Agents

The NetLogo world is made up of agents. Agents are beings that can follow instructions. Each agent can carry out its own activity, all simultaneously.

In NetLogo, there are four types of agents: turtles, patches, links, and the observer. Turtles are agents that move around in the world. The world is two dimensional and is divided up into a grid of patches. Each patch is a square piece of "ground" over which turtles can move. Links are agents that connect two turtles. The observer doesn't have a location -- you can imagine it as looking out over the world of turtles and patches.

When NetLogo starts up, there are no turtles yet. The observer can make new turtles. Patches can make new turtles too. (Patches can't move, but otherwise they're just as "alive" as turtles and the observer are.)

Patches have coordinates. The patch at coordinates (0, 0) is called the origin and the coordinates of the other patches are the horizontal and vertical distances from this one. We call the patch's coordinates pxcor and pycor. Just like in the standard mathematical coordinate plane, pxcor increases as you move to the right and pycor increases as you move up.

The total number of patches is determined by the settings min-pxcor, max-pxcor, min-pycor and max-pycor. When NetLogo starts up, min-pxcor, max-pxcor, min-pycor and max-pycor are -16, 16, -16, and 16 respectively. This means that pxcor and pycor both range from -16 to 16, so there are 33 times 33, or 1089 patches total. (You can change the number of patches with the Settings button.)

Turtles have coordinates too: xcor and ycor. A patch's coordinates are always integers, but a turtle's coordinates can have decimals. This means that a turtle can be positioned at any point within its patch; it doesn't have to be in the center of the patch.

Links do not have coordinates, instead they have two endpoints (each a turtle). Links appear between the two endpoints, along the shortest path possible even if that means wrapping around the world.

The way the world of patches is connected can change. By default the world is a torus which means it isn't bounded, but "wraps" -- so when a turtle moves past the edge of the world, it disappears and reappears on the opposite edge and every patch has the same number of "neighbor" patches; if you're a patch on the edge of the world, some of your "neighbors" are on the opposite edge. However, you can change the wrap settings with the Settings button. If wrapping is not allowed in a given direction then in that direction (x or y) the world is bounded. Patches along that boundary will have fewer than 8 neighbors and turtles will not move beyond the edge of the world.

Agentsets

An agentset is exactly what its name implies, a set of agents. An agentset can contain either turtles, patches or links, but not more than one type at once.

An agentset is not in any particular order. In fact, it's always in a random order. And every time you use it, the agentset is in a different random order. This helps you keep your model from treating any particular turtles, patches or links differently from any others (unless you want them to be). Since the order is random every time, no one agent always gets to go first.

You've seen the turtles primitive, which reports the agentset of all turtles, the patches primitive, which reports the agentset of all patches and the links primitive which reports the agentset of all links.

But what's powerful about the agentset concept is that you can construct agentsets that contain only some turtles, some patches or some links. For example, all the red turtles, or the patches with pxcor evenly divisible by five, or the turtles in the first quadrant that are on a green patch or the links connected to turtle 0. These agentsets can then be used by ask or by various reporters that take agentsets as inputs.

One way is to use turtles-here or turtles-at, to make an agentset containing only the turtles on my patch, or only the turtles on some other particular patch. There's also turtles-on so you can get the set of turtles standing on a given patch or set of patches, or the set of turtles standing on the same patch as a given turtle or set of turtles.

Once you have created an agentset, here are some simple things you can do:

• Use ask to make the agents in the agentset do something
• Use any? to see if the agentset is empty
• Use all? to see if every agent in an agentset satisfies a condition.
• Use count to find out exactly how many agents are in the set

See the Models Library for many more examples, and consult the NetLogo Dictionary for more information about all of the agentset primitives.

More examples of using agentsets are provided in the individual entries for these primitives in the NetLogo Dictionary. In developing familiarity with programming in NetLogo, it is important to begin to think of compound commands in terms of how each element passes information to the next one. Agentsets are an important part of this conceptual scheme and provide the NetLogo developer with a lot of power and flexibility, as well as being more similar to natural language.

Earlier, we said that agentsets are always in random order, a different random order every time. If you need your agents to do something in a fixed order, you need to make a list of the agents instead.

Breeds

NetLogo allows you to define different "breeds" of turtles and breeds of links. Once you have defined breeds, you can go on and make the different breeds behave differently. For example, you could have breeds called sheep and wolves, and have the wolves try to eat the sheep or you could have link breeds called streets and sidewalks where foot traffic is routed on sidewalks and car traffic is routed on streets.

