5.5 Continuing Saga: Nice Spider Patterns

Now that you know how to write loops, let's return to the spider. Recall Program 3.13, in which we asked the spider to draw a box using a large number of Step commands. This is much more easily done using a loop. Program 5.10 shows how the box can be drawn using a single loop with three Step commands in the loop body. The loop counter runs from 1 to 4 because the body is repeated for each of the four sides. The basic algorithm is

1. FOR each of four sides LOOP

2.Draw one side of the box

END LOOP;

Step 2 is refined to

2.1 Step

2.2 Step

2.3 Step

2.4 Turn

We omit the sample run because the output is very similar to the original from Program 3.13.

Program 5.10
Drawing a Box with a Loop

WITH Spider;
PROCEDURE Draw_Box_with_1_Loop IS
------------------------------------------------------------------------
--| Draw 4 x 4 box with spider - use loop
--| Author: Michael B. Feldman, The George Washington University
--| Last Modified: July 1995
------------------------------------------------------------------------

BEGIN -- Draw_Box_with_1_Loop

  Spider.Start;

  FOR Side IN 1..4 LOOP
    Spider.Step;
    Spider.Step;
    Spider.Step;
    Spider.Right;
  END LOOP;

  Spider.Quit;

END Draw_Box_with_1_Loop;

A different refinement of step 2 takes note of the repetitious nature of the Step commands:

2.1 FOR each of three steps LOOP

2.2 Step

END LOOP;

2.3 Turn

so that the overall algorithm is now:

1. FOR each of four sides LOOP

2. Draw one side of the box

2.1 FOR each of three steps LOOP

2.2 Step

END LOOP;

2.3 Turn

END LOOP;

and this algorithm is implemented in Program 5.11. Note that we could very easily change the number of steps per side just by changing the upper bound of the inner loop.

Program 5.11
Drawing a Box with Nested Loops

WITH Spider;
PROCEDURE Draw_Box_with_Loops IS
------------------------------------------------------------------------
--| Draw 4 x 4 box with spider - use nested loops
--| Author: Michael B. Feldman, The George Washington University
--| Last Modified: July 1995
------------------------------------------------------------------------

BEGIN -- Draw_Box_with_Loops

  Spider.Start;

  FOR Side IN 1..4 LOOP
    FOR Count IN 1..5 LOOP
      Spider.Step;
    END LOOP;
    Spider.Right;
  END LOOP;

  Spider.Quit;

END Draw_Box_with_Loops;

Finally, consider what happens if we make the upper bound of the inner loop different for each iteration of the outer loop. Specifically, suppose we set this upper bound to be the current value of the outer counter. The resulting shape would be a spiral. This is shown in Program 5.12, together with some sample output that shows a 10-segment spiral pattern. Be sure you trace this program carefully to understand how and why the spiral is drawn.

Program 5.12
Drawing a Spiral

WITH Spider;
PROCEDURE Spiral IS
------------------------------------------------------------------------
--| Draw spiral pattern with spider - use nested loops
--| Author: Michael B. Feldman, The George Washington University
--| Last Modified: July 1995
------------------------------------------------------------------------

BEGIN -- Spiral

  Spider.Start;
  Spider.Blue;

  FOR Line IN 1..10 LOOP
    FOR Count IN 1..Line LOOP
      Spider.Step;
    END LOOP;
    Spider.Right;
  END LOOP;

  Spider.Quit;

END Spiral;

Sample Run

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                   |. . . . . . . . . . . . . . . . . . . . .|
                   |. . . . . . X X X X X X X X X X * . . . .|
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