9.3 Strings
“Alright,
everybody! Show me what you have!”. With that opening statement,
the class began. The teacher have them filled this table:
L0 D C D C
L1 D D C C
A A A=AopB A A=AopB
B B=(B*10)+C B C B
C D 0 D 0
OP OP op OP op
What that
table means is that L0 and L1 denote the type of commands that is
entered. D for digits and C for aritmetic operation. L0 is the last
keypress, while L1 is the previous keypress (what was previously L0).
A,B,C stands for variables, where A and B are variables for
calculation and C is the digit being entered. OP is the variable
holding the aritmetic operator, where 'op' lowercase is the actual
operator.
“Now, that
you have learned string operation, L0 and L1 are a piece of cake!”
said the teacher. “Here is the modified main program. You see how
simple this can be.”
REM
CALCULATOR
CLS:CLEAR
DIM OP$[5]
@INIT
A=0:B=0:C=0:OP=0
OP$[0]=”NOP”:OP$[1]=”ADD”
OP$[2]=”SUB”:OP$[3]=”MUL”
OP$[4]=”DIV”
L0=0:L1=0
@MAIN
VSYNC 1:CLS
?”A=
“;A:?”B= “;B:?”C= “;C
?”OP=
“;OP$[OP]
?”L0=
“;L0”?”L1= “;L1
@MAIN1
CH$=INKEY$:IF
CH$==”” GOTO @MAIN1
IF CH$==”C”
GOTO @INIT
IF CH$==”E”
THEN B=0:GOTO @MAIN
L1=L0:L0=0:'0=D,1=C
IF
INSTR(“0123456789”,CH$)<0 THEN L0=1
ON (L1*2+L0)
GOTO @ADD,@ADC,@ACD,@ACC
@ADD
GOTO @MAIN
@ADC
GOTO @MAIN
@ACD
GOTO @MAIN
@ACC
GOTO @MAIN
“And there
you go. All you have to do is fill in the blanks. Now that you have
all the table set up, it's easy! Of course, if you fill in the table
wrong, then you will have trouble doing it. Do you understand now why
I'm not grading your homework? I'd rather see you suffer all the bugs
you yourself created!
“And then I
will see you try to overcome the bugs. The way you approach your
debugging will reveal your character. If you blame yourself, then
you'll be alright. If you blame the tool, then you will never make
it! Gya ha ha ha!”
@ADD
C=INSTR(“0123456789”,CH$)
B=(B*10)+C
GOTO @MAIN
@ADC
IF OP==0
THEN A=B
IF OP==1
THEN A=A+B
IF OP==2
THEN A=A-B
IF OP==3
THEN A=A*B
IF OP==4
THEN A=A/B
C=0
IF
INSTR(“=+-*/”,CH$)>0 THEN OP=INSTR(“=+-*/”,CH$)
GOTO @MAIN
@ACD
C=INSTR(“0123456789”,CH$)
B=C
GOTO @MAIN
@ACC
GOTO @ADC
GOTO @MAIN
Filling the
routines then becomes a simple exercise. The students need to be
careful regarding some order or execution, but otherwise, it's not a
problem. The problem, of course, is in design, and with bad design,
comes bad program. Peter, the smart one in the class, has actually
programmed in the routines just to be sure what he has actually
works. In fact, that's the best way to do it. The other students,
being beginners, never thought to do it that way. They still think
that coding remains the most difficult phase of computer programming.
The whole
process of filling in data takes less than 10 minutes. Testing, of
course, takes longer. One by one, however, the students finally
managed to get their program going. There are two special commands
involved: All Clear (AC) and Clear Entry (CE). These corresponded
with character 'C' and 'E' respectively. If you see the code, they
are very simple. 'C' will simply shuffle the program execution to
@INIT function, whereas 'E' will simply reset B to 0.
“Alright.”
the teacher said. “Now that everybody got it, the next step is to
create a graphical calculator. Now, don't worry about it because it
doesn't involve anything you haven't done. In fact, if you remember
your piano playing program, then it's the same exact thing. The only
thing different is the key or button arrangement. So, what I want you
to do now is to draw some character text on screen and shape that as
calculators. So, get out your journal and start drawing!
“Remember,
program design starts on paper! Those of you who tries to shortcut
the step will find out that the whole process will take 2 to 3 times
longer. If you don't believe me, just try it! Gya ha ha ha!”
The students
dutifully drew the design on their journal. A 4 by 4 design emerged.
In the end, there was no place to put the equal sign command, and the
function is placed on the display instead. The final code is listed
in Appendix G.
And so it
goes. The students finally managed to draw a rough draft of the
calculator, and it did start to look like a real calculator.
Certainly there are some features that are missing, and some of the
calculation behavior isn't quite right, but these are just little
nits. The fact is, even beginning students are perfectly capable of
doing a calculator program that is not dependent upon INPUT commands.
“Right,
guys. Now for homework, I want you to think very carefully about how
strings work because I will take the character patterns away from
you. You will have to use character arrays for your on-screen
keyboard input.”
The students
all moaned their displeasure.
The teacher
banged on their desk. “What's this? You're all complaining on such
a little task? Well, well, why don't I add this to your homework,
then: Memory button. Remember, you have 4 different operations on it.
Memory Add, Memory Clear, Memory Reset, and Memory Substract.”
The students
regretted their outburst already, but the teacher isn't finished yet.
