I’ve read several tutorials on Object-Oriented Programming (OOP), and none of them has been as clear as I thought they should be. So it took me a long time to figure out OOP, and now I feel like I should pass on the knowledge. Here it is:

Objects

One of the main difficulties in OOP is thinking about it. So start trying to wrap your mind around an object. Basically, an object is a specialized data structure than can hold information, and do things with it. In other words, it’s a thing represented by code. And you aren’t limited to just one thing, you can have as many as you want, just as you can have as many numbers, lists or dictionaries as you want.

Holding Information and Doing Things

You can give objects attributes. What’s even cooler is that you can access them really specifically:

fido = Dog("Golden Retriever")
fido.breed

Assuming you’ve made a “Dog” class, fido.breed will be a string saying “Golden Retriever.” You can also change the data (it’d be pretty pointless if you couldn’t…) :

>>> fido = Dog("Golden Retriever")
>>> fido.showBreed()
It's a Golden Retriever
>>> fido.breed = "Labrador"
>>> fido.showBreed()
It's a Labrador
>>> fido.changeBreed("Dalmatian")
>>> fido.showBreed()
It's a Dalmatian
>>> fido.breed
'Dalmatian'

Here we make a Golden Retriever named fido. We have it call a method (OOP-speak for function) to say what breed it is, and then change it’s breed variable to “Labrador.” After that, we use a different method (yes, objects can have multiple methods) to change the breed to “Dalmatian,” and have fido tell us that he is, in fact, a Dalmatian. If you feel like having to prefix everything with “fido.” is annoying, don’t worry: There’s a reason. We could just as easily have:

>>> fido = Dog("Golden Retriever")
>>> spot = Dog("Mutt")
>>> fido.showBreed()
It's a Golden Retriever
>>> spot.showBreed()
It's a Mutt

If we just used a generic ’showBreed()’ method, Python wouldn’t know whether we were talking about fido or spot, or some other dog. The <name><dot> system is a pretty efficient way of telling Python who’s doing what, or who’s remembering what.

Look back over this section and make sure you understand it well: We made an instance of the Dog class, which held information (fido.breed) and did things (fido.showBreed()), we also made another instance of the same Dog class (spot), which held different, but similar information (Mutt instead of Golden Retriever), and did similar things (said what breed he, spot, was, as opposed to what fido was).

Obviously stuff can get a lot more complicated, but if you understand that, you’re doing fine!

~Read Part 2 to learn how to make classes~

Consider the following code:

1->A

If A = 1
Disp "A=1"

Let’s study it a bit. The first line just stores 1 as A. Standard stuff. The second line is freaky, and the third line just displays “A=1″. Standard stuff. Let’s go back to the freaky part. It introduces one of the more powerful commands in TI-BASIC: If. What it says, is that if A equals 1, to do something. The syntax isn’t nearly as remarkable as the code, just If [value][thingy][value]. Thingy can be anything from the ‘Test’ menu. =, <, >, etc.

Note: Due to the constraints of this keyboard, the less-than-or-equal-to sign will be written as <=, and the greater-than-or-equal-to sign will be written as >=. Also, the not-equal-to sign will be written as !=.

If you’re looking for ‘if’, it’s under ‘Ctl’, not ‘I/O’. And this is the first time we’re using ‘Ctl’…Yay! OK, digression over

If’s power comes from the fact that it introduces uncertainty into the program. This means the program can rocket off in a lot more directions, which means: Sweeeeeeet.

Since you handled that chunk of code so beautifully, let’s consider this one too.

Input A
If A=1
Disp "A=1"
Disp "Where is this?"

I hope you’re wondering if the ‘Where is this?’ line is part of the if statement or not. It’s not. The only command that’s conditional is the one immediately after the if line. Which is really useful for a code snippet like this:

If A=1
Disp "A is 1"
If A=2
Disp "A is 2"
If A>2
Disp "A is pretty big"
If A<1
Disp "A is pretty small"

That's a lot cleaner than the following method, which is also legal.

