CS108L Computer Science for All
Week 6 Video Summary
Abstraction in Modeling and Simulation
• Abstraction is Simplification
▫ Why?
• “Real world” problems are too complicated
• Must simply in order to model
• Ignoring some details or parts so that you can focus on what is
important to you at that time.
• As your interests change the amount of abstraction you need can
change too.
▫ Involves assumptions (things that you take to be true).
• Before abstraction
• During abstraction
• After abstraction during model development
• Assumptions are what the modeler assumes when moving from the real world
problem to a model.
• Sometimes a simplification
• Sometimes a specification
• Assumptions need to be carefully considered
• Affect the outcome of the model
• Should be validated
Variables and Scope
 Variable
• A container that holds a value that can be used and changed.
• Three Steps for Using Variables
▫ Declare – allocates the space and sets the variable name
▫ Initialize – set the initial value of the variable
▫ Get/Set– Use or change the value as the program is executed
• There are three types of NetLogo variables / they differ in scope
• Local Variables– used in the block where declared
• Agent Variables– used by specific type of agents
• Global Variable– used anywhere in the program
• Local Variables
• Can only be used in the procedure or command block where it is declared
• Can be used by any agent
• How to Use:
▫ Declare and initialize local variables using the let command
▫ Use the local variable in an expression or
▫ Modify a local variable using the set command
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Agent Variables
▫ Three types of Agent Variables
 Turtle Variables
• Each turtle has its own value for every turtle variable
 Patch Variables
• Each patch has its own value for every patch variable
 Link Variables
• Each link has its own value for every link variable
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Built-In Agent Variables
▫ Some agent variables are built-in to NetLogo
 Turtles: color, heading, xcor, ycor, etc.
 Patches: pcolor, pxcor, pycor, etc.
 Links: color, shape, etc.
User-defined variables
▫ Can make user defined agent variables by declaring them at the
beginning of the program
turtles-own [ energy ]
patches-own [ grass? ]
links-own [ strength ]
How to use an agent variable
▫ Declare the agent variable if necessary (do not declare built-in
variables)
▫ Change/modify the variable using the set command
▫ Generally can only be used/set by the specific agent
 Turtles  Turtle Variables
 Patches  Patch Variable
 Link  Link Variables
Exceptions
▫ An agent can set a different agent's variable by using the ask
command.
ask patches […….
ask turtle 5 [ set color blue] ;; changes turtle 5’s color to blue
…]
▫ One agent can read another agent’s variable using the of command
show [color] of turtle 5 ;; prints turtle 5’s color
▫ A turtle can read and set patch variables of the patch it is standing on
directly
ask turtles [set pcolor blue] ;;makes patches under all turtles blue
Global Variables
• The is only one value of each global variable at any time
• You can declare a global variable by
▫ Using an input device on the interface tab (switch, slider, chooser or
input box)
▫ In the code by using the globals keyword at the beginning of the code
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globals [NumTurtles]
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Can be used
▫ At any location in any procedure in the code
▫ By any agent
▫ Use by using the variable name
Modified by using
▫ In the Code, with the set command
set NumTurtles 100
▫ On the Interface tab with switch, slider, chooser or input box
Interface Input in NetLogo
• Can input Global Variables from the Interface tab of NetLogo
▫ A quick way to change variable value
▫ No recoding
• Four ways:
▫ Sliders
▫ Switches
▫ Choosers
▫ Input Boxes
• Sliders
▫ Slider sets a global variable value to a NUMBER
▫ Slider has a range of values
• from the minimum value
• to the maximum value.
▫ Move the slider to the desired value
• Increase by moving the slider to the right
• Decrease by moving to the left
▫ The current value of the variable is given on the right below the sliders
• Switches
▫ Switches are used to set the global variable to a boolean value (On/Off)
• Choosers
o Can set the Global variable to be any data type: strings, numbers,
booleans, or lists.
o Lets you choose from a list of choices.
o List is in a drop down menu.
 Input Boxes
o Input Boxes are used to enter the value global variables that contain
strings, numbers or colors.
• Strings can be
• simple strings
• commands/reporters (checks syntax)
• Any type number can be entered (more variety than sliders)
• Color input boxes offer a color chooser using the NetLogo color table.
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Interface Output in NetLogo
• Gathering data is an important part of modeling
• Easiest to Output to the Interface Tab
• Four ways/locations
▫ Monitor
▫ Graph
▫ Command Center
▫ Output Area
• Monitor
• Outputting to a Monitor is easiest.
• Requires no in-code output commands
• Just need the variable name
• Outputs the value of a variable
• Updated continuously as the model runs
• Graph
• Outputting to a Graph is slightly harder but…
 Can output more than one variable at a time
 A line for each variable
 Graphical information can be easier to understand
 Provides a history of output
• Outputs the value of an variable
• Updated continuously as the model runs
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