Slides and model files will be at
www.is.wayne.edu/drbowen >
NetLogo workshop
NetLogo Workshop
Complexity And Business Analytics
October 10, 2008, UM-Dearborn
David Bowen, WSU Physics
d.r.bowen@wayne.edu
As a result of this workshop, (I
hope) you will be able to:
12:30 – 2:10
• Describe agent-based
modeling
• Download & install
NetLogo
• Find & use existing
models installed with
NetLogo & on
NetLogo website
10/10/08
2:30 – 4:10
• Modify an existing
NetLogo model
• Build a new NetLogo
model
NetLogo Workshop
2
NetLogo Workshop
Part 1
• Agent-based modeling
• Download and Install
• Find and run
Agent-Based Modeling
• What is agent-based modeling?
 Simulating a set of acting/interacting agents
 Examples of agents: people, organs, molecules,
planets, family units, corporations, ribosomes
 Modeler assigns (programs):
o Characteristics or properties of agent
 Homogenous or diverse agents
o Rules or methods of interaction
 Example: Preferentially link to the most popular agent (the
one that already has the most links)
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Agent-Based Modeling
• What is agent-based modeling?
 Agent-based modeling environment/toolkit:
o Steps agents through actions/interactions
o Displays agents, charts indicators, saves data
 NetLogo has four basic types of agents A: always
o A Observer: sees all
o A Turtles (agents), modeler can create subtypes
o S Links between turtles, can have subtypes
o A Patches or pieces of background
 Can be “invisible” but sometimes important (grass in Sheep
and Wolves)
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Agent-Based Modeling
• Where is agent-based modeling used?
 Everywhere, e.g. all academic departments
 Natural systems (science, engineering etc.)
o Many existing simulation techniques, question is
purpose, strengths, weaknesses
 Social systems (humanities, social science,
business/marketing etc.)
o Often the best or only laboratory available
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Agent-Based Modeling
• Types of use
 Real-world simulation, optimization
 Research
 Classroom: learning, demo, homework
• Alternatives to making your own models
 Models Library downloads with installation
 Publicly available models
 Modify and existing model
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Agent-Based Modeling
• About NetLogo
 Uri Wilensky et al. at Center for Connected
Learning and Computer-Based Modeling at
Northwestern University
 “Low floors, high ceilings”
 Free download
 Java-based (run anywhere) but private Java copy
 Discussion forums, technical support,
continuous development
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Download and Install
•
•
•
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URL: ccl.northwestern.edu/netlogo/
Notice: mail lists, Resources (more models)
Download on the left
Save to desktop (because of rights on these
computers, elsewhere you can accept the default)
• Double-click on downloaded file to install
(NetLogo4.0.3Installer.exe)
• Location: up-arrow to desktop, add \NetLogo >
Next
• Accept all other defaults
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Run
• Start > All Programs > NetLogo >
NetLogo 4.0.3
• Notice:
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File menu item (top left) – Models Library, etc
Interface – “the model”
Information – How to use & change the model
Procedures – the code
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Models Library
• File > Models Library
 Checked except for (unverified)
• Biology > Heatbugs (famous)
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Double-click model name or picture to open
Setup then Go (Go again pauses)
Information – what the model is doing
Procedures
o Turtles-own establishes properties (characteristics)
o Button name starts “to” procedure with that name
o ; for comment, can be whole or last part of line
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Models Library
• File > Models Library > Computer Science
> Cellular Automata > Life
• Cellular Automaton
 Here, patches = cells, no turtles
o Dark = cell alive, background color = dead
 Cell with 3 live neighbors comes to life
 Live cell with two live neighbors continues to
live
o Otherwise cell dies
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Models Library
• Social Science > Party (famous in another
form)
 Schelling model – two types of people
o Here, men and women
o Originally, Black and White
o Otherwise the same – properties and methods
 How comfortable are these agents with those of
the other type – slider
 Even a slight preference can lead to complete
segregation
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Models Library
• File > Models Library > Networks > Small
Worlds (famous – six degrees of separation)
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Uses links
Path along links from one agent to another
Setup = ring, links to 2 nearest on each side
Average path length for all pairs of agents
Even a few links across the ring reduces the
average path length significantly
 Significant in organizations, e.g. businesses
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Models Library
• File > Open > Biology >
Wolf Sheep Predation
• Different kinds of agents, including patches
• Click grass? Switch to On
• Boom-and-bust cycles very common
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Models on NetLogo site
• URL: ccl.northwestern.edu/netlogo/ >
Community Models
• Possible problems if models are written for
earlier versions of NetLogo
 My Two-color gas works with 4.0.3
 My Brownian motion doesn’t
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NetLogo Models on the Web
• Solves problems with old versions since
Java Virtual Machine (VM) downloaded
also (but different versions can coexist)
• www.is.wayne.edu/drbowen/ > Classroom
Models
 Brownian motion works
 Big Bang model
• Other things NetLogo can do
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Extensions
• Array, Table, GoGo, Profiler, GIS, Sample,
and Sound are in the standard installation
• In Extensions subfolder of NetLogo
installation folder
• Each is explained in Help > NetLogoUser
Manual > Extensions
• Some (GIS, Table) come with existing
models
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NetLogo Workshop
Part 2
• Modify
• Roll your own
Programming Resources
• Help > User Manual
 Tutorials
 Interface Guide – World, Menu items, Controls
o World - Settings: size, wrapping Vs reflecting
 Programming Guide – big picture
o Types: observer (starts off), turtle, patch, link
o What can I do with agents?
