132 The VIC 20
User
Guide
3540
(lET
Cf:
IF
C$-""
THEN
3S48
35Se
NM~·VAL(C')
3560
IF
NM~(LOr.
OR
NM~)HI~
THEN
3~48
3570
PRINT
C'J
3590
RETURN
Write a short program that sets values for HI% and LO%, and then
goes to subroutine 3500. Add the previous subroutine and run it.
Can you change the subroutine so that it accepts two-digit input? Try to
write this modified program for yourself.
If
you cannot do it, wait until the
next section, where you will find the necessary subroutine in the program
that controls the input
of
a date.
ENTERING
A
VALID
DATE
Most programs at some point need relatively simple data
input-more
than a simple
yes
or
no, but less
than
a full screen display. Consider a date.
You must be careful with such apparently simple data entry. In all
likelihood, the date will be just one item in a
data
entry sequence.
By
carefully designing
data
entry for each small item, you can avoid having to
restart a long
data
entry sequence whenever the operator messes
up
a single
entry.
The date
is
to be entered as follows:
MM-DD-YY
-'-ll§-'-~Lyear
Separator
Day
of
the month
Separator
Month
The dashes separating the month, day, and year could be slashes
or
any
other appropriate character. In Europe, the day
of
the month precedes the
month.
You should program
data
entry so
that
it
is
pleasing to the operator's
eye. The operator should be able to see immediately where
data
is
to be
entered, what type
of
data
is
required, and how far the data entry process
has proceeded. A good way of showing where data
is
to be entered
is
to
display the entry line in inverse video.
For
example, the program
that
asks
for a date might create the following display:
To Next Page
Loading...
