Radio Shack TRS-80 User Manual page 20

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!)