Then - Radio Shack TRS-80 User Manual

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.
.
.,-,•.'•••,
'"
'
"-"