StarLogo

1. StarLogo Building a Modeling Construction Kit for Kids Andrew Begel University of California, Berkeley Agent Simulation Workshop October 16, 1999 The StarLogo Team at MIT: Prof. Mitchel Resnick Brian Silverman Andrew Begel Bill Thies Vanessa Colella

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 • 1990’s: *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 60’s • 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

7. 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.

8. 1 2 3 4 5 Cmd #1 Cmd #1 Cmd #1 Cmd #1 Cmd #1 Cmd #1 Cmd #1 Cmd #1 Cmd #1 Cmd #1 Cmd #2 Cmd #2 Cmd #2 Cmd #2 Cmd #2 Cmd #3 Cmd #3 Cmd #3 Cmd #3 Cmd #3 Cmd #2 Cmd #2 Cmd #2 Cmd #2 Cmd #2 Cmd #4 Cmd #4 Cmd #4 Cmd #4 Cmd #4 Cmd #4 Cmd #4 Cmd #4 Cmd #4 Cmd #4 Cmd #3 Cmd #3 Cmd #3 Cmd #3 Cmd #3 Cmd #5 Cmd #5 Cmd #5 Cmd #5 Cmd #5 Cmd #5 Cmd #5 Cmd #5 Cmd #5 Cmd #5 Cmd #6 Cmd #6 Cmd #6 Cmd #6 Cmd #6 Cmd #6 Cmd #6 Cmd #6 Cmd #6 Cmd #6 StarLogo Parallelism { Job #1 { Job #2 time

9. 1 2 3 4 5 Cmd #1 Cmd #1 Cmd #1 Cmd #1 Cmd #1 Cmd #1 Cmd #1 Cmd #1 Cmd #1 Cmd #1 Cmd #2 Cmd #2 Cmd #2 Cmd #2 Cmd #2 Cmd #3 Cmd #3 Cmd #3 Cmd #3 Cmd #3 Cmd #2 Cmd #2 Cmd #2 Cmd #2 Cmd #2 Cmd #4 Cmd #4 Cmd #4 Cmd #4 Cmd #4 Cmd #4 Cmd #4 Cmd #4 Cmd #4 Cmd #4 Cmd #3 Cmd #3 Cmd #3 Cmd #3 Cmd #3 Cmd #5 Cmd #5 Cmd #5 Cmd #5 Cmd #5 Cmd #5 Cmd #5 Cmd #5 Cmd #5 Cmd #5 Cmd #6 Cmd #6 Cmd #6 Cmd #6 Cmd #6 Cmd #6 Cmd #6 Cmd #6 Cmd #6 Cmd #6 CM2 *Logo Parallelism (SIMD) { Job #1 { Job #2 time Turtles run commands in lockstep. Each job executes in series.

10. 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. 1 2 3 4 5 Cmd #1 Cmd #1 Cmd #1 Cmd #1 Cmd #1 Cmd #1 Cmd #1 Cmd #1 Cmd #1 Cmd #1 Cmd #2 Cmd #2 Cmd #2 Cmd #2 Cmd #2 Cmd #3 Cmd #3 Cmd #3 Cmd #3 Cmd #3 Cmd #2 Cmd #2 Cmd #2 Cmd #2 Cmd #2 Cmd #4 Cmd #4 Cmd #4 Cmd #4 Cmd #4 Cmd #4 Cmd #4 Cmd #4 Cmd #4 Cmd #4 Cmd #3 Cmd #3 Cmd #3 Cmd #3 Cmd #3 Cmd #3 Cmd #5 Cmd #5 Cmd #5 Cmd #5 Cmd #5 Cmd #5 Cmd #5 Cmd #5 Cmd #5 Cmd #5 Cmd #6 Cmd #6 Cmd #6 Cmd #6 Cmd #6 Cmd #6 Cmd #6 Cmd #6 Cmd #6 Cmd #6 MacStarLogo Parallelism { { time Each job executes in series. Turtles are switched one after another. Turtles may get out of sync.

12. 1 2 3 4 5 Cmd #1 Cmd #1 Cmd #1 Cmd #1 Cmd #1 Cmd #1 Cmd #1 Cmd #1 Cmd #1 Cmd #1 Cmd #2 Cmd #2 Cmd #2 Cmd #2 Cmd #2 Cmd #2 Cmd #2 Cmd #2 Cmd #2 Cmd #2 Cmd #3 Cmd #3 Cmd #3 Cmd #3 Cmd #3 Cmd #3 Cmd #3 Cmd #3 Cmd #3 Cmd #3 Cmd #4 Cmd #4 Cmd #4 Cmd #4 Cmd #4 Cmd #4 Cmd #4 Cmd #4 Cmd #4 Cmd #4 Cmd #2 Cmd #1 Cmd #1 Cmd #2 Cmd #2 Cmd #1 Cmd #2 Cmd #2 Cmd #1 Cmd #1 Cmd #5 Cmd #5 Cmd #5 Cmd #5 Cmd #5 Cmd #5 Cmd #5 Cmd #5 Cmd #5 Cmd #5 StarLogo for Java Parallelism time All jobs are scheduled in parallel. Commands are switched one after another. Jobs may get out of sync.

13. 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. Patch Parallelism

14. 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

15. 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.

16. 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 waitdelay at the end. • When everything stops, monitors are run once more to show current values.

17. 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.

18. 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

19. 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).

20. 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

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

22. 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

23. 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

24. 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 doesn’t mean that turtle #2 talks to turtle #1.

25. 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

26. 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 partnerone-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

27. Grab! • StarLogo for Java breeds [girls guys] turtles-own [mygene child-gene father-color] to procreate if breed = girls [grabone-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

28. 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).

29. 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.

30. 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

31. 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

32. 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