Simulation and Modeling: Predator-Prey Processing Lab IS 101Y/CMSC 101 Computational Thinking and Design Tuesday, September 23, 2014 Carolyn Seaman Susan Martin University of Maryland, Baltimore County

What Does That Mean?? “It looks like you’re mixing “active” and “static” modes.” “static” mode is when your Processing program is just a list of instructions – those instructions are run once and then the program ends “active” mode is when you define the setup() and/or draw() functions and the program runs continuously looking for input If you have any functions defined, you can’t have commands outside those functions – you need to use setup() and draw() If you want anything to happen after setup() runs (e.g., when a key is pressed), you must have a draw() function (even if it’s empty)

Today’s Concepts Simulating processes Using data: Visualizing data (generating graphs) Outputting data (for use by Excel or other analysis programs) Code reading: Understanding Modifying

Simulating Processes Process model: We can represent “states” as collections of variables Semester game: Happiness, grades, wealth We can represent “actions” as choices from a list, parameter settings,... Semester game: Hours studying, hours in class... State1State2State3 Action1Action2Action3...

Data Representations for Simulations States: set of variables x, y, z,... at each time t x 1, x 2, x 3,... (values at time 1, 2, 3...) Can represent the sequence of values for state variable x as an array x[], indexed by time Multiple state variables  multiple arrays We’ll talk about arrays next time!! Alternative: if we don’t need to remember all of the values, we can just represent one or two values for each state variable: currentX currentX and prevX (if we want to measure/track change over time)

Visualizing Data How might we want to see (say) happiness, grades, and wealth over time? One way: table of numbers Another way: as a graph TimeHappinessGradesWealth 190%60%$ %70%$ %70%$ %80%$ %90%$200

Outputting Data Tables in Processing // open file for recording data PrintWriter OUTPUT = createWriter (FILENAME); Example: PrintWriter datafile= createWriter ("outfile.csv”); // print to output file OUTPUT.println (OUTPUTSTRING); Example: datafile.println (”time, happiness, grades, wealth”); Better example: datefile.println (“Your happiness level for this week is “, happiness);

Visualizing Data in Processing Graphs are just lines! Tricky part: figuring out how a particular state variable value will map to a screen location Time value has to “scale” to the width of the screen State variable value has to “scale” to the height of the screen

Graphing in Processing: Quadratic Example // Graph the function y=x^2 - 10, x= // Range of function: [-10, 390] void setup() { float x, prevX; float y, prevY; size (500, 500); // Generate the first y value and save it as prevY prevY = (-20*-20) - 10; // for each value of x, draw a line from the // previous (x, y) to the new (x,y) for ( x=-19 ; x<=20 ; x++ ) { y = x*x – 10; drawLine (x-1, prevY, x, y); prevY = y; } void drawLine (float prevX, float prevY, float x, float y) { line (screenX (prevX), screenY (prevY), screenX (x), screenY (y)); } // Scaling to the display window // Let's say y=400 will appear at screenY = 50 // y=0 will appear at screenY = 450 // y=-10 will appear at screenY=460 // So screenY = 450 – y // Let's put x=-20 at screenX = 50 // x=0 at screenX = 250 // x=20 at screenX = 450 // So screenX = (x+20)* float screenX (float x) { return ( (x+20)* ); } float screenY (float y) { return ( 450-y ); }

The Predator-Prey Cycle Lotka-Volterra equations “Actions” (fixed): birth rate (b pred, b prey ) and death rate (d pred, d prey ) of predators and prey States: population (n pred, n prey ) of predators and prey n prey = n prey + b prey *n prey - (d prey *n prey *n pred ) n pred = n pred + b pred *n pred *n prey - (d pred *n pred )

Code Reading First, just read the code, by yourself Break it up into chunks Recognize patterns from the Quadratic example Read the comments for clues Pair up – designate #1 and #2 #1 explains the code to #2, #2 corrects and asks questions Together, answer the following questions: What part of the code draws the blue line? What part of the code makes sure the graph doesn’t go up above the top of the screen? What part of the code makes sure the graph doesn’t go off the right hand edge of the screen? Where in the code calculates the numbers of predators and prey?

Modifying Code Download the.pde file from Blackboard Make the following modifications: Make the predator line green and the prey line yellow Ask the user to input the maximum number of cycles Replace the “while” loop with a “for” loop (the program should behave exactly the same way) Draw X and Y axes with meaningful labels and markings Test all your modifications by changing the values of height, width, border, max_cycles, max_pred, max_prey – it should still work, but the graph will look different