StarLogo

1
StarLogo
Building a Modeling Construction Kit for Kids
The StarLogo Team at MIT Prof. Mitchel
Resnick Brian Silverman Andrew Begel Bill
Thies Vanessa Colella

• Andrew Begel
• University of California, Berkeley
• Agent Simulation Workshop
• October 16, 1999

2
Big Ideas

• StarLogo a programmable modeling environment
• Intended for non-expert users and non-programmers
• Great for kids, great for researchers!
• Emphasis on decentralized behaviors with local
  interactions.

3
Talk Outline

• History of StarLogo
• Models of Parallelism
• Parallel Communication
• Parallel Debugging
• StarLogo Workshop
• StarLogo for Java

4
History

• 1990s Logo on the Connection Machine 2 (a
  massively parallel computer)
• 1994 MacStarLogo on 68K and PPC Macs
• 1999 StarLogo in Java

5
Logo

• Developed by Feurzeig and Papert in 60s
• Based on Lisp
• simpler syntax
• incorporates elements of natural language.
• Interactive programming environment

6
Turtle Logo

• Turtle can move around a grid-based world.
• The turtle is an object to think with.
• body syntonics
• Example Code
• to square
• pendown
• repeat 4 forward 10 right 90
• end

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StarLogo

• Thousands of turtles instead of just one (can be
  organized in groups called breeds).
• Background grid of patches can run Logo code.
• The user is the observer and can discover and
  modify global characteristics of the model.

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StarLogo Parallelism

Job 1

Job 2
time
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CM2 Logo Parallelism (SIMD)

Job 1

Job 2
time
Turtles run commands in lockstep. Each job
executes in series.
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Simulating Parallelism

• How do you simulate parallelism on a computer
  with one processor?
• Our goal is realistic looking parallelism.
• Preemptive multi-threading
• Switch threads every n milliseconds.
• Cooperative multi-threading
• Switch threads at carefully chosen program
  points.
• Fine-granularity vs. coarse granularity
• We context switch after each command, but not
  each reporter.

11
MacStarLogo Parallelism


time
Each job executes in series. Turtles are switched
one after another. Turtles may get out of sync.
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StarLogo for Java Parallelism
time
All jobs are scheduled in parallel. Commands are
switched one after another. Jobs may get out of
sync.
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Patch Parallelism

• CM2
• All patches execute the same code in lockstep.
• Mac
• Each patch runs through the code one by one.
• Context switch after each patch has finished.
• Java
• Patches may no longer run code.

14
Observer Execution

• Theres only one observer.
• Its like a lifeguard sitting in a high chair at
  (0, 0).
• May view and modify global characteristics of the
  model.
• Create turtles.
• Gather statistics about turtles and patches.
• Performs various auxiliary functions
• Plotting, Movies, File I/O, Data Collection

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Putting It All Together

• In MacStarLogo, how do we run the turtles,
  patches and observer?

• Forever buttons
• In a loop,
• Run turtles as many times as you can for 1/60th
  of a second
• Run patches once
• Run one observer forever button

• Command Center and Buttons
• Observer code interrupts loop.
• Turtle or patch commands are run after forever
  button code have finished running once.
• Only one command center function may be running
  at any time.

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Putting It All Together (2)

• In StarLogo for Java
• All jobs are scheduled in a round-robin queue.
• Each job has equal priority.
• Forever buttons are the same as normal buttons,
  but the code has a loop button-code around
  it.
• Monitors spawn jobs, too.
• While anything is running, monitors are run in a
  loop with a wait delay at the end.
• When everything stops, monitors are run once more
  to show current values.

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Model Timing

• How do you relate real time (in seconds,
  minutes, hours, days or years) to model time
  (in observer/turtle commands)?
• Answer Its not easy.
• StarLogo is qualitative, not quantitative.
• One idea Use the observer to time how long the
  turtles take to finish one cycle.

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Parallel Communication

• Goal Turtles must communicate with each other.
• Message passing
• Action at a distance
• How can we do it?
• 1. Set a global variable
• 2. Set a patch variable
• 3. Set a turtle variable

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Communicating Through Globals

• Visible from anywhere in the world.
• Theres only one copy, so it better not change
  quickly (else only monotonically) in order that
  all turtles have a chance to see it.
• Example (next slide).

