run. FastSTI has a reporting facility to do write out demographic and statistical information at any stage of a simulation.

Reports are written to a single output file in CSV format. If you’re running multiple simulations, the report lines from each simulation will be interleaved with each other throughout the file. But each report line has enough information for you to know which simulation produced it.

You’ll usually need to use a spreadsheet program or a scripting language like R or Python to process the output CSV file. See the scripts directory for examples of report processing code that you make using FastSTI much easier for you.

Reporting is just another event in FastSTI. There are a few provided: _report - which outputs statistics, _write_agents_csv - which outputs every agent in the simulation, _write_living_agents_csv - which outputs every living agent in the simulation, and _write_dead_agents_csv - which outputs every dead agent in the simulation. There are also events to write the headers for the output CSV files.

The report_frequency parameter specifies how frequently to run these events during a simulation.

Use the events that output agents carefully. If you’re executing _write_agents_csv during the simulation, having a report_frequency of 1 will slow your simulation down and use huge amounts of hard drive space.

A typical report setup for a simulation looks something like this:

results_file = results.csv
agents_output_file = agents_out.csv
report_frequency = 365
events>; _report
during_events = <other events>; _report;  _write_living_agents_csv
after_events = <other events>; _report; write_agents_csv


If you don’t need to know details about individual agents, then it’s best not to execute the events that output agents. Your simulation will be faster and use much less disk space. Then you can also have much more frequent report writing. E.g.

results_file = results.csv
report_frequency = 10