Simpleton Geek Teaches Computer Programming Book 2 Ch 9B

9.2 Arithmetic: Add, sub, mul,div,mod
The students were all sitting in their seats. The teacher was standing in front of the blackboard. “Today, we are going to teach the computer how to calculate! That means addition, substraction, multiplication, division, and maybe even some modular arithmetic.”

The students were confused. “Didn't we learn all that before?”

“Hey, what did I tell you about more than one way of doing things? Have you forgotten that already?” The students kept silent.

“Alright, take a look of this program. You see three variables, A, B and C. Furthermore, there's another variable. This one denotes the arithmetic operator. That's add, substract, multiply, and divide.”

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”

@MAIN

CLS

?”A= “;A:?”B= “;B:?”C= “;C

?”OP= “;OP$[OP]:?

?”MAIN MENU”

?”1. INPUT A”

?”2. INPUT B”

?”3. SET OP TO ADD”

?”4. SET OP TO SUB”

?”5. SET OP TO MULTIPLY”

?”6. SET OP TO DIVIDE”

?”7. CALCULATE”

?”WHICH?”

LINPUT CH$:CH=(VAL(CH$))%8

ON CH GOTO @NOP,@STA,@STB,@ADD,@SUB,@MUL,@DIV,@CAL

@NOP

GOTO @MAINEND

@STA

GOTO @MAINEND

@STB

GOTO @MAINEND

@ADD

GOTO @MAINEND

@SUB

GOTO @MAINEND

@MUL

GOTO @MAINEND

@DIV

GOTO @MAINEND

@CAL

GOTO @MAINEND

@MAINEND

GOTO @MAIN

“There's the main structure of the program. All you have to do is fill in the blanks. You have 10 minutes! Go do it!”

The students went all panicky, but quickly entered the program. 10 minutes later, a few struggling students got their heads whacked by the teacher. Most, however, managed to finish the program. It's all very easy:

@NOP: OP=0

@STA: INPUT A

@STB: INPUT B

@ADD: OP=1

@SUB: OP=2

@MUL: OP=3

@DIV: OP=4

@CAL:IF OP==1 THEN C=A+B

IF OP==2 THEN C=A-B

IF OP==3 THEN C=A*B

IF OP==4 THEN C=A/B

The students tried out their program. All managed to have the program running correctly. It's all very easy. It's just a question of how quickly you can get the program running. The teacher then compared their program with the existing one provided as a sample demo (SAMPLE2) program.

“Now, count the number of lines, which one is shorter? How about convenience? Would you like to enter the numbers repeatedly or would you like to just enter the numbers once? You already know how long it takes you to do this. How long do you think it takes them to do this? One more thing. I want you to draw the tables of operations in your journal. Not that I'll be grading this since this is so easy! Gya ha ha ha!”

“As you notice, this is where most people stop working. Once you have the functionalities of a calculator, they simply stop programming and move on to another program. Well, that's not good enough! We want to have a program that is not only works as expected, but also easy and convenient to use! Therefore, you are going to learn how to do a calculator, like a real calculator. Notice that there is no 'INPUT A' or 'INPUT B' on the calculator. So, we're going to do the same. We will simply concentrate on the buttons, and see if we can duplicate the interface. That means INKEY$.

“Now, if you first begin using the calculator, everything is zero. When you press the operator button, just assign the operator. Simple enough. If you press the equal sign button, that means calculate. Also simple. But what happens when you press the number button? There are different responses.

1. Add the number to the display

2. Clear the display and show the number.

“That's right. 2 different actions, depending upon whether or not you are simply adding digits to the number or you are inputting a new number. So, can anyone tell me the difference?”

The student were listening intently to the lecture, and expected the teacher to tell them the difference. They simply didn't expect to solve the problem by themselves.

All was quite. What would be the difference between adding a digit and inputting new number? Finally, Peter raised his hand. “You add a new number after an operator.” The teacher smiled.

“You got it! It's so simple, isn't it? Take a look at this: 23*45+67. You reset the display when the last key was an operator. What does that mean in our program? We set a flag to clear the display when the last key was an operator. That's it! Now, modify the program so that instead of INPUT, we use INKEY$. Also see if you can do automatic calculation whenever we press any of the four arithmetical operators!”

@INIT

L=0:'0 NORMAL, 1-CLEAR DISP

@MAIN

VSYNC 1:CLS

?”A= “;A:?”B= “;B:?”C= “;C

?”L= “;L:?”OP= “;OP$[OP]:?

