Exercise 1.3.1

1
Exercise 1.3.1

• A) Find out how tock in that figure works
  manually. Enter expression expressions such as
  (tock 0) and (tock 1) into DrScheme's
  Interactions window and see what their results
  and their effects on the canvas are. Hint
  Execute the animation but comment out the
  expression (on-tick-event tock).
• B) Make the UFO animation slower and/or faster.
• C) Render the UFO as a yellow shape. Add an
  antenna. Make the background blue.
• D) Swap the two drawing actions in tock. Does
  this make a difference? Why? Why not?

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1.3.1.A Solution / Explanation

• Type in some example of tock manually
• (tock 0) erases the 0th UFO and draws the 1st
  UFO.
• (tock 1) erases the 1st UFO and draws the 2nd
  UFO.
• In general, tock erases the current UFO and draws
  the next one.
• Since we do not want to sit at the computer and
  keep typing (tock 2), (tock 3), (tock 4), etc.,
  we use a predefined function called
  on-tick-event.
• The on-tick-event function takes another function
  (like tock) and automatically runs it after every
  clock tick.

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1.3.1B,C Solution / Explanation

• To slow down the animation, increase the time
  between clock ticks. For example, (big-bang 1
  0).
• To speed up the animation, decrease the time
  between clock ticks. For example, (big-bang .3
  0).
• To make the UFO yellow, change the color in the
  ufo-draw function (from green) to yellow.
• To make the background blue, we will simply draw
  a solid blue rectangle the same size as the
  canvas
• (draw-solid-rect0 0 0 100 200 blue)

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1.3.1C Solution / Explanation

• To add an antenna, we add a line (say, 15 pixels
  long) that extends from the top of the circle.
• Since the center is (X, Y) and the radius is 10,
  the top point on the circle is (X, Y 10).
• ---In Scheme, that is (X t) and (- (Y t) 10).
• Since we draw the line 15 pixels up from that
  point, the line goes up to (X, Y 25).
• ---In Scheme, that is (X t) and (- (Y t) 25).
• So we have,
• (draw-solid-line0 (X t) (- (Y t) 10) (X t) (- (Y
  t) 25) yellow)

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1.3.1D Solution / Explanation

• (define (tock t)
• (draw (ufo-draw ( t 1))
• (ufo-erase t)
• produce ( t 1)))
• Swapping the 2 drawing actions causes
• --part of the antenna may get erased
• --if the clock ticks are slow enough, you will
  be able to (incorrectly) have 2 UFOs on the
  screen at the same time
• There is no other difference as the 2 actions
  occur simultaneously, at least to human eyes.

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In conclusion

• We have written an animation program for moving
  the UFO across the screen.
• This program includes several Scheme functions
  that we wrote, like X, Y, ufo-paint, and tock.
  It also includes several predefined functions
  like , , draw-solid-rect0, big-bang, and
  on-tick-event.
• In HW4, you will write an animation for moving
  the shots across the screen.
• Then, you will combine the two animations, and
  complete the UFO game.
• For the rest of the semester, we will write
  shorter programs (each with only one, or a few,
  Scheme functions).