Radio Shack TRS-80 User Manual page 162

Turn
to
page 217 of
Appendix
A
and
find the
Exponentiation Subroutine.
Type
it
in
slowly
(as if
we
needed
to say that)
and
carefully.
Now
type
in
the following
"demonstration program".
10
PRINT "SEEKING THE VALUE OF
X
TO THE
Y
POWER"
20
INPUT
" X =
"
;X
30
INPUT
" Y
=
"
;
Y
40
GOSUB 30120
50
PRINT "THE ANSWER
IS";P
60
GOTO
10
And RUN
.
(Use [BREAK]
when you want
to
get
out of
the
program.)
Without
trying to
understand the
mathematics behind
it,
let's
trace
the flow of the
program
control
from main program
to
the various subroutines
and back.
Lines
20 and 30 provide values for
X
and
Y.
Line
40
transfers
program
control
from
the
main program
to
the
Exponentiation Sub-
routine
.
Line 30140
calls
the log subroutine to obtain log(X).
Line
30230
returns
L
= log(X) to the
exponentiation subroutine.
(Note that control passes
to the
statement immediately
following the
last
GOSUB
command
—
even though
that state-
ment
is
on the
same
line in this case.)
Y L
Line
30140
now
calls
the exponentiation subroutine to
compute
e
Finally,
Line 30150 adjusts the
magnitude
of
P
(don't ask questions!)
and
returns the
computed
value
P =
X
v
to the
main program, Line
50,
for output.
Now
let's
go
off
on
a
tangent
about
Christmas trees.
Selecting a Christmas tree
in
the
middle of a forest
on
a
snowy
evening in
December can
be a trying process. Especially
when you're seeking a
tree that's
exactly
28
feet
tall
(the tree
is
going
to be set
up
in
a park
downtown).
You
can climb
up
each
tree,
attach a 28-foot
tape
measure, climb back
down
and check to see
if
the tape
just
touches the
ground
—
and
160
It's
along one -afteryou type
it
in
and
get-
it
running
properly,
you^l want
to save
it
on
tape for
lateruse.
Note that the input variable
X
is
changed by the
subroutine.
Suppose
we
need
this
input value later
in
the program.
We
can't
refertothis original
value
by
calling
it
X,
:
because
X
has take
on
a
new
value,
The way
around
this
common
problem
is
to use a
"dummy
variable" to
hold the original
value of
X.
The
dummy
variable
assignment
must
;
be before the entry
to
the subroutine:
'.
35
S-X
Adding
thisline to
our
resident
program
saves
the value for later use.
For example,
we
can
now
change
line
59
to
.
%> PRINT
S;.*"I«THE";Y;"=
";P.
.
Watch out
-for
subroutines
which change the ..-•.
.value-of
theisput
variable.
T&e
lowly
"dummy
variable"
ts
quite
handy
in
such
cases.