Extended Exercise: Moving Pictures

In sections  and , we studied how to move individual shapes. A picture, however, isn't just a single shape but a whole collection of them. Considering that we have to draw, translate, and clear pictures, and that we may wish to change a picture or manage several pictures at the same time, it is best to collect all of the parts of a picture into a single piece of data. Because pictures may consist of a varying number of items, a list representation for pictures naturally suggests itself.

Exercise 10.3.1

Provide a data definition that describes the class of lists of shapes. The class of shapes was defined in exercise .

Create a sample list that represents the face of figure  and name it FACE. Its basic dimensions are gathered in the following table:

The table assumes a canvas of size 300 by 100.

Develop the template fun-for-losh, which outlines functions that consume a list-of-shapes. Solution

Exercise 10.3.2

Use the template fun-for-losh to develop the function draw-losh. It consumes a list-of-shapes, draws each item on the list, and returns true. Remember to use (start n m) to create the canvas before the function is used. Solution

Exercise 10.3.3

Use the template fun-for-losh to develop translate-losh. The function consumes a list-of-shapes and a number delta. The result is a list of shapes where each of them has been moved by delta pixels in the x direction. The function has no effect on the canvas. Solution

Exercise 10.3.4

Use the template fun-for-losh to develop clear-losh. The function consumes a list-of-shapes, erases each item on the list from the canvas, and returns true. Solution

Exercise 10.3.5

Develop the function draw-and-clear-picture. It consumes a picture. Its effect is to draw the picture, sleep for a while, and to clear the picture. Solution

Exercise 10.3.6

Develop the function move-picture. It consumes a number (delta) and a picture. It draws the picture, sleeps for a while, clears the picture and then produces a translated version. The result should be moved by delta pixels.

Test the function with expressions like these:

(start 500 100)

(draw-losh 
  (move-picture -5 
    (move-picture 23 
      (move-picture 10 FACE))))

(stop) 

When the function is fully tested, use the teachpack arrow.ss and evaluate the expression:

(start 500 100)

(control-left-right FACE 100 move-picture draw-losh)