Radio Shack TRS-80 User Manual page 161

You'll
probably want
to
try
"graphing" other
curves using the subroutines, so
we'll go
into a
little
detail
on
how
our
concentric
circles
program
works.
Line 10
gives us a nice
clear
screen to
start with.
Line 20
sets
up
a
loop which
increments the Radius
R
from
2 to
22
in steps of 4.
Line
30 sets
up
a "nested" loop which
increments
the X-coordinate of
our graph from
—
R
toR.
Line 40 computes
the Y-coordinate of our graph
as
a function of the radius
R
and
the X-
coordinate A.
The
square root subroutine
is
called.
Lines
50 and 60
center the
circle
on
the Display
and "draw
it".
Line 50 produces
the lower
half
of the
ci&le,
and
line 60
produces
the
upper
half.
Lines 70, 80 and 90
.
.
.
you
can
figure
them
out for yourself.
From One
Subroutine
to
Another
So
far
we've used
GOSUB
commands
in
the
main program
to call subroutines.
Now
let's
be
neighborly
—
and
let
one subroutine
call
on
another.
Suppose
we.
want
to
compute
3
U
—
that's
3 times
itself
11
times.
We
can compute
it
directly as
3*3*3*3*3*3*3*3*3*3*3,
right?
But what
about
3 1 '
3
-
that
is,
3 to the
11.3
power? Simple
multiplication
isn't
going to get us
anywhere on
this
one.
It
looks
like a
job
for
Supersub!
That's
our Exponential Subroutine,
which
actually
calls
on two
other subroutines before
it's
through
—
one
for
logs,
one
for
anti-logs.
(These
are
not the opposing
sides
in
a conservation
dispute.
They're extremely
useful
math
functions.)
Supersub derives
its
number-crunching power from
a rather complex-looking equation:
x Y
=
e
Y*logX
(You
don't have to understand
it,
but
it's
nice to
know
it's
there.)
All
we
have to
do
is
provide the subroutine with the values
for
X
and Y,
in this case 3
and
11.3,
and the
rest
is
automatic.
The subroutine goes
and
gets log
3, etc.,
and
returns to our
main program
with the
final
answer.
W«'reustog
natural logs
and
auti;l«gs, as opposed;
to
common
log^ Bememoer,
this
is
a
classy
operation!
159