2. Practice - An Introduction to NetLogo

The observer in NetLogo acts as an overseer, responsible for managing and modifying the environment and agents without being an agent itself. It can execute commands to create, move, or modify turtles, patches, and links, as well as control the simulation by adjusting global settings, running procedures, and monitoring overall behavior.

Opening NetLogo presents a minimalist interface with a blank project, including an empty grid window by default. The easiest way to begin interacting with this environment is through the Command Center.

By default, you are acting as the observer, which grants full control over the entire environment with a global perspective. In observer mode, commands are executed at the global level and can directly manipulate the environment, agents, and simulation settings.

This code may appear simple, but it performs multiple actions behind the scenes:

The command inspect turtle is used to inspect all properties of a turtle.

In this window, you can observe additional properties of the turtle that were not mentioned previously:

There is also an input field at the bottom of the Inspect window, which allows you to directly modify properties or execute commands for the selected agent or object (such as a turtle, patch, or link) in the simulation.

For example, to change the label of a turtle, you can use the following command:

The set command modifies the properties of the agent selected in the Inspect window. However, if you want to do this from the observer level, you need to use the ask command to specify which agent you are targeting:

This observer-level command is slightly modified from the original, with the inclusion of the ask keyword, along with the breed (turtle) and the unique identifier (who) of the agent in question. The ask command in NetLogo allows the observer to direct specific agents (or groups of agents) to perform actions.

Commands are procedures that don't have any output, but only side effects on the environment.

This code defines a procedure called go, which performs the following actions:

This command can then be called in the Command Center with the following line:

Is there another way to interact with the Code Pane from the Interface Tab? Yes, through Interface Elements, which allow users to modify and interact with the simulation without directly changing the code. The most notable elements include:

Reporters are procedures that compute a value and report it.

The above code defines a reporter named double that performs the following actions:

This reporter can be called in the Command Center or within other procedures to compute the double of a given number. For example:

In the code above, num acts as an input parameter to the command.

There isn't an official NetLogo style guide. Nonetheless the official documentation is fairly consistent and follows some good habits:

Variables in NetLogo can be divided into three main groups:

When asking to update an agent variables a subset of all the agents, called agentset, can be used. An agentset contains one or more agents, all of the same type, and it's always randomly ordered.

The one-of primitive in NetLogo randomly selects one agent from a given set of agents, such as turtles, patches, or links. For example, one-of turtles randomly selects one turtle from the current set of turtles. Additionally, you can create subsets of agents using conditions (e.g., turtles with [color = red]) and then instruct these specific subsets with targeted commands.

Conditionals in NetLogo allow agents to make decisions based on specific criteria using commands like if, ifelse, and ifelse-value. For example, if pcolor = black [ set pcolor white ] changes a patch's color to white only if its current color is black.

Conditions are logical expressions (=, <, >, and, or, etc.) that evaluate to true or false.

Loops in NetLogo allow repeated execution of commands, enabling dynamic and iterative behaviors. Common looping constructs include repeat, which runs a block of commands a fixed number of times, and while, which runs as long as a specified condition is true.

Lists in NetLogo are ordered collections of items, which can include numbers, strings, agents, or other lists. They are data structures that support operations like adding, removing, or accessing elements.

Some examples:

The output will the firsts element or item in the list, which is blue.

my-list will contain the values of [1, 99, 3] after using replace-item.

The lput primitive command adds an element to the end of a list, while fput adds an element to the beginning.

The flexibility of NetLogo and its agent-centered way of building models quickly escalates to complex models that are difficult to work with.

Try to keep your structure as close as possible to:

Even though NetLogo is not a higher-order language we can simulate this behavior using anonymous procedures/reporters.

Anonymous procedures assigned to variables (tasks):

Higher-order reporter:

Unlike traditional procedures or reporters, anonymous ones are not stored in the Code Tab and cannot be reused unless redefined. While NetLogo doesn’t directly support higher-order functions, anonymous procedures allow for similar behavior in many cases.

In NetLogo breeds are a way to "subclass" the turtle type.

After a breed has been created, the ask command can be used with the breed name (e.g., ask hunters) to execute actions for agents of that specific breed. All commands and properties applicable to turtles can also be used with the newly defined breed.

Tasks: