Contents:

NOTE: Use the palgo version on the web here, not the version on the CS that came with the lab book.  The version on the web has a number of bugs fixed.

Exercise 53 of Ch. 8 (p. 279); answer in back of text

Review the program examples that come with the Palgo package.

Try Exercises 1 and 2 from Lab 8 of the Meyer book.

Fill in the code in the subroutine L so that an 'L' shape is drawn whose top left pixel is at (x, y) and which has height h pixels and width w pixels.   Assume h and w are least 2, so the result does not collapse into a single segment or a point.  When you run the program, it should look exactly like the picture below.  Two possible solutions are at the end of this web page.

   define L(x,y,h,w)
     /* code here */
   end

   /* include, but do not change anything below this line */
   color("red")
   L(0,0,7,4)
   color("blue")
   L(2,1,5,3)
   color("green")
   L(5,5,2,2)
   end

1. (6 points)
   
   For this problem, you can use Palgo to do the work for you if you'd like. Though on a test, you would certainly be expected to do such a problem on your own, so please make sure that you fully understand the answer.
   1. What value or sequence of values will be printed when running the following segment of code?

         val = 3
         while val < 6
           print(val)
           val = val + 1
         end
      

   2. What value or sequence of values will be printed when running the following segment of code?

         val = 11
         while val >= 7
           val = val -1
           print(val)
         end
      

2. (4 points)
   Modify the PlotPoint subroutine in the following Palgo program, so that it only plots a point in the case that x>2*y.

   define PlotPoint(x,y)
     draw(x,y)
   end
   
   /* include, but do not change anything below this line */
   color("red")
   repeat 1000 times
     PlotPoint(random(20),random(20))
     wait(100)
   end
   
   

3. (5 points)
   Modify the PlotPoint subroutine in the following Palgo program, so that it always draws the point x,y, however chooses the color based on the following rules:
   blue: whenever x <= y AND x <= (19-y)
   red: whenever y < x <= (19-y)
   green: whenever y < x AND (19-y) < x
   yellow: whenever (19-y) < x <= y

   define PlotPoint(x,y)
     draw(x,y)
   end
   
   /* include, but do not change anything below this line */
   color("red")
   repeat 1000 times
     PlotPoint(random(20),random(20))
     wait(100)
   end
   
   

   A typical result of running such a program is the following. Please be careful in handling the boundaries between colors precisely.
   

4. (5 points)
   Write a Palgo program which draws a red X across the entire grid, as shown here:
   

    define rectangle(leftX,topY,width,height)
        ...
    end

Once you have tested this routine to see that it works, then you can modify the "Random Colored Dots" program to develop a random colored rectangles program, as described in the "Deeper Investigation" section of Lab 8 of the Meyer text.

Two of many possible solutions for the subroutine in the last practice problem follow. It is easy to make the height or width be off by one.   Hopefully you corrected it if you saw that your original code made a dimension one too big.

     define L(x,y,h,w)
       highY = y + h - 1
       highX = x + w - 1
        /* let corner be drawn by the second loop */
       while y < highY
         draw(x,y)
         y = y+1
       end
       while x <= highX
         draw(x,y)
         x = x+1
       end
     end

     define L(x,y,h,w)
       goto(x,y)
       down(h-1)
       right(w)
     end