Commodore PET User Manual page 249

Enlarging the Square
Lefs take the small square we just animated and en large it so that it forms a
boundary one space from the perimeter of the screen:
The PET screen is 40 spaces wide by 25 spaces long. If the rectangle's sides are to
be one space from the perimeter. then the square should be 38 spaces wide by 23
spaces long:
40-2 (1 space for each side)
=
38
25-2 (1 space for each side)
=
23
With just a few changes to the animated small square program we can draw
the larger rectangle to form the screen boundary. FOR... NEXT loops were used in
the previous animation program to print a string of graphics for each side. FOR
1-1 to 5. To enlarge the square. change the value of the TO index to 35 for the
horizontal sides and 21 for the vertical sides. leaving spaces for the corners.
40-2 (1 spa ce for each side) - 2(corners)
=
36
25-2 (1 space for each side) - 2(corners)
=
21
15
FOR
l =-= 1
TO
36;?
11-11 ;
~
Horizontal sides
25
FOR
I=-=l
TO
21 ;?"
100";
35
FOR
I=l
TO
36:
?""".,";
45
FOR
I=-=l
TO
21:
?"1
In";
Verticalsides
Make these changes in your program and try a trial RUN.
That was simple. Because Vou have just created a boundary around the
edge of the screen. the last statement of the program to move the cursor out of
the square is unwanted. Instead. delete line 50 and program the cursor to move
inside of the box and print something; vou do not want a boundary surrounding
an empty screen. Be sure not to program the cursor to go beneath the square.
because the screen will scroll up. and vou will lose the top of the square. Program
something inside the box. type RUN and watch it go!
236