You define turtle breeds using the breed keyword, at the top of the Procedures tab, before any procedures:

breed [wolves wolf]
breed [sheep a-sheep]

The order in which breeds are declared is also the order order in which they are layered in the view. So breeds defined later will appear on top of breeds defined earlier; in this example, sheep will be drawn over wolves.

When you define a breed such as sheep, an agentset for that breed is automatically created, so that all of the agentset capabilities described above are immediately available with the sheep agentset.

A turtle's breed agentset is stored in the breed turtle variable. So you can test a turtle's breed, like this:

if breed = wolves [ ... ]

Note also that turtles can change breeds. A wolf doesn't have to remain a wolf its whole life. Let's change a random wolf into a sheep:

ask one-of wolves [ set breed sheep ]

The set-default-shape primitive is useful for associating certain turtle shapes with certain breeds. See the section on shapes below.

Buttons

Buttons in the interface tab provide an easy way to control the model. Typically a model will have at least a "setup" button, to set up the initial state of the world, and a "go" button to make the model run continuously. Some models will have additional buttons that perform other actions.

A button contains some NetLogo code. That code is run when you press the button.

A button may be either a "once button", or a "forever button". You can control this by editing the button and checking or unchecking the "Forever" checkbox. Once buttons run their code once, then stop and pop back up. Forever buttons keep running their code over and over again, until either the code hits the stop command, or you press the button again to stop it. If you stop the button, the code doesn't get interrupted. The button waits until the code has finished, then pops up.

Normally, a button is labeled with the code that it runs. For example, a button that says "go" on it usually contains the code "go", which means "run the go procedure". (Procedures are defined in the Procedures tab; see below.) But you can also edit a button and enter a "display name" for the button, which is a text that appears on the button instead of the code. You might use this feature if you think the actual code would be confusing to your users.

When you put code in a button, you must also specify which agents you want to run that code. You can choose to have the observer run the code, or all turtles, or all patches, or all links. (If you want the code to be run by only some turtles or some patches, you could make an observer button, and then have the observer use the ask command to ask only some of the turtles or patches to do something.)

When you edit a button, you have the option to assign an "action key". This makes that key on the keyboard behave just like a button press. If the button is a forever button, it will stay down until the key is pressed again (or the button is clicked). Action keys are particularly useful for games or any model where rapid triggering of buttons is needed.

Turtle, patch, and link forever buttons

There is a subtle difference between putting commands in a turtle, patch or link forever button, and putting the same commands in an observer button that does ask turtles, ask patches or ask links. An "ask" doesn't complete until all of the agents have finished running all of the commands in the "ask". So the agents, as they all run the commands concurrently, can be out of sync with each other, but they all sync up again at the end of the ask. The same isn't true of turtle, patch and link forever buttons. Since ask was not used, each turtle or patch runs the given code over and over again, so they can become (and remain) out of sync with each other.

Lists

In the simplest models, each variable holds only one piece of information, usually a number or a string. The list feature lets you store multiple pieces of information in a single variable by collecting those pieces of information in a list. Each value in the list can be any type of value: a number, or a string, an agent or agentset, or even another list.

Lists allow for the convenient packaging of information in NetLogo. If your agents carry out a repetitive calculation on multiple variables, it might be easier to have a list variable, instead of multiple number variables. Several primitives simplify the process of performing the same computation on each value in a list.

The NetLogo Dictionary has a section that lists all of the list-related primitives.

Turtle shapes

In NetLogo, turtle shapes are vector shapes. They are built up from basic geometric shapes; squares, circles, and lines, rather than a grid of pixels. Vector shapes are fully scalable and rotatable. NetLogo caches bitmap images of vector shapes size 1, 1.5, and 2 in order to speed up execution.

A turtle's shape is stored in its shape variable and can be set using the set command.

New turtles have a shape of "default". The set-default-shape primitive is useful for changing the default turtle shape to a different shape, or having a different default turtle shape for each breed of turtle.

The shapes primitive reports a list of currently available turtle shapes in the model. This is useful if, for example, you want to assign a random shape to a turtle:

ask turtles [ set shape one-of shapes ]

Use the Turtle Shapes Editor to create your own turtle shapes, or to add shapes to your model from our shapes library, or to transfer shapes between models.