 NetLogo Dictionary – the nitty gritty
o Alphabetical list
o By type
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Modify One
• File > Model Library > Computer Science >
Cellular Automata > Life
• Procedures – scan to numbers 3 and 2
• On the way
 patches-own – creates properties for patches
o Built-in: pxcor, pycor, pcolor
o living? - ? makes this logical (true/false only)
• 3  4, 2  3
• File > Save As a new name, e.g. LifeDB on
desktop
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Modify Life
• Add a slider to vary number on interface
• Click on down-arrow to select slider
 Global variable init-number, 1 to 5, value 2
 Then right-click to Select, squeeze it in
• In Procedures, 2 init-number,
3  init-number + 1 / Save, Setup then Go
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Modify Life
• You now load this modified version from
the desktop or by double-clicking the
desktop icon
 “Back home” you can keep the .NLOGO file in
any folder you choose
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Modify Small Worlds
• File > Models Library > Networks >
Small Worlds
• On interface, put in a slider:
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Global variable init-links
Minimum 1
Increment 1
Maximum 3
Value 2
• Save on desktop under a new name
 Add initials?
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Modify Small Worlds
• Procedures, scroll through (bigger than
Life)
 Find to setup, then see make-turtles then
wire-them (this creates the links)
 About 75% down (Find?) find to wire-them
 What we want to do:
o Always link to n+1
o If init-links = 2 or 3 then also link to n + 2
o If init-links = 3 then also link to n + 3
 Dictionary: if condition [ commands ]
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Modify Small Worlds
• Error when running with init-links = 1;
divide by zero
 Divide by clustering-coefficient-of-lattice 
divide by dum, protect dum from being zero
o We might eventually want a better fix
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Modifying Models
• Find a model that does something close to
what you want, and modify it to suit
• For these modified models, we should go
back and update the information
• Structured and commented code helps – do
this for yourself, too
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Roll Your Own
• Decisions needed
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System to be modeled
What do the agents represent?
What are the rules of action and interaction?
How will you approximate these rules for entry
into the modeling environment?
 What does a tick (cycle) represent?
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Other
• You might want to change the World
settings
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Size
Location of 0, 0
Wrapping or box
Patch size (also determines size of world
screen)
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Programming in NetLogo
• Case-insensitive
• To set a equal to b: set a b
• Arithmetic operators must have space on
either side: set a 3 * b
• Can use parentheses to control order
• Use [ ] to block code statements
• White space ignored after minimum space
• Routines start with to, end with end
• No number types
• Logical variables end in ?
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Programming in NetLogo
• Context: who is acting?
 ask turtles [ commands ] sets context to turtles
o Agents can ask other agents or even other types of
agents
 ask turtles [ … ask links [ … ]… ]
 Only that particular agent can find the value or
change a value that it owns
 If one agent (e.g. a turtle) is acting, other turtles
is set of all other turtles, one-of other turtles is a
random other
 stop to stop asking
• breed to subtype an agent: breed [ cats cat ]
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Simple Model
• Model will:
 Setup
o Create turtles with random headings (this is the
default), positions, speed
 Go
o Let turtles go with speed and headings
o Randomly change speed and headings
 Sliders to control speed range and size of
heading changes
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Simple Model
• Built-in turtle variables (each turtle has its own)
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breed (sub-type)
Color (defaults to random for each turtle)
Heading (degrees, 0 = up, 90 = right)
hidden?
label, label-color
pen-mode, pen-size
shape
size
Who (a number)
xcor, ycor (by default 0, 0)
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Network Model
• Model will:
 Setup
o Create a set of turtles with random positions
o Create a random set of links between the turtles
 Each turtle has same chance of having links
 Go Once
o Pick a random turtle and a random one of its links,
and replace it within one attached preferentially
 Higher chance of linking to a turtle with the most links
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Network Model
• Start new model, save it to desktop as
Network
• Go to Model Library, open Network >
Preferential Attachment, copy layout and
limit-magnitude to clipboard and then into
Procedures window, and resave
 Will make a better World plot
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Network Model
• Agentset – a list set of agents
 Defaults to a random order but can be sorted
 Can be “traversed” – go through all agents in
the list by asking
 Can also pick a random agent from the list
 If we want angentset to persist, must set it to a
name we have previously declared in globals or
locally
• If we have picked a turtle, other turtles is an
agentset of all of the other turtles
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Agentsets and Lists
• Agentset: an unordered set of agents of any
type, including breeds
 Order is randomized on each use
 Can ask agentset [ list of commands ]
 Current agent in “ask” sequence: self
• List: an ordered list of any objects,
including agents
 Order is always the same unless you change it
 Can be sorted, etc.
 Current list item in “for each” sequence: ?
Return to slide 36
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Jumpers
• Transport
• Agents jump by patches left to right
• Have to wait at each patch for its individual wait
count, can’t jump on top of another
• Setup:
 Initialize patches with wait counts, color-code
• Go:
 Turtles jump to right if wait count reached and that
patch is empty
 After jumping off last patch on right, turtle dies
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And…
• 3D
• Numerical modeling (modeling number of
agents, etc.)
• Flying behind a turtle
• Back
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