Radio Shack TRS-80 User Manual page 119

Note
that
we
have used the
same
trick as
above (under
Details)
to avoid negative values
ofY.
What
happens
if
you change
line
40 to
40
RESET (X+126.Y)
and
RUN?
Then;
40
RESEKX+125.Y)
And RUN.
Don't
just
sit
there
—
try
them! See what you
almost missed?
EXERCISE
20-3
:
Change
the
last
two programs
so that they
move
the dot
up
and
to the
left
respectively.
[%
Now,
let's
have the dot
move down
until
it
strikes
a
barrier.
The program
is:
10
INPUT
"HORIZONTAL STARTING POINT
(0
TO 127)
"(X
20
INPUT
"VERTICAL STARTING POINT
(0
TO 47)
";Y
30
INPUT
"
LOWER
BARRIER"
;K
40
CLS
50
FOR M=0 TO 127
60 SET (M,K)
70
NEXT
M
80 RESET
(
X
,
Y+47)
90
SET(X,Y)
100 Y=Y+1
I
117