Section 2 - Commodore Computers

154 COMPUTE! May. 1Q82.lssue 1982. Issue24
24
1000 :
1500 RETURN
And RUN it. Well, it doesn't do anything. AS A$ is a
useless string, a long chain of blanks, nOt not exactlyy
our intention. ntion.
(3) Let's position the cursor over the string.
Move It it back lO to ':0" "0" oon n "ONE" by inserting three
left cursors wlthlll within the quotes after "E". RUN it.
There is hope, but note the unacceptable length of
the string.
. (4) "Ve We cann limit it PET's wish for characters by
pnntlllg printing a comma ma after the "ONE". But that's the
last thing I need in my neat display. Let's convince
the PET, instead, that its screen width is not 40
characters (or 80 in 80xx), but onlyy three. Butterfield's
memory map indicates that information is
held in 213. I don't know what it is in Original
PETs, those maps were never printed in COMPUTE!.
(Perhaps an equivalent of 213 does not exist. I If I'
Butter-
anyone would like to contribute an accurate crossreference
type map for all these systems, please
send one in).
Type two lines:
1020 POKE213,3 (0 to squeeze the screen width
1080 POKE213,39 (0 to restore it back.
(PET counts from zero)
This works. Incidentally, line e 1080 may not be
cross-
needed, but out of overabundance of caution
I'll
keep it. PET's self preservation instinct seems lO to
restore reSlOre 213 for subsequent screen work. Query
whether 80-column window facility might be useful.
(5) We're not out of the woods yet. What will
happen if "ONE" were followed by a string of
blanks, a typical sight in a left adjusted alphabetic
information? rmation' Modify two twO lines to lo be:
100 ?LEFT$("ONE" + +" "10 I 0 sp'\10) sp", I + +" "10 t 0 left";:
GOSUB1000
GOSUBIOOO
1020 POKE213.10
POKE213,10
RUN it. Trailing g blanks are handled correctly.
y.
(6) What at about leading blanks you might see
in a right-justified number? number' Change one line:
100 ?RIGHT$("IO sp"+"472", IO)+ "lO left";!
100 ?RIGHT$("10 sp" + "472",10> + "10 left";:
GOSUBI000
GOSUBIOOO
and RUN it. Oops! The value is intact, but PET
and RUN it. Oops! The va lu e is il~tact, but PET
stripped the leading spaces, spaces. as it always does,in
INPUT. I This T result may be satisfactory tisfaclOry for many
applications, lOa sloppy for others. Let's handle it.
applications, loo sloppy for others. Let's handle it.
(7) One fix x might involve printing a phony
character r in the e first position. But BUllhen, then, if we wanted
to use the value in a numeric variable, able, we'd have to LO
strip the character. Instead, we'll use a harmless
strip the character. Instead. we'll use a harmless
quote by typing 100 PRINT CHR$(34) + RIGHT$
GHTS
... etc. and in crease the screen width lo I I in line
... etc. and increase the screen width to 11 in line
1020. Unfortunately y this places a character on the
screen. Unavoidable. RUN it. Not bad. . The T leading
blanks are there, the value is all right, but the origi
blanks are there, the value is all right, but the original
string is useless with the in-quote cursors.
In fa ct, any formatting command o n the same
lin e, i. e. prio r lo goin g into 1000, wi ll print this
nal string is useless with the in-quote cursors.
In fact, any formatting command on the same
line, i.e. prior to going into 1000, will print this
way. TAB(x) will yield x right cursors. That's the
way. TAB(x) will yield x right cursors. That's the
way PET is built. We could POKEE 205 lo to disable
the quote mode, but that gets messy in some applicall
cations. We could close the quote, but that increases
the space penalty on the line by another character.
A less limiting way is needed.
(8) Remember that all we reallyy wa want is lo to
move the cursorr left. PET docsn't doesn't care how. TThe
~n memory elllo r.y map ind~ indicates G~tes the current cursor positionn
IS is heldm in 198 (Original n gln 226?). 22(5?). To move the cursor
into the first position we'll put a zero there. I Inn line
100 delete 10 left cursors and its surrounding unding
quotes. Type 1005 POKE 198,0 lo to put cursor in the
first posltlon, position, and 1040 POKE 198,1111 sothata
a
subsequent field prints in a correct position. RUN
It. it. It reallyy works now. We can handle leading g and
trailIn trailing g blanks and anything in between. Right)
Right?
(9) I Not Ot qU quite. Ite: How about a null string? I Itt cann
happen. If a condition is true you might be printing
h ~,~ ,~~n. I f a con~ llI on is true you might be printing
a "*", '.: II if false you'lI you'll pnnt print nothlllg nothing and use TAB lo to
position to the next item, or skip line if it is at the
end. Our 1000 subroutine can't predict the future,
and will allemptlO attempt to process a null string with all its
IIlherent inherent troubles. Let's see what happens. Type
WO 100 PRINl PRINT "" ;:GOS~ ;:GOSUB1000andB don't type, type/but
Imagllle, imagine, 11100 PRINT INl TAB(!O)"whatever". TAB(!0)"whatever". RUN it.
Very nasty. There is actually more wrong than the
eye can see. If I fthe PET is alive you u have just learned,
lor ror instance, IIl stancc, (he the easiest way of causing g active files
to vanish into thin air (look at the table in $0251-
026E 026F on the Upgrade and 4.0 systems). Tragic
results, to lo say the least.
(10) ( A quote may help. TypeT 100 PRINT
CHR$(34) + "M;:GOSUB "";:GOSUB 1000. Once O again, this I.his
gives a correct result, ult. but subsequent TAB or
cursor characters will cause trouble. This was not Il ot
very important illl porlant with numbers. It's vital that it doesn't
happen here.
(11) ( II ) While we correct this mishap, mishap.let·s let's make
the routine a bit more general for furl further testing.
Type NEW and the code in lines 10-260, adjusting
198, 213 and screen width of 39 to lo your u r system. systcm. PI
is the first position ot of the field. . P2 is the final linal posi­
tion n after a string, string. null or otherwise, is primed. printed.
You can n try the code on various things within the
quotes in line 30. When en done, type two quotes
there (" '*) ") and convince yourself that it it can't work.
rk.
Then remove REM from line e 190 and try it. This
works. Replace REM in 190 and remove REM in
200. This also works with some space penalty.
Either method methocl should work for a particular
application tion - the choice is is a function of further use
for r the field. . Lines 190 or 200 simply check if the
cursor has moved. If I r not, t, the subroutine utine detects a
null string g and makes the needed adjustments by
faking a different screen width. In I n case of 200, it it
must also reset 198, since the PR I IT statement
must also reset 198, since the PRINT statement