Modelling and Simulations Agent-Based

Agent-Based

Modelling and Simulations

NetLogo

Agostino Poggi

Introduction

♦ is a programmable modeling environment for simulating complex systems

♦ Modelers can give instructions to hundreds or

thousands of independent agents operating in parallel

♦ Was created by Uri Wilensky in 1999

♦ Current release is 5.2

System Features

♦ Cross-platform

 Runs on different operating systems (e.g., Windows, Linux, MacOS, …)

♦ Web-enabled

 Runs within a web browser, but can be download and run locally

♦ Models can be saved as applets to be embedded in web pages

Language Features

♦ Is fully programmable

♦ Has a simple language structure

♦ Is a Logo dialect extended to support agents and parallelism

♦ Supports a unlimited numbers of agents and variables

♦ Supports double precision arithmetic

♦ Provides many built-in primitives

Agents

♦ The world is two dimensional and is divided up into a grid of patches

♦ Patches are agents that represent a square piece of the world over which turtles can move

♦ Turtles are agents that move around in the world

♦ Links are agents connecting pairs of turtles

♦ The observer is an agent that can create turtles and have read/write access to all the agents and

variables

Patches (1/2)

♦ Have a color defined by the pcolor variable

♦ Have a location defined by the pxcor and pycor constant variables

♦ The patch in the center of the world has coordinates (0, 0)

♦ The total number of patches is determined by the max-pxcor - min-pxcor and max-pycor - min-pycor pairs of variables

♦ Usually every patch has the same number of neighbor patches (toroidal world)

Patches (2/2)

Turtles

♦ Have an index defined by the who variable

♦ Have a color defined by the color variable

♦ Have a location defined by the xcor and ycor variables

 Its value can be a decimal number: it can be positioned at any point within a patch, default position is the center of the patch

♦ Have a direction defined by the heading variable

 Its value is a number greater than or equal to 0 and less than 360 (0 is north, 90 is east, …)

♦ Can be divided in breeds

♦ The total number of turtles is determined by the count turtles command

Links

♦ Represent “edges” on networks

♦ Can “move”, “die”, “grow”, …

♦ Have a start node and an end node represented by two turtles

♦ Can be directed or undirected

♦ Can be divided in breeds

♦ Can have different colors, thickness, …

Observer

♦ Can create new agents and controls

♦ Gives commands to patches, turtles and links

♦ Can change the values of variables

♦ Can produce information about the state of the simulations

♦ Is the interpreter between the user and the agents

Breed

♦ Allows to defines a subtype of turtles and links breed [wolves wolf]

breed [sheep a-sheep]

directed-link-breed [unis uni]

undirected-link-breed [bis bi]

♦ Each turtle has a breed variable if breed = wolves [ ... ]

♦ Each turtle can change breed set breed sheep

♦ Each breed can a specific default shape set-default-shape wolves “wolf"

Agent Set

♦ Is an homogeneous set of agents

♦ Some agent sets are defined by NetLogo environment turtles, patches and links

♦ Subset of turtles, patches and links are defined by the use of some specific commands

♦ Agent sets are assigned through the commands turtle-set, patch-set and link-set

Data Types

♦ Number values are used for defining calculations and are all floating point numbers

♦ Boolean values are used for defining conditions and correspond to the true and false constants that can be combined with the and, or, not and xor operators

♦ String values are used for defining names and

represent immutable sequence of characters that are create with double quotes and concatenate with the word command

List

♦ Allows to store multiple pieces of information in a single value

♦ Each piece of information can be values of any

possible type (i.e., a boolean, a number, a string, an agent, an agent set or even another list)

♦ Pieces of information are enclosed between square brackets

♦ Each piece of information is identified by an integer starting at 0

Variables

♦ Each turtle, patch and link has its set of variables

♦ Some variables are defined by NetLogo environment

♦ The turtles-own, patches-own and links-own keywords allow to define new variables

turtles-own [ energy speed ]

♦ Variables value can be modified using the set command set color red

set pcolor red

♦ A turtle can also read and set variables of its patch

Global & Local Variables

♦ If a variable is a global variable, there is only one

value for the variable and all the agents can access it

♦ A variable is made global by using the globals keyword at the beginning of the program code

globals [ clock ]

♦ Moreover, it is made global by adding at the interface level either a switch or a slider

♦ Global variables can by read and set at any time by any agent

♦ Local variable are defined inside procedures let limit 10

Tick Counter

♦ Usually simulations are discrete and time passes in discrete steps defined by a built-in tick counter

♦ The tick command advances the tick counter by 1 and usually also update the view

♦ The clear-all command clears the tick counter along with everything else

♦ The reset-ticks command resets only the tick counter

♦ The current value of the tick counter is provided by the ticks report and is shown in the NetLogo GUI above the view

