8. Case Study: Catch

8.1. Getting started

In our first case study we will build a small video game using the facilities in the graphics package. The game will shoot a ball across a window from left to right and you will manipulate a mitt at the right side of the window to catch it.

8.2. Using while to move a ball

while statements can be used with graphics to add motion to a program. The following program moves a black ball across an 800 x 600 pixel graphics window. Add this to a file named pitch.py:

from graphics import *
from time import sleep

def main():
    window = GraphWin("Catch", 800, 600)
    window.setBackground("yellow")

    ball_x = 10
    ball_y = 300
    center = Point(ball_x, ball_y)
    ball = Circle(center, 10)
    ball.setFill("black")
    ball.draw(window)
    dx = 4
    dy = 1

    while ball_x < 810:
        ball.move(dx, dy)
        ball_x += dx
        ball_y += dy
        sleep(1/120)

    window.getMouse()
    window.close()

main()

As the ball moves across the screen, you will see a graphics window that looks like this:

ball on yellow background

Trace the first few iterations of this program to be sure you see what is happening to the variables x and y.

Some new things to learn about graphics from this example:

  • GraphWin takes arguments for the title, width and height of the graphics window and it returns a window that you can draw on.
  • window = GraphWin stores the graphics window in the variable named window
  • window.setBackground calls a method of the window object called setBackground with a string representing the name of a color as an argument. The result is that the background of the window is painted that color.
  • center = Point stores a point in a variable named center
  • Point takes two arguments: the x and y values of the position of the point on the graphics window.
  • ball = Circle stores the circle in a variable named ball so that it can be referenced later.
  • Circle takes two arguments: the first argument is a Point and the second argument is the radius of the circle.
  • ball.setFill calls a method of the ball object that fills the ball with whatever color is passed as an argument.
  • ball.draw calls a method of the ball object that draws the ball into the graphics window provided as an argument.
  • The ball.move method moves the ball a distance dx in the x direction and dy in the y direction.
  • The sleep function is used to delay the action in a graphics program. The argument to sleep is the number of seconds to wait. Here, we wait 1/120 of a second: a small amount, but enough to make the ball move smoothly across the screen. Try the program without the sleep function and see how fast the ball moves across the screen.
  • window.getMouse() waits for the user to click the mouse until moving until the next line. Without this line, the graphics window will close immediately after finishing the loop making it hard to see what just happened.
  • window.close() closes the graphics window

8.3. Varying the pitches

To make our game more interesting, we want to be able to vary the speed and direction of the ball. We can use the randint function in the random library to choose a random number from a range between low and high. To see how this works, run the following program:

from random import randint

def main():
    i = 0
    while i < 10:
        print(randint(-5, 5))
        i += 1

main()

Each time the function is called a more or less random integer is chosen between -5 and 5. When we ran this program we got:

-2
-1
-4
1
-2
3
-5
-3
4
-5

You will probably get a different sequence of numbers.

Let’s use randint to vary the direction of the ball. Replace the line in pitch.py that assigns 1 to y:

dy = 1

with an assignment to a random number between -4 and 4. Be sure to add the line from random import randint to the top of your program.

dy = randint(-4, 4)

8.4. Making the ball bounce

Running this new version of the program, you will notice that ball frequently goes off either the top or bottom edges of the screen before it completes its journey. To prevent this, let’s make the ball bounce off the edges by changing the sign of dy and sending the ball back in the opposite verticle direction.

Add the following as the first line of the body of the while loop in pitch.py:

if ball_y >= 590 or ball_y <= 10:
    dy *= -1

Run the program several times to see how it behaves.

