# 2. Charts

This step Illustrates how define monitors and charts in GAMA. In addition, it illustrates how to define a stopping condition for the simulation.

## Formulation

- Definition of new global variables: current_hour, nb_people_infected, nb_people_not_infected, infected_rate
- Definition of a monitor to follow the current hour and the nb of people infected
- Definition of a series chart to follow the number of people infected and not infected
- Definition of a stopping condition (when infected rate = 1)

## Model Definition

### global variables

In order to define dynamic variable able to update itself, we use the **update** facet of variable definition.
Indeed, at each simulation step, all the agents (and the world agent) apply for each dynamic variable (in their definition order) its update expression.
We define 4 new variables:

**current hour**(int) : current simulation step (**cycle**) / 60 mod 24**nb_people_infected**(int): nb of people with is_infected = true (use of the**list count condition**operator that count the number of elements of the list for which the condition is true)**nb_people_not_infected**(int): nb_people - nb of people infected**infected_rate**(float): nb of people infected / nb of people

```
global{
...
int current_hour update: (cycle / 60) mod 24;
int nb_people_infected <- nb_infected_init update: people count (each.is_infected);
int nb_people_not_infected <- nb_people - nb_infected_init update: nb_people - nb_people_infected;
float infected_rate update: nb_people_infected/nb_people;
...
}
```

### stopping condition

We add a new reflex that stops the simulation if the **infected_rate** is equal to 1. To stop the simulation, we apply the **halt** action.

```
global {
...
reflex end_simulation when: infected_rate = 1.0 {
do halt;
}
}
```

Note that it would have been possible to use the **pause** action that pauses the simulation instead of the **halt** action that stops the simulation.

### monitor

A monitor allows to follow the value of an arbitrary expression in GAML. It has to be defined in an output section. A monitor is defined as follows:

```
monitor monitor_name value: an_expression refresh:every(nb_steps);
```

With:

- value: mandatory, its value will be displayed in the monitor.
- refresh: bool, optional : if the expression is true, compute (default is true).

In this model, we define 2 monitors to follow the value of the variable **current_hour** and **infected_rate**:

```
experiment main_experiment type:gui{
...
output {
monitor "Current hour" value: current_hour;
monitor "Infected people rate" value: infected_rate;
...
}
}
```

### chart

GAMA can display various chart types:

- Time series
- Pie charts
- Histograms

A chart must be defined in a display : it behaves exactly like any other layer. Definition of a chart :

```
chart chart_name type: chart_type {
[data]
}
```

The data to draw are define inside the chart block:

```
data data_legend value: data_value
```

We add a new display called **chart** refresh every 10 simulation steps.
Inside this display, we define a chart of type *series*:

- "Species evolution"; background : white; size : {1, 0.5}; position : {0, 0}
- data1: susceptible; color : green
- data2: infected; color : red

```
experiment main_experiment type:gui{
...
output {
...
display chart refresh:every(10) {
chart "Disease spreading" type: series {
data "susceptible" value: nb_people_not_infected color: #green;
data "infected" value: nb_people_infected color: #red;
}
}
}
}
```

## Complete Model

```
model SI_city
global{
int nb_people <- 500;
float step <- 1 #minutes;
geometry shape<-envelope(square(500 #m));
float infection_distance <- 2.0 #m;
float proba_infection <- 0.05;
int nb_infected_init <- 5;
int current_hour update: (cycle / 60) mod 24;
int nb_people_infected <- nb_infected_init update: people count (each.is_infected);
int nb_people_not_infected <- nb_people - nb_infected_init update: nb_people - nb_people_infected;
float infected_rate update: nb_people_infected/length(people);
init{
create people number:nb_people {
speed <- 5.0 #km/#h;
}
ask nb_infected_init among people {
is_infected <- true;
}
}
reflex end_simulation when: infected_rate = 1.0 {
do halt;
}
}
species people skills:[moving]{
bool is_infected <- false;
reflex move{
do wander;
}
reflex infect when: is_infected{
ask people at_distance infection_distance {
if flip(proba_infection) {
is_infected <- true;
}
}
}
aspect circle{
draw circle(5) color:is_infected ? #red : #green;
}
}
experiment main_experiment type:gui{
parameter "Infection distance" var: infection_distance;
parameter "Proba infection" var: proba_infection min: 0.0 max: 1.0;
parameter "Nb people infected at init" var: nb_infected_init ;
output {
monitor "Current hour" value: current_hour;
monitor "Infected people rate" value: infected_rate;
display map {
species people aspect:circle;
}
display chart refresh:every(10) {
chart "Disease spreading" type: series {
data "susceptible" value: nb_people_not_infected color: #green;
data "infected" value: nb_people_infected color: #red;
}
}
}
}
```