HW 7

User-Interface Programming

Introduction.  In this assignment you will take an existing, working program and add some user controls to it.

DebuggingExercises: There are no debugging exercises this week.

Programming Exercise. Examine the program FoxesAndRabbits.py which implements a predator-prey simulation. In the world of this simulation animals live a predictable life -- each year they try to eat to gain energy. If they are old enough and have enough energy they reproduce. When they become too old they die. This cycle is represented by the method oneYear(self) of class Animal. Class Fox and class Rabbit are subclasses of Animal, so they both inherit this method.

Animals are kept in an AnimalList, which is comparable to the AnimationLists that we kept in other animated programs. When animals are first born they are put in a NewAnimalList rather than the main animal list, and only enter the main list after their first year. This prevents a situation in which new rabbits are born into the end of the AnimalList and then have babies themselves which further expand the AnimalList. If left unchecked this could become a cycle in which the AnimationList gets longer and longer and we never return to the beginning of the list.

In the simulation foxes are represensted by red squares, rabbits by blue squares, and grass by green ones. The rabbits eat only grass. The foxes eat only rabbits. Each animal can only eat if its food is in a square that is adjacent to it. The animals eat by moving to the square containing the food. Rabbits, of course, are killed when the foxes eat them. The grass grows back as soon as the rabbit on it mvoes on. Rabbits only move by finding an adjacent grass square and moving to it. If foxes don't find an adjancent rabbit they can still move in a random direction to any empty adjacent square as they search for food.

If you run the simulation you will see that the only user control it provides is a Quit button. The main() function initializes a few variables, class function RandomlySeedGrid() to put an initial assortment of foxes and rabbits only the grid, and then immediately starts running the simulation one year at a time. Your job is to make the following changes:

• There is a variable PAUSE that is set to False in main(). Change this to initialize PAUSE to True, so the program starts in pause-mode. Then add a Run button; this should have a callback function that changes PAUSE to False. Your callback function should first declare PAUSE to be global, so the system knows that it is dealing with the global PAUSE variable and not a variable local to this function. Add another button labeled Paue. The callback for this one should chage PAUSE back to True.
  
• Remove the call in main() to RandomlySeedGrid(). Add a button called "Seed" that calls this function. Now when you start up the program the entire grid should be green until you click the Seed button.
  
• Give scale widgets that allow the user to enter the number of Foxes and the number of Rabbits to use for seeding the grid. These should have callback functions that change the varaibles InitialNumberRabbits and InitialNumber Foxes which are used in RandomlySeedGrid().Note that if you give these IntVar control variables, you can control the initial value of the slider. For example, if the IntVar you use is called "variable", then variable.set(30) will set the slider to 30.
  
• After all of these changes your program should behave as follows:
  It starts up paused.
  It allows the user to set the number of foxes and rabbits at the start.
  When you hit the Run button the simulation starts; you can pause it with the Pause button.
  
• Feel free to add more controls. You might want to give the user the optionof changing the number of children in a litter of either Foxes or Rabbits. You might want to add controls for the age at which the animals die or the probability that they actually have chilcren.

Program BasicGraphics.py has code for buttons and scale widgets. You can use this as a template for the controls you need to add for this program.

This homework should be handed in by midnight of Friday, May 9. This is the last day of classes. I will accept late work up to the end of Reading Period, but no later.