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globals season fall winter spring summer to
go every 10 change-seasons end to
change-seasons case season fall set season
winter winter set season spring spring
set season summer summer set season
fall end to grow-grass case season spring
repeat 100 plant-grass summer repeat 85
plant-grass fall turn-all-grass-brown
winter kill-all-grass end
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Communicating Through Patches

• Only visible on that patch
• Useful for communicating information to all
  turtles on that location (i.e. infection)
• Example (next slide)

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patches-own sick-here? turtles-own sick? to
infect ifelse sick? set sick-here? true
Im sick. if sick-here? set sick?
true healthy wiggle end to wiggle right
random 100 left random 100 if sick? and
count-turtles-here-with sick? 1 set
sick-here? false forward 1 end
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Communication via Turtles

• Directly read and modify other turtles
  variables.
• turtles-own dead?
• to kill turtle-id
• set dead?-of turtle-id true
• end
• to check-if-dead
• if dead? die
• end

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Turtle-TurtleCommunication Issues

• Must be able to find a turtle to talk to it.
• one-of-turtles-here, one-of-frogs,
  one-of-turtles-with color red
• Must remember its name to talk to it more than
  once.
• i.e. one-of-turtles-here changes over time.
• Other turtles never stop moving.
• Communication is asymmetric.
• Just because turtle 1 talks to turtle 2 doesnt
  mean that turtle 2 talks to turtle 1.

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Example Turtle Mating

• Buggy MacStarLogo code
• breeds girls guys
• turtles-own father-color mygene child-gene
• to procreate
• ask-girls
• if count-guys-here gt 1
• setfather-color color-of one-of-guys-here
• setchild-gene
• combine mygene mygene-of
  one-of-guys-here
• hatch ifelse (random 2) 0
• setbreed guys
• setbreed girls
• setmygene child-gene
• setcolor father-color
• end

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Example Turtle Mating (2)

• Correct MacStarLogo code
• breeds girls guys
• turtles-own partner father-color mygene
  child-gene
• to procreate
• ask-girls
• if count-guys-here gt 1
• set partner one-of-guys-here
• setfather-color color-of partner
• setchild-gene
• combine mygene mygene-of partner
• hatch ifelse (random 2) 0
• setbreed guys
• setbreed girls
• setmygene child-gene
• setcolor father-color
• end

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Grab!

• StarLogo for Java
• breeds girls guys
• turtles-own mygene child-gene father-color
• to procreate
• if breed girls
• grab one-of-guys-here
• set father-color color-of partner
• set child-gene
• combine mygene mygene-of partner
• hatch ifelse (random 2) 0
• set breed guys
• set breed girls
• set mygene child-gene
• set color father-color
• end

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Parallel Debugging

• In MacStarLogo, with 2000 turtles, how do you
  figure out if something went wrong?
• Stack overflow (too many nested functions) and
  divide by zero in turtles and patches are
  ignored.
• Unexpected behaviors due to not knowing how the
  compiler interpreted your code.
• Look at turtle or patch state
• Oops, no print capability for turtles or patches.
• Use turtle monitors to view all variables for a
  turtle.
• Use command center to ask turtles or patches to
  set observer variables (or set turtle variables
  that are visible from the turtle monitor).

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Parallel Debugging (2)

• Java StarLogo
• Simpler programming model (separate turtle and
  observer procedures) to eliminate certain kinds
  of programming bugs.
• Turtles and observer can use print (output shows
  up in the appropriate command center).
• Runtime errors in turtles and observer pop up in
  a dialog box.
• (What happens if all 2000 turtles have the error?
  2000 dialog boxes?)
• Much better compiler error messages. They even
  report the line number of the error!
• Turtle monitors and patch monitors will be added
  soon.

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StarLogo for Java New Features

• Works on PC, Mac and Unix!
• Rectangular (non-square) patch grid.
• Turtles and observer can play sounds.
• (count, one-of, list-of)-(turtles, breeds)-(here,
  at, towards) reporters.
• 64-bit double math.
• Unlimited number of turtles and number of
  variables.
• All math and list operations work for both
  turtles and observer.
• New primitives case, let, loop, wait-until,
  random-gaussian, pick, kill, nmin, nmin4, nmax,
  nmax4, diffuse4

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Workshops

• Teacher and student workshops held at Santa Fe
  Institute in Summer 99.
• Learning through Adaptive Agent Computer
  Models(Pictures http//www.taumoda.com/web/sfi99
  /)
• Run by Vanessa Colella, Eric Klopfer and Monica
  Linden from MIT, Larry Latour from U. Maine, and
  Nigel Snoad from SFI
• Project Building (StarLogo Workbook Challenges)
• Group Activities (StarPeople)
• Predator/Prey Badge Activity

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Whats next?

• Jan 2000 Finish StarLogo for Java 1.0
• Plotting, Shapes, Paint Tools, Turtle and Patch
  Monitors, Output and Information Windows
• StarLogo Project Web Player
• GIS Support
• Finish StarLogo Workbook
• For more information
• http//www.media.mit.edu/starlogo