“Come to think of it, that's still too easy. So, not only you will
have to come up with 8 memory banks, but also a way to read a script
so you can do the operation automatically.”
“Teacher,
what do you mean a 'script'?” asked a student.
“What I
mean is that you have automatic input. Coming from a string. You need
to parse the string and automatically give the answer.”
Peter raised
his hand. “Parse the string? Isn't parsing a difficult thing to do?
As in Graduate Student level?”
The teacher
waved his hand, dismissing Peter's concern. “Nah. I learned it in
College Senior level. So, should be fine.”
All the
students looked at one another. College Level? Yikes! What kind of
trouble have their teacher got them into? They couldn't believe it!
“Will this
be graded?” asked Peter. There is hope yet at salvation.
“Of course.
If you managed to do this, you will be amply rewarded.” The teacher
nodded his head. “Of course, if you fail miserably, then you will
be severely punished, considering that it will have the highest
scoring criteria. So, good luck on working on that problem. Gya ha ha
ha!” The bell rang, and there's nothing more to say on the subject.
On the next
day, the students compared their notes. The first criteria turns out
to be easy. Instead of using PRINT and CHKCHR, the solution is simply
use a string array instead of PRINT for the row, and use MID$ for the
column. That's it. It's very trivial to do.
The second
solution is a bit harder. However, the AC/CE button provided a hint
on the solution. It's a simple matter to just add more command to it.
The most important function is READ/WRITE command equivalent. What
about this 8 bank memory? Another array solves the problem. Simply by
setting aside 8 characters for each bank, the memory bank problem is
trivially solved. What remains are the task of Memory Add, Sub, Read,
and Clear. An extension of AC/CE buttons solved that one also.
Of course,
the students only had so much time in a day, and they were unable to
solve the last problem. Parsing! A whole book can, and have been,
written on the subject. In the end, they gave up and on the next day,
they entered the class with fear, just waiting for the hammer to
fall.
There was
nothing they could do. How can mere high school students compete with
University Level students? How can they, as beginner, do what 4 year
college students are expected to do? Impossible!
Of course,
this doesn't sit well with the teacher. “What do you mean you have
no idea what to do? Well, well, it's lucky that I have my favorite
stamp with me, today.” The students coiled in terror!
“So
everybody got the first problem? Of course, it only scored 5 points.
That's so easy. All you have to do is create a string array, and then
check against it. Like this:”
DIM SC$[25]
@DIS
CLS:PNLTYPE
“OFF”
SC$[0]=”====================”
SC$[1]=SC$[0]:SC$[2]=SC$[0]
SC$[3]=”111112222233333+++++”
SC$[4]=SC$[3]:SC$[5]=SC$[3]:SC$[6]=3:SC$[7]=3
SC$[8]=”444445555566666-----”
and so on...
'Checking
CH$=MID$(SC$[TCHY/8],TCHX/8,1)
For memory
bank problem, simply set aside a memory array. Then set some keys to
select them. “C” and “E” are taken, but “a”-”h” are
not. As for Memory key operations, “M”-”P” are available.
This simply lends itself to this implementation:
DIM
MM[8]:'MEMORY BANK STORAGE
MI=0:'MEMORY
BANK INDEX
IF
INSTR('abcdefgh',CH$)>=0 THEN MI=INSTR('abcdefgh',CH$):GOTO MAIN
IF CH$==”M”
THEN MM[MI]=MM[MI]+B:GOTO @MAIN
IF CH$==”N”
THEN MM[MI]=MM[MI]-B:GOTO @MAIN
IF CH$==”M”
THEN MM[MI]=0:GOTO @MAIN
IF CH$==”M”
THEN B=MM[MI]:GOTO @MAIN
IF CH$==”C”
GOTO @INIT
“Does
everybody got that? They are all so easy, if you cannot do it, you
should be ashamed of yourself! Well, now that you've got 15 points
out of 100, ready to get the other 85 points?” the teacher asked.
Of course, this is where none of the students have any solution
whatsoever. The teacher gleefully put 15 points for everybody. “What
a bunch of losers! Gya ha ha ha!”
All the
students are just so frustrated. They simply do not know what to do.
Instead of resuming teaching, however, the teacher simply went back
to his desk, put his feet up, and went to sleep. Snoozing. At first,
the student were patiently waiting for the teacher to wake up. After
a while, it's clear that the students are left to fend for
themselves.
None of the
students know what to do. Some started doodling. Others decided to
take a snooze as well. Only Peter was alert. He knew that the teacher
did this for a reason. What could that be?
What was the
assignment? To have automated entry? That's parsing. Parsing is
difficult. But since the teacher said that it isn't impossible, that
means it's possible, and within the limits of the lectures so far.
So, how can that be?
It was not
after Peter stared at the problem for 20 minutes that he looked at
this line one more time:
CH$=MID$(SC$[TCHY/8],TCHX/8,1)
The key isn't
in TCHX/TCHY. The key is in CH$! That's one single character. What's
a single character? LEFT$(S$,1). If he can find a way to pull a
single character each time...
IS$=”1+2+3+4+5=”
@LOOP
IF LEN(I$)
THEN CH$=LEFT$(IS$,1) ELSE END
IS$=RIGHT$(IS$,LEN(IS$)-1)
?CH$:GOTO
@LOOP
Peter
confidently raised his hand. “Teacher! I got it!” All the rest of
the students couldn't believe it!