8.5. Responding to the keyboard

The following program creates a circle (which will become our mitt in the game of catch) which responds to keyboard input. Pressing the j or k keys moves the mitt up and down, respectively. Add this to a file named mitt.py:

from graphics import *

def main():
    window = GraphWin("Catch", 800, 600)
    window.setBackground("yellow")

    mitt_x = 780
    mitt_y = 300
    mitt = Circle(Point(mitt_x, mitt_y), 20)
    mitt.draw(window)
    pressed_escape = False  # a flag variable to indicate that Escape was pressed

    while not pressed_escape:
        key_pressed = window.checkKey()
        if key_pressed == 'k' and mitt_y <= 580:
            dy += 5
        elif key_pressed == 'j' and mitt_y >= 20:
            dy -= 5
        elif key_pressed == 'Escape':
            pressed_escape = True
        else:
            dy = 0

        mitt.move(0, dy)
        mitt_y += dy

    window.close()

main()

Run mitt.py, pressing j and k to move up and down the screen.

8.6. Checking for collisions

The following program moves two balls toward each other from opposite sides of the screen. When they collide, both balls disappear and the program ends and waits for the user to click the mouse in the window:

from graphics import *
from time import *

def distance(x1, y1, x2, y2):
    return ((x2 - x1)**2 + (y2 - y1)**2)**0.5

def main():
    window = GraphWin("Catch", 800, 600)
    window.setBackground("yellow")

    ball1_x = 10
    ball1_y = 300
    ball1 = Circle(Point(ball1_x, ball1_y), 10)
    ball1.setFill("black")
    ball1_dx = 4
    ball1.draw(window)

    ball2_x = 790
    ball2_y = 300
    ball2 = Circle(Point(ball2_x, ball2_y), 10)
    ball2.setFill("black")
    ball2_dx = -4
    ball2.draw(window)

    collision = False  # have the balls collided?

    while ball1_x < 810 and not collision:
        ball1.move(ball1_dx, 0)
        ball2.move(ball2_dx, 0)
        ball1_x += ball1_dx
        ball2_x += ball2_dx
        if distance(ball1_x, ball1_y, ball2_x, ball2_y) <= 20:
            ball1.undraw()
            ball2.undraw()
            collision = True
        else:
            sleep(1/120)

    window.getMouse()
    window.close()

main()

Put this program in a file named collide.py and run it.

8.7. Putting the pieces together

In order to combine the moving ball, moving mitt, and collision detection, we need a single while loop that does each of these things in turn:

from graphics import *
from random import randint
from time import *

def distance(x1, y1, x2, y2):
    return ((x2 - x1)**2 + (y2 - y1)**2)**0.5

def main():
    window = GraphWin("Catch", 800, 600)
    window.setBackground("yellow")

    ball_x = 10
    ball_y = 300
    ball = Circle(Point(ball_x, ball_y), 10)
    ball.setFill("black")
    dx = 4
    dy = randint(-4, 4)
    ball.draw(window)

    mitt_x = 790
    mitt_y = 300
    mitt = Circle(Point(mitt_x, mitt_y), 20)
    mitt_dy = 0
    mitt.draw(window)

    # the game is over if the user presses Escape, misses the ball,
    # or catches the ball.
    game_over = False

    while not game_over:
        #move the ball
        if ball_y >= 590 or ball_y < 10:
            dy *= -1
        ball_x += dx
        ball_y += dy
        ball.move(dx, dy)
        if ball_x > 810:
            game_over = True  # the ball went past the paddle

        #check the mitt
        key_pressed = window.checkKey()
        if key_pressed == 'k' and mitt_y <= 580:
            mitt_dy += 5
        elif key_pressed == 'j' and mitt_y >= 20:
            mitt_dy -= 5
        elif key_pressed == 'Escape':
            game_over = True  # the user pressed Escape
        else:
            mitt_dy = 0

        if not game_over:
          mitt_y += mitt_dy
          mitt.move(0, mitt_dy)

          #check if we caught the ball
          if distance(ball_x, ball_y, mitt_x, mitt_y) <= 30:
              ball.undraw()
              game_over = True
          else:
              sleep(1/120)

    window.getMouse()
    window.close()

main()

Put this program in a file named catch.py and run it several times. Be sure to catch the ball on some runs and miss it on others.

