Radio Shack TRS-80

120 IF
YO-1
THEN 80
130 Y
=Y-2*D
140
D=-D
150
GOTO
90
999
60TD
999
Since we did not
explain
in detail how
that
fairly
simple
program
worked,
take
time now to
see if you can
follow
it through. When
you have
it figured
out,
tackle
this exercise:
EXERCISE
22-1
: Using
PRINT AT
statement(s),
cause
the word
"PING"
to appear near
the
ball
each
time it
bounces
off either the top
or
bottom
boundary.
Isn't
it
amazing
how
close we are to
getting to
some
of the
actual
video
games
that are
all
the rage?
—
and
yet it's
really so
simple
and
logical.
Merely
for
Display
Purposes
A good way to
get a
feel for PRINT AT
(or any
feature) is
to look
at a
fairly
simple
program
which
illustrates
its use.
This program lays
out a
graph
format on
the screen. What you
do
with it beyond
that
point depends on
your
own needs
and interests,
but
it
is worth
entering,
studying
and
getting
a feel for its use. Type:
10
CLS
20 K
=
900
30 F .X
=
1T059
40
P.ATK+X, "
.
"
;
50 N.Xi
K
=
964
60
FOR
Y
=
TO
13
70 P.ATY*64
+
5
,
"
.
"
;
80
NEXT Y
136
One noteworthy
procedure
in this program is
the
use of
trailing
semicolons
after
PRINT AT
state-
ments. The
reason^again, is
that
theprinting is
taking piaee in the
last
line on
the screen so the
carriage
return
vrbald
activate £
scroll. We
there-
.
fore
have
to
suppress
the carriage
return with the
•.semicolon.
. .,-,•.'•••,
'"
'
"-"