Radio Shack TRS-80 - 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.