In
the
vertical
direction,
there are 48
light-block
addresses numbered from to 47. is
at
the top
and 47 is at
the bottom.
The statement
"SET
(X,Y)"
whitens the
block which is the Xth
block
from the left in
the
horizontal
direction and
the Yth one down from the top
in the vertical direction.
And,
you've
figured
out that RESET
works the same way except
that
it "turns the light off".
Let's
try it out. This
program will lighten
any
one block of your choosing.
Type:
10
INPUT "HORIZONTAL ADDRESS (0 TO
127) IS";X
20 INPUT
"VERTICAL ADDRESS
(0
TO
47)
IS"
(Y
30
CLS
40
SET (X, Y)
and RUN
many times
using
various values of X and
Y.
What happens if X
=
150?
Try it. How about if
Y
=
60?
Try it, too.
The block
just moves
off
the end (or
bottom) of
the screen and starts
over with the address count. We
call
this "wrap
around".
You may
have noticed
that
if a block is lit
in the
upper left-hand corner, the READY
and
the prompt
(>)
destroy
it. Try X
=
6 and
Y
=
6.
Then X
=
5
and Y
=
5. We can
avoid
this
problem by not
returning
control to the prompt
—
by
adding
99 GOTO
99
at the end of
the
program. After running the
program, this
line
locks
the Computer in an
endless loop. To break
the loop,
press
| BREAK]
key.
You
should put an
endless loop at
the
end
of every
graphics program. Do it here,
then try
X
=
5
and Y
=
5.
Now try
X
=
and
Y
=
0.
Remember the |BREAK|
key to stop a
properly "locked
out"
graphics program,
before starting
another.
While we
have a key that RESETs
every
block on the screen to "OFF" in one
operation
(the
JCLEAR
J
key), we
don't
have a similar key to turn
them
all
"ON".
However, we
can
easily write a program
that
"lights", "whitens" or "paints" the
entire
screen.
It uses one
Clear (not really a must, but
always
a
good
habit to use one for graphics
programs), two
FOR-NEXT loops and one
endless
"locking loop". Type this:
10
CLS
108
To Next Page
Loading...
