Line 398 The statement
PRINT@
A,"
";
erases the old asterisk
by
printing a space over it. The second statement
PRINT@
AT,
"4";
prints the asterisk
at
the new position. The semicolons at the end of these PRINT@
statements are required to prevent scrolling when printing in the bottom row.
Line 318 The variable A stores the
"old"
asterisk position, and AT stores the
"new"
asterisk position. This assignment statement updates the
"old"
asterisk position.
Execution is returned to Line
200
to allow repeated movement
of
the asterisk.
Experiment
#9
Move in Four Directions
Execute the previous program and hold down
CU.
The asterisk should move
continuously
to
the left. When the asterisk reaches the left margin, it jumps over to
the right and up one line. Continue to hold down
rn
until the asterisk reaches the
upper left corner
of
the display.
If
you try to move past the upper left corner, the
program will terminate execution with
an
error message
?FC
Error
in
300
This "Function
Call"
error occurs because the PRINT@ position goes negative
if
rn
is pressed with the asterisk in the upper left corner (PRINT@ position 0).
The program can be modified
to
prevent this error from occurring. At the same time,
it
is relatively easy to allow the asterisk to be moved vertically. Enter the following
changes into the program:
230
IF
A$="E"
THEN
AT=AT-£10
2£10
IF
A$="X"
THEN
AT=AT+£10
250
IF
AT)=0
AND
AT(=318
GOTO
300
260
PRINT
CHR$(7l;
:
AT=A
:
GOTO
200
List the program to confirm that it
is
now:
100
CLS
: A = 100 :
AT
= A : PRINT @ A,
"*"
200
A$
=
INKEY$
:
IF
A$
=
""
THEN
200
210
IF
A$
=
"0"
THEN
AT
=
AT
+ 1
220
IF
A$
=
"S"
THEN
AT
=
AT
- 1
230
IF
A$="E"
THEN
AT=AT-£10
2£10
IF
A$="X"
THEN
AT=AT+£10
250
IF
AT>=0
AND
AT>=318
GO
TO
300
260
PRINT
CHR$(7l;
:
AT=A
:
GOTO
200
300
PRINT @ A, "
";
:
PRINT
@ AT,
"*";
310
A =
AT
:
GO
TO
200
Execute this program.
The asterisk should appear in the center
of
the display. Press
CIJ
and the asterisk
should move up. Press the
00
and the asterisk should move down.
120
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