Turtles Creation create-turtles 10

♦ Asks the Observer to create 10 turtles create-wolves 10

♦ Asks the Observer to create 10 wolves sprout 10

♦ Asks the current patch to create 10 turtles hatch 10

♦ Asks the current turtle to create 10 turtles

Links Creation create-link-with turtle 2

♦ Creates an undirected link between the caller and turtle 2

create-link-to turtle 2

♦ Creates a directed link from the caller to turtle 2 create-link-from turtle 2

♦ Creates a directed link from turtle 2 to the caller create-bi-with turtle 2

♦ Creates an undirected link (bis breed) between the caller and turtle 2

create-uni-to turtle 2

♦ Creates a directed link (unis breed) from the caller to turtle 2

Agent Set Creation (1/2) other turtles

♦ All the turtles excluding the current turtles-here

♦ The turtles on the current patch other turtles here

♦ The turtles on the current patch excluding the current patches at-points [[1 0] [0 1] [1 0] [0 1]] (neighbors4)

♦ The patches around the current patch turtles-on neighbors

♦ The turtles standing on the eight surrounding patches

Agent Set Creation (2/2) turtles in-radius 3

♦ The turtles less than 3 patches away patches with [pcolor = red]

♦ All the red patched

other turtles here with [color = red]

♦ The other red turtles on the current patch

turtles with [(xcor>0) and (ycor>0)and(pcolor=green)]

♦ The green turtles with coordinates greater than 0 [mylinks] of turtle 0

♦ The links connected to turtle 0

Agent Set Uses (1/3) ask turtles [ die ]

♦ Makes all the agents in the set do something ask one-of links [ die ]

♦ Picks a random agent from the set

ask min-one-of turtles [energy] [ die ]

♦ Picks the agent in the set with min/max of some attribute

show count turtles

♦ Finds out exactly how many agents there are in the set

Agent Set Uses (2/3)

show mean [energy] of turtles with [color = red]

♦ Computes (mean, median, min, max, sum,

standard-deviation, variance) over some attribute of the agents in the set

if any? turtles [ show count turtles ]

♦ Sees if the set of agents is empty

if member? self turtles [show "Yes, it's member !"]

♦ Sees whether the current agent is in a set

if all? turtles [color = red] [show "everyone is red!"]

♦ Sees if all the agents in a set satisfy a condition

Agent Set Uses (3/3) histogram [color] of turtles

♦ Makes a distribution histogram of a variable in a set ask (turtle-set self turtle 2 turtles with [color = red])

[ die ]

♦ Creates new sets from other sets and individual agents if (set1 = set2) [show "Identical sets !"]

♦ Compares (=, !=, <, >) two sets

Working with Lists (1/4) set mylist [2 7 5 Bob [3 0 -2]]

♦ Creates a constant list

set random-list list (random 10) (random 20)

♦ Creates a list of two random integers set mylist replace-item 2 mylist 10

♦ Replaces the third item of mylist with 10 set mylist lput 42 mylist

♦ Adds 42 as last item of mylist

Working with Lists (2/4) set mylist fput 12 mylist

♦ Adds 12 as first item of mylist set mylist but-last mylist

♦ Removes the last item from mylist set mylist but-first mylist

♦ Removes the first item from mylist foreach mylist show

♦ Shows the items of mylist

Working with Lists (3/4) map round [1.2 2.2 2.7]

♦ Builds the list [1 2 3]

map [? * ?] [1 2 3]

♦ Builds the list [1 4 9] (? Means the current item) member? 2 [1 2 3]

♦ Checks if 2 is member of the list [1 2 3]

position 2 [1 2 3]

♦ Gets the position of 2 in the list [1 2 3]

Working with Lists (4/4) sort turtles

♦ Builds a list of turtles sorted in ascending order by who variable

sort-by [[size] of ?1 < [size] of ?2] turtles

♦ Builds a list of turtles sorted in ascending order by size variable

list [1 2 3] [4 5 6]

♦ Builds the list [[1 2 3] [4 5 6]]

sentence [1 2 3] [4 5 6]

♦ Builds the list [1 2 3 4 5 6]

Commands & Reporters (1/3)

♦ Commands are actions for the agents to carry out

♦ Reporters carry out some operation and report a result either to a command or another reporter

♦ Commands and reporters defined by NetLogo environment are called primitives

♦ User’s defined commands and reporters are called procedures

Commands & Reporters (2/3)

♦ Procedures can take inputs value

♦ Each procedure is composed of

 The keyword to or to-report

 The name of the procedure

 The optional arguments

 A sequence of commands

 The keyword end

♦ After its definition, a procedure can be used elsewhere in the program

Commands & Reporters (3/3)

Conditionals

Loops

File I/O (1/3) file-open myfile.txt

♦ Opens a file file-close

♦ Closes a file file-close-all

♦ Closes all the files user-file

♦ Opens a window for opening a file user-new-file

♦ Opens a window for creating a file

File I/O (2/3)

File I/O (3/3) file-read

♦ Reads the next token in the file and interprets it in the appropriate type

file-read-line

♦ Reads the next line in the file as a string file-read-characters 4

♦ Reads the next 4 characters (including newlines and spaces) in the file as a string

file-at-end?

