Radio Shack TRS-80 User Manual page 76

We
can insert
temporary
print lines
anywhere
in
any program
so
we
follow every step
in
its
execution.
The Computer
can actually
overwhelm
us
with
data,
but by
carefully indicat-
ing
what we want
to
know,
we
can observe the inner
details
of the
calculations. Start
by
adding
this line:
92
PRINT " #92 M = "
;M
...
and
RUN
The
essentials
of
this
"test"
or "debugging"
or "flag" line are:
1. It
PRINTs
something.
2.
The
print
tells
the
line
number,
for analysis
and easy location for later erasure,
3. It tells
the
name
of the variable
you
are
watching
at
that point
in
the
program,
4. It
gives
the
value
of that variable
at
that point.
It
is
most
helpful
of
all
when
inserted
in
FOR-NEXT
loops
—
so
:
97
PRINT
'*
#97
A
=
" ;A
...and
RUN.
Wow!
The
data really
comes
thick and
fast!
Hard
to
keep track of so
much
information,
and
we've barely
begun. This
tells
what
is
happening during each
pass
of the loop.
Is
there
some
way
to
make
it
more
readable? Sure.
Can you
think of
a
way?
Yes, there are
lots
of ways. Indenting
is
just
one simple
way
to keep the answers separated
from
the trouble shooting data.
Retype
lines
92 and 97
as
follows:
"
#92
M
= " }M
"
#97
A
=
" ;A
92
PRINT
"
97
PRINT
"
...
and
RUN.
Ahhh.
How
sweet
it is.
That
is
so easy to read,
let's
monitor
some more
points
in
the pro-
gram. Type
in:
125
PRINT "
"
," " ,"
#125 M =
"
;
M
135 PRINT
*'
" ," "
,
" #135
M =
"
;
M
... and
RUN.
74
.
This
'^agglftg"
is :
such
a
wonderful troubleshootr
:
ing tool in
stubborn programs
that
you
will
warit
to
make a habit of n«ver
forgetting to
use
it
when
-
thegoing
gsts
tough.