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simulation of a predator turtle chasing its prey

import random
import time
def chase():

  # Create an amphitheater in which our turtles will fight to the death
  amphitheater = makeWorld()

  # Create two turtles: a predator and its prey
  predator = makeTurtle(amphitheater)
  prey = makeTurtle(amphitheater)

  # Position the turtles in their starting positions.
  # The predator is aiming at the prey; the prey is pointing South-East.
  penUp(predator)
  penUp(prey)
  x1 = random.choice(range(0, amphitheater.getWidth()/4))
  y1 = random.choice(range(0, amphitheater.getHeight()/4))
  x2 = amphitheater.getWidth()/2
  y2 = amphitheater.getHeight()/2
  moveTo(predator, x1, y1)
  moveTo(prey, x2, y2)
  turnToFace(predator, x2, y2)
  turnToFace(prey, amphitheater.getWidth(), amphitheater.getHeight())

  # Have the predator chase the prey and the prey run away until the
  # predator has closed to within tooth distance.
  while distanceBetween(predator, prey) > 5:

    # The prey panics and keeps changing direction.
    evasiveAngle = random.choice(range(-90, 90))
    turn(prey, evasiveAngle)
    forward(prey, 40)

    # The predator aims at where the prey is, not where it is moving to.
    # But if it is fast enough, it won't keep going round in a circle.
    turnToFace(predator, prey.getXPos(), prey.getYPos())

    # We need to stop the predator overshooting.
    # See what happens if you take this out.
    closingDistance = int(min(distanceBetween(predator, prey), 50))

    # Close in on the victim
    forward(predator, closingDistance)

    # Pause half a second so that we can watch what happens at leisure.
    time.sleep(0.5)

# Compute the Euclidean distance between turtles. Very like the distance between
# colors, except that they have three dimensions (RGB). Turtles live in a
# two-dimensional plane.
def distanceBetween(turtle1, turtle2):
  x1 = turtle1.getXPos()
  x2 = turtle2.getXPos()
  y1 = turtle1.getYPos()
  y2 = turtle2.getYPos()
  dist = math.sqrt((x1-x2)**2 + (y1-y2)**2)
  return dist

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• Spring 2007 materials last edited on 21 August 2007 at 10:44 pm by user-1121vkp.dsl.mindspring.com