♦ Checks if there are no more characters left to read in the current file

Graphical User Interface

toolbar

Toolbar Elements (1/2)

Button Allow the execution of a procedure

Slider Is a global variable which is accessible by all agents

Switch Is a visual representation for a true/false global variable Chooser Allows to choose a value for a global variable from a list

of choices

Input Is a global variable that contains a string or a number Monitor Displays display the current value of a variable

Plot Displays the history of the value of a variable

Output Is a scrolling area of text which can be used to create a log of activity in the model

Note Allows the addition of text labels to the Interface tab

Toolbar Elements (2/2)

button

switcher

slider output

plot

Code View

Interface View (1/3)

Interface View (2/3)

Interface View (3/3)

Model Setting

Info View

Model Library

Turtle Shape Editor

A Multi-Colors World (1/2) patches-own [ soil-type ]

to setup clear-all

ask patches [

set soil-type random 4

ifelse soil-type = 0 [ set pcolor YELLOW ] [ ifelse soil-type = 1 [ set pcolor RED ]

[ ifelse soil-type = 0 [ set pcolor GREEN ] [ set pcolor BLUE ]

] ] end]

A Multi-Colors World (2/2)

Population Evolution (1/3) breed [persons person]

persons-own [ age ] to setup

clear-all

set-default-shape persons "person“

reset-ticks end

to go

… end

Population Evolution (2/3) to go

create-persons 10 [ set xcor random-xcor set ycor random-ycor set color RED

set age 0 ]

ask persons [ set age (age + 1) ] ask persons with [age > 10]

[ die ]

tickplot count persons end

Population Evolution (3/3)

Random Turtle Movement (1/2) to setup

clear-all

create-turtles 100 reset-ticks

end to go

ask turtles [

fd 10 rt random 10 lt random 10 show absolute-value xcor ]

endtick

to-report absolute-value [number]

ifelse number >= 0 [ report number ] [ report (- number) ] end

Random Turtle Movement (2/2)

Prey Predator Model (1/7)

breed [sheep a-sheep]

breed [wolves wolf]

sheep-own [ age ]

wolves-own [ energy age ] to setup

...

end to go

...

end

Prey Predator Model (2/7)

to setup clear-all

set-default-shape sheep "sheep"

create-sheep initial-sheep [ set color white

set size 1.5

setxy random-xcor random-ycor set age 0 ]

set-default-shape wolves "wolf"

create-wolves initial-wolves [ set color brown

set size 1.5

setxy random-xcor random-ycor set age 0

set energy random (3 * energy-per-sheep-eaten) ]

Prey Predator Model (3/7) to go

if not any? turtles [ stop ] ask turtles [ take-a-step ]

ask sheep [ try-to-make-bunnies ] ask wolves [

set energy (energy - 1) try-to-eat

try-to-make-kittens ]

ask turtles [set age (age + 1) ]

ask sheep [ if age > max-sheep-age [ die ] ] ask wolves [

if age > max-wolves-age [ die ] if energy < 0 [ die ] ]

endtick

Prey Predator Model (4/7) to take-a-step

right random 50 left random 50 forward 1

end

to try-to-make-bunnies

if random-float 200 < sheep-birth-rate [ hatch-sheep 1 [

rt random-float 360 fd 1

set age 0 ] ]

end

Prey Predator Model (5/7) to try-to-eat

let prey one-of sheep-here if prey != nobody [

ask prey [ die ]

set energy energy + energy-per-sheep-eaten ] end

to try-to-make-kittens

if energy > wolves-energy-to-reproduce [ hatch-wolves 1 [

rt random-float 360 fd 1set age 0

set energy energy / 2 ] set energy energy / 2 end]

Prey Predator Model (6/7)

Prey Predator Model (7/7)

Walking on a Network (1/4) breed [nodes node]

breed [walkers walker]

walkers-own [location]

to setup end …

to layout

layout-spring nodes links 0.5 2 1 end

to go end ---

Resistance to change in their length for a change of 1 unit

Zero force length

Repulsion force that 2

nodes at a distance of 1 unit will exert on each other

Walking on a Network (2/4) to setup

clear-all

set-default-shape nodes "circle"

create-nodes 30 [ set color blue ]

ask nodes [ create-link-with one-of other nodes ] repeat 500 [ layout ]

ask nodes [ setxy 0.95 * xcor 0.95 * ycor ] create-walkers 5 [

set color red

set location one-of nodes move-to location

]reset-ticks end

Walking on a Network (3/4) to go

ask links [ set thickness 0 ] ask walkers [

let new-location one-of [link-neighbors] of location ask [link-with new-location] of location [

set thickness 0.5 ]

face new-location move-to new-location set location new-location

] tick end

Walking on a Network (4/4)