8.8. Displaying text

This program displays scores for both a player and the computer on the graphics screen. It generates a random number of 0 or 1 (like flipping a coin) and adds a point to the player if the value is 1 and to the computer if it is not. It then updates the display on the screen.

from graphics import *
from random import randint
from time import sleep

def main():
    window = GraphWin("Catch", 800, 600)
    window.setBackground("yellow")

    player_score = 0
    comp_score = 0

    player = Text(Point(110, 570), "Player: %d Points" % player_score)
    player.setSize(24)
    player.draw(window)
    computer = Text(Point(640, 570), "Computer: %d Points" % comp_score)
    computer.setSize(24)
    computer.draw(window)

    while player_score < 5 and comp_score < 5:
        sleep(1)
        winner = randint(0, 1)
        if winner == 1:
            player_score += 1
            player.setText("Player: %d Points" % (player_score))
        else:
            comp_score += 1
            computer.setText("Computer: %d Points" % (comp_score))

    if player_score == 5:
        result_text = "Player Wins!"
    else:
        result_text = "Computer Wins!"

    result = Text(Point(340, 290), result_text)
    result.setSize(32)
    result.draw(window)

    window.getMouse()
    window.close()

main()

Put this program in a file named scores.py and run it.

We can now modify catch.py to diplay the winner. Immediately after the if ball_x > 810: conditional, add the following:

text = Text(Point(340, 290), "Computer Wins!")
text.setSize(32)
text.draw(window)

It is left as an excercise to display when the player wins.

8.9. Abstraction

Our program is getting a bit complex. To make matters worse, we are about to increase its complexity. The next stage of development requires a nested loop. The outer loop will handle repeating rounds of play until either the player or the computer reaches a winning score. The inner loop will be the one we already have, which plays a single round, moving the ball and mitt, and determining if a catch or a miss has occured.

Research suggests there are clear limits to our ability to process cognitive tasks (see George A. Miller’s The Magical Number Seven, Plus or Minus Two: Some Limits on our Capacity for Processing Information). The more complex a program becomes, the more difficult it is for even an experienced programmer to develop and maintain.

To handle increasing complexity, we can wrap groups of related statements in functions, using abstraction to hide program details. This allows us to mentally treat a group of programming statements as a single concept, freeing up mental bandwidth for further tasks. The ability to use abstraction is one of the most powerful ideas in computer programming.

Here is a completed version of catch.py:

from graphics import *
from random import randint
from time import *

def distance(x1, y1, x2, y2):
    return ((x2 - x1)**2 + (y2 - y1)**2)**0.5

def play_round(window):
    """
    Play one round of catch. Returns "PLAYER" if the player wins the round,
    "COMPUTER" if the computer wins the round, and "QUIT" if the player
    pressed Escape and wants to quit.
    """
    ball_x = 10
    ball_y = randint(20, 280)
    ball = Circle(Point(ball_x, ball_y), 10)
    ball.setFill("black")
    dx = 4
    dy = randint(-5, 5)
    ball.draw(window)

    mitt_x = 790
    mitt_y = randint(20, 280)
    mitt = Circle(Point(mitt_x, mitt_y), 20)
    mitt_dy = 0
    mitt.draw(window)

    game_over = False
    result = ""

    while not game_over:
        #move the ball
        if ball_y >= 590 or ball_y < 10:
            dy *= -1
        ball_x += dx
        ball_y += dy
        ball.move(dx, dy)
        if ball_x > 810:
            ball.undraw()
            mitt.undraw()
            result = "COMPUTER"
            game_over = True

        #check the mitt
        key_pressed = window.checkKey()
        if key_pressed == 'k' and mitt_y <= 580:
            mitt_dy += 5
        elif key_pressed == 'j' and mitt_y >= 20:
            mitt_dy -= 5
        elif key_pressed == 'Escape':
            result = "QUIT"
            game_over = True
        else:
            mitt_dy = 0

        if not game_over:
            mitt_y += mitt_dy
            mitt.move(0, mitt_dy)

