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
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Big Ideas
• StarLogo: a programmable modeling
environment
• Intended for non-expert users and nonprogrammers
– Great for kids, great for researchers!
• Emphasis on decentralized behaviors with
local interactions.
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Talk Outline
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History of StarLogo
Models of Parallelism
Parallel Communication
Parallel Debugging
StarLogo Workshop
StarLogo for Java
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History
• 1990’s:
• 1994:
• 1999:
*Logo on the Connection Machine 2
(a massively parallel computer)
MacStarLogo on 68K and PPC Macs
StarLogo in Java
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Logo
• Developed by Feurzeig and Papert in 60’s
• Based on Lisp
– simpler syntax
– incorporates elements of natural language.
• Interactive programming environment
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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
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time
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CM2 *Logo Parallelism (SIMD)
Job #1
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Turtles run commands in lockstep.
Each job executes in series.
time
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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.
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MacStarLogo Parallelism
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Each job executes in series.
Turtles are switched one after another.
Turtles may get out of sync.
time
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StarLogo for Java Parallelism
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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.
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Observer Execution
• There’s only one observer.
• It’s 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,
1. Run turtles as many times
as you can for 1/60th of a
second
2. Run patches once
3. 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: It’s 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.
• There’s 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 [set season
spring [set season
summer [set season
end
winter]
spring]
summer]
fall]]
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] ;; I’m 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-Turtle
Communication 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 doesn’t
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 > 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 > 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
o 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
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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, randomgaussian, 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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What’s 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
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