Radio Shack TRS-80

Putting the Beast
to
Work
We
will
now
use the
Computer
for
some
very simple
problem-solving.
That
means
using
equations
—
oh
-
panic. But
then, an equation
is just
a
little
statement
that
says what's
on one side of the
equals
sign
amounts
to the same
as what's on
the
other side.
That
can't
get too bad (it
says
here).
We're going
to use that old
standby
equation,
"Distance
traveled
equals
Rate of
travel
times Time
spent traveling."
If it's
been a few years,
you might
want
to
sit
on the
end of
a
log
and contemplate
that
for
awhile.
To
shorten
the equation,
lets
choose
letters (called
variables)
to stand
for
the three
quan-
tities.
Then we can rewrite
the
equation
as a BASIC
statement acceptable
to
the
TRS-80:
40
D
=
R
*
T
What's
that
40
doing
there?
That's
the
program line
number.
Remember,
every
step in a
pro-
gram has
to have one. We
chose
40,
but
another
number
would have
done just as well.
The
extra spaces in the
line
are there
just to make
the
equation
easier
for us to
read;
the TRS-80
ignores them.
Later, when
you
write very long
programs,
you'll
probably
want to eliminate
extra
spaces,
because they
take up memory
space. For
now, they
may
be helpful,
so leave
them
in.
We can use
any of
the
26
letters
from A through Z
to identify
the
values we
know as well
as
those we want
to
figure
out.
Whenever you can,
it's
a
good idea
to chose
letters
that remind
you of the things
they
stand for
—
like the D,
R, and T of the
Distance,
Rate, Time
equation.
To further
complicate
this very
simple
example,
we will
point
out now
that there's
an
optional
way of writing
the equation,
using the
BASIC
statement
LET:
40
LET D
=
R
*
T
This use
of LET
reminds us that
making D equal
R
times
T was
our choice, rather
than an
eternal
truth like
1
+
1
=
2,
Some computers are
fussy,
and
always
require
the
use of LET
with
programmed
equations.
Your
TRS-80 says,
"Have it your
way".
Okay
—
let's complete
the
program.
Assume:
Distance
(in miles)
=
Rate (in
miles per
hour)
multipled by
Time
(in
hours).
How far is it
from
Boston
to San Diego if
a jet
plane
traveling
at an
average
speed
of
500
miles
per
hour
makes the
trip in 6 hours?
18
Remember,
we have
to
use
the
*
for
multqriica-
tton.
.
,'
."/.
Here's
what
line 40
means
to t&e Computer;
"Tate
the
value^of Rand
T^ multiply
mem
together^
arul
assign the resulting
value to
the
vari-
able
IX
So
until further
notice,
5
fe
equalto
the
result
of
R
times T."
We
could not reverse the
equation
and write,
f^T^
p.
This
would have
no meaning
for
the;
Computer. Hemember,
the left
hand
side of the
1
equation
is reserved
for variable
names (which-
:
;eyer
'tetter
we choose).
The right
hand side iatihe--.
piaee
to
put
math
expressions involving
numbers,
operators,
and
known
variables.
'
(Yes,
I
ktiow^ybu
aim'd^'ttiat'oheln your.'
:
hS
hut that's
not
the point!)