            #check if we caught the ball
            if distance(ball_x, ball_y, mitt_x, mitt_y) <= 30:
                ball.undraw()
                mitt.undraw()
                game_over = True
                result = "PLAYER"
            else:
                sleep(1/120)

    return result

def play_game(window):
    """
    Play multiple rounds of the game until someone wins 5 games. Returns "PLAYER"
    if the player wins the game, "COMPUTER" if the computer wins the game, and
    "QUIT" if the player pressed Escape and wants to quit.
    """
    player_score = 0
    comp_score = 0

    game_result = ""   # keep track of the final 5 game winner

    while game_result != "":
        pmsg = Text(Point(110, 570), "Player: %d Points" % player_score)
        pmsg.setSize(24)
        pmsg.draw(window)
        cmsg = Text(Point(640, 570), "Computer: %d Points" % comp_score)
        cmsg.setSize(24)
        cmsg.draw(window)
        sleep(3)
        pmsg.undraw()
        cmsg.undraw()

        round_result = play_round(window)

        if round_result == "PLAYER":
            player_score += 1
            if player_score == 5:
                game_result = "PLAYER"
        elif round_result == "COMPUTER":
            comp_score += 1
            if comp_score == 5:
                game_result = "COMPUTER"

        else:
            game_result = "QUIT"

    return game_result

def main():
    window = GraphWin("Catch", 800, 600)
    window.setBackground("yellow")

    result = play_game(window)

    if result == "PLAYER":
        result_text = "Player Wins!"
    else:
        result_text = "Computer Wins!"
    result = Text(Point(340, 290), result_text)
    result.setSize(32)
    result.draw(window)

    window.getMouse()
    window.close()


main()

Some new things to learn from this example:

  • Following good organizational practices makes programs easier to read. Use the following organization in your programs:

    • imports
    • function definitions
    • main body of the program
  • We took the version of the program developed in section 8.8 and wrapped it in a function named play_round().

  • A new function, play_game(), creates variables for player_score and comp_score. Using a while loop, it repeatedly calls play_round, checking the result of each call and updating the score appropriately. Finally, when either the player or computer reach 5 points, play_game returns the winner to the main body of the program, which then displays the winner and then quits.

  • There are two variables named result—one in the play_game function and one in the main body of the program. While they have the same name, they are in different namespaces, and bear no relation to each other. Each function creates its own namespace, and names defined within the body of the function are not visible to code outside the function body. Namespaces will be discussed in greater detail in the next chapter.

  • Throughout this chapter we have used hardcoded values for convenience and clarity. In general you should avoid hardcoding in your programs. For graphics programs this generally means defining values in terms of the width or height of the graphics window. For instance, we might define a variable ball_radius = width/80 and then use this variable when we create the ball. Then we could replace a value like 590 with the more general expression window_height - ball_radius. If all the relevant quantities are defined in terms of the window’s width and height, we can change just these two values and see everything scale nicely.

8.10. The break statement

The break statement is used to immediately leave the body of a loop. The following program implements a simple guessing game:

from random import randint

def main():
    number = randint(1, 1000)
    guesses = 1
    guess = int(input("Guess the number between 1 and 1000: "))

    while guess != number:
        if guess > number:
            print("Too high!")
        else:
            print("Too low!")
        guess = int(input("Guess the number between 1 and 1000: "))
        guesses += 1

    print("\n\nCongratulations, you got it in %d guesses!\n\n" % (guesses))

main()

We can rewrite this program to eliminate the duplication of the input statement by using a “sentinel” or “flag” variable to indicate when the user has guessed the number.

from random import randint

def main():
    number = randint(1, 1000)
    guesses = 0
    guessed = False  # the flag variable is False until we guess correctly

    while not guessed:
        guess = int(input("Guess the number between 1 and 1000: "))
        guesses += 1
        if guess > number:
            print("Too high!")
        elif guess < number:
            print("Too low!")
        else:
            print("\n\nCongratulations, you got it in %d guesses!\n\n" % (guesses))
            guessed = True   # set the flag variable to True

main()

This program makes use of the mathematical law of trichotomy (given real numbers a and b, a > b, a < b, or a == b). While both versions of the program are 15 lines long, it could be argued that the logic in the second version is clearer.

Put this program in a file named guess.py.