@MAIN1

C=0:CH$=INKEY$:IF CH$=”” GOTO @MAIN1

IF CH$==”+” THEN L=1:GOTO @ADD

IF CH$==”-” THEN L=1:GOTO @SUB

IF CH$==”*” THEN L=1:GOTO @MUL

IF CH$==”/” THEN L=1:GOTO @DIV

IF CH$==”=” THEN L=1:GOTO @CAL

C=VAL(CH$)

IF L==1 THEN A=B:B=C

IF L==0 THEN B=B*10+C

L=0

GOTO @MAIN

@CAL:IF OP==1 THEN B=A+B

IF OP==2 THEN B=A-B

IF OP==3 THEN B=A*B

IF OP==4 THEN B=A/B

GOTO @MAIN

“You see we do not do anything different in algorithmic sense. Everything is still the same. Later on, I'll show you a bit more advance knowledge. Now get to it! you have 15 minutes!”

After all the students finished the program and tested it to their satisfaction, the teacher pointed out that their program is actually a poorly designed one.

“You can see, after all, that you need to press equal sign everytime you want to calculate. If you look at the regular calculator, then you see that it calculates automatically after every calculation. So, in that sense, your program works as designed. No one can say that your program is buggy, because it's not. It performs as designed. The program isn't bad, it's the design that is bad!

“So, for your next project, you want to duplicate the function of regular calculator as close as possible. I'll tell you right now that your program must be thrown away and you need to build the new program from scratch!”

All the students moaned in disappointment. “Teacher, why must we rebuild the whole thing? Can't we just modify the existing program?”

“Anything is possible.” said the teacher. “It's just a question of time and money. Sometimes the core (the main program flow) is so different, that it's easier and faster to just do the whole program from scratch. The thing is, sometimes you have gone down the wrong path so far, that you need to redo the whole thing.”

All the students aren't convinced at all. That's just double the work! All that work for nothing!

“Here's the key: You cannot just throw existing program willy-nilly. You must first understand what the problem is, what your solution is, and what your plan is going to be. Once you have those, then you can make the decision whether starting from scratch is the right thing to do. Since I have done the work already, I KNEW that starting from scratch is the right thing to do. So, do your homework!”

“But before that, I want to teach you something really quick: STRING. That is string variables and what you can do with it. Now, you already know what string is. You've been using it with LINPUT, so you have the basic understanding of what it is already. That's why I'm confident that the 15 minutes of class time that we have left is more than sufficient to teach you everything you need to know about strings.”

The teacher then smiled, “Of course, if it turns out that you haven't been doing your homework, and that your understanding of string variables is faulty, then you will be clueless and will not be able to do your homework properly. But in that case, it's your fault! Gya ha ha ha!”

The students are all scared and confused. Learning a whole new concept in 15 minutes? Is that even possible? But since there's no time left on the clock, the students don't have any choice but to go along with the lesson.

“Now, tell me what you know about string.” the teacher started.

“Strings are variables that has value between two quotes.”

“Great. Now let's say there's a string with the value: 'HELLO AND GOODBYE'. Now I want you to tell me the following:

1. The length of the string.

2. The first 5 characters of the string

3. The last 3 characters of the string

4. The string from 7 to 9 position

5. Finding the position of substring “GOOD”

6. Finding the position of substring “END”

“That's should be enough for now. In fact, it's so easy, I don't have to say anything other than it's all described on Help page #32. So get to it! I'll be scoring your work this time around. After all, there's nothing better to make the students fail the quiz than giving precious little time to do it. Gya ha ha ha!”

All the students moaned and get to work immediately. In fact, the answers are all obvious. It's just straight implementation of the commands in help page.

1. LEN(“HELLO AND GOODBYE”)

2. LEFT$(“HELLO AND GOODBYE”,5)

3. RIGHT$(“HELLO AND GOODBYE”,3)

4. MID$(“HELLO AND GOODBYE”,7,3)

5. INSTR(“HELLO AND GOODBYE”,“GOOD”)

6. INSTR(“HELLO AND GOODBYE”,“END”)

The students barely managed to type those into the computer, just to make sure that they have it correct. Of course, smart students would put C$=”HELLO AND GOODBYE”, and use C$ as the variable inside the commands. Doing it that way certainly saved quite a lot of time.

Some of the students actually managed to miss the instruction. Typing 'AND' instead of 'END' for example. Others still didn't follow the instructions too closely and switch MID$, LEFT$, and RIGHT$ in various orders. With predictable bad scores on the quiz.

Just because there's basic understanding on how string works, does not mean the students can just ignore the tasks presented in front of them. They still have to pay attention to details.

“Time's up! Let's see those papers! For homework, I want you to fill in this table. Make sure to do this properly. It is of the utmost importance that you do it right!”

“The grading is that important?” asked a student.

“Oh, I'm not grading it. If you do it wrong, you will certainly suffer greatly tomorrow. Gya ha ha ha!”