If A=1
Then
Disp "A is 1"
End
If A=2
Then
Disp "A is 2"
End
If A>2
Then
Disp "A is pretty big"
End
If A<1
Then
Disp "A is pretty small"
End

You’re probably wondering why that’s legal. It’s stinking huge! Which brings us back to our problem–if only lasts for a line. If-Then-End fixes that problem. Then means that all of the following instructions are only to happen if the statement is true, and End means that an end has been put to the conditional instructions. There’s no arguments required for Then or End (an argument is something that has to be put with it to make the syntax legal.) So let’s look at our ‘Guess!’ program again. Last time we’d gotten this far:

Disp "Guess!","","(GPL) 2008", "T. Macdonald"

#Note that going from the outline to the implementation there was a better idea--putting 'Guess!' and the GPL on separate lines.
Input "Guess:".G
#Oh no! We don't know how to make a random number!
#Oh no! We don't know how to say if the number is too big or too small!
#(That's why the tutorial's not done. Stay tuned.)

And now we do know how to say if a number is too big or too small!
So
#I'm not copying all the code, we'll pick it up here:
Input "Guess:", G
If G<R
Disp "Too small!"
If G>R
Disp "Too big!"
If G=R
Disp "You win!"

Unfortunately, that’s really all we can do with that, as of right now.
Next time we’ll talk about Else.

Assignment: Write a program that will ask the user for his age, and then will classify them as “Kid” (12 and under), “Teenager” (13-19) or “Adult” (20+).

The somewhat astute among us have noticed that in Round *, there was the following line of code:

Input "Guess:",G

And that I haven’t explained how that works.

The pretty astute among us have also noticed that when they run a program such as the following:

Input G

All that’s shown is a question mark, and it’s not very visually appealing.

The quite astute among us have already realized that the way to get rid of the ugly question mark, and replace it with something more attractive is to put the attractive message in quotes, follow it with a comma, and then tack the variable name to the end of it.

Those who are not feeling very astute should be please that the astute have shared their thoughts, and we’re all back on a level playing field.

Finally, the EXTREMELY astute people are saying to themselves “It is such a drag when there’s like four variables that the user has to put in, and I have to make four Input statements. Four! Can you imagine? Surely there’s an easier way.”

Meet Input’s big brother ‘Prompt’. (TI-BASIC is very family-oriented. Output is Disp’s big brother, Input is Prompt’s little brother, Else is If’s big broth–oh, we haven’t gotten there yet. Round three is coming.)

Prompt is great for math. Take the Pythagorean Theorem:

Prompt A,B
A^2+B^2->D
sqrt(D)->C
Disp C

Which will display:

A=?
B=?
(The solution, which depends on the input)

So Prompt is just a quick and easy way of getting a bunch of variables inputted. Would I use it for games, or polished programs? No. Do I use it for quickly making a program that’ll do a mathematical formula? Always.

I’ve been thinking, and I realized it’d be nice to add another goal to the TI-BASIC series. So I’m adding a sample program.

About the program: It’s called ‘Guess!’ (I am really creative), and it’s been around since I was in the fifth grade, in one form or another. It consists of guessing a number from 1-100, and being told if it’s too big or too small, and then guessing again, and being told again, etcetera, until you guess the number.

Technical description: Before you make a program, it’s a really really really really really really really really good idea to know exactly what you want it to do. So here’s the ‘outline’ (remember that # and everything after it is a comment):

Guess! (GPL 2008 T. Macdonald) #Title and license (GPL is sort of like a Copyright)

Guess: #User guesses a number

Too big! #If the number’s too big

Too small! #If it’s too small

You win! #If the user guesses it

And that’s it. Believe it or not, the minute it took to write that saved a lot of wasted coding.

So let’s implement what we already know:

Disp "Guess!","","(GPL) 2008", "T. Macdonald"

#Note that going from the outline to the implementation there was a better idea--putting 'Guess!' and the GPL on separate lines.
Input "Guess:".G
#Oh no! We don't know how to make a random number!
#Oh no! We don't know how to say if the number is too big or too small!
#(That's why the tutorial's not done. Stay tuned.)

If you have a program that consists solely of displaying things, and it’s not some kind of thrilling narrative or story, then you have problems.
What you need is input.
So here’s how to do it:

Input X
Disp X

Wasn’t that tricky? When you