8.11. Glossary

abstraction
Generalization by reducing the information content of a concept. Functions in Python can be used to group a number of program statements with a single name, abstracting out the details and making the program easier to understand.
constant
A numerical value that does not change during the execution of a program. It is conventional to use names with all uppercase letters to repesent constants, though Python programs rely on the discipline of the programmers to enforce this, since there is no language mechanism to support true constants in Python.
nested loop
A loop inside the body of another loop.
random
Having no specific pattern. Unpredictable. Computers are designed to be predicatable, and it is not possible to get a truly random value from a computer. Certain functions produce sequences of values that appear as if they were random, and it is these psuedorandom values that we get from Python.
trichotomy
Given any real numbers a and b, exactly one of the following relations holds: a < b, a > b, or a = b. Thus when you can establish that two of the relations are false, you can assume the remaining one is true.

8.12. Exercises

  1. What happens when you press the key while running mitt.py? List the two lines from the program that produce this behavior and explain how they work.

  2. What is the name of the counter variable in guess.py? With a proper strategy, the maximum number of guesses required to arrive at the correct number should be 11. What is this strategy?

  3. What happens when the mitt in mitt.py gets to the top or bottom of the graphics window? List the lines from the program that control this behavior and explain in detail how they work.

  4. Change the value of ball1_dx in collide.py to 2. How does the program behave differently? Now change ball1_dx back to 4 and set ball2_dx to -2. Explain in detail how these changes effect the behavior of the program.

  5. Comment out (put a # in front of the statement) the break statement in collide.py. Do you notice any change in the behavior of the program? Now also comment out the ball1.undraw() statement. What happens now? Experiment with commenting and uncommenting the two undraw statements and the break statement until you can describe specifically how these statements work together to produce the desired behavior in the program.

  6. Where can you add the lines

    text = Text(Point(340, 290), "Player Wins!")
    text.setSize(32)
    text.draw(window)
    

    to the version of catch.py in section 8.8 so that the program displays this message when the ball is caught?

  7. Trace the flow of execution in the final version of catch.py when you press the Escape key during the execution of play_round. What happens when you press this key? Why?

  8. List the main body of the final version of catch.py. Describe in detail what each line of code does. Which statement calls the function that starts the game?

  9. Identify the function responsible for displaying the ball and the mitt. What other operations are provided by this function?

  10. Which function keeps track of the score? Is this also the function that displays the score? Justify your answer by discussing specific parts of the code which implement these operations.

8.13. Project: pong.py

Pong was one of the first commercial video games. With a capital P it is a registered trademark, but pong is used to refer any of the table tennis like paddle and ball video games.

catch.py already contains all the programming tools we need to develop our own version of pong. Incrementally changing catch.py into pong.py is the goal of this project, which you will accomplish by completing the following series of exercises:

  1. Copy catch.py to pong1.py and change the ball into a paddle by using Rectangle instead of the Circle. You can look at the graphics library reference for more information on Rectangle. Make the adjustments needed to keep the paddle on the screen.

  2. Copy pong1.py to pong2.py. Replace the distance function with a boolean function hit(ball_x, ball_y, radius, paddle_x, paddle_y, height) that returns True when the vertical coordinate of the ball (ball_y) is between the bottom and top of the paddle, and the horizontal location of the ball (ball_x) is less than or equal to the radius (radius) away from the front of the paddle. Use hit to determine when the ball hits the paddle, and make the ball bounce back in the opposite horizontal direction when hit returns True. Your completed function should pass these doctests:

    def hit(ball_x, ball_y, radius, paddle_x, paddle_y, height):
        """
          >>> hit(760, 100, 10, 780, 100, 100)
          False
          >>> hit(770, 100, 10, 780, 100, 100)
          True
          >>> hit(770, 200, 10, 780, 100, 100)
          True
          >>> hit(770, 210, 10, 780, 100, 100)
          False
        """
    

    Finally, change the scoring logic to give the player a point when the ball goes off the screen on the left.

  3. Copy pong2.py to pong3.py. Add a new paddle on the left side of the screen which moves up when 'a' is pressed and down when 's' is pressed. Change the starting point for the ball to the center of the screen, (400, 300), and make it randomly move to the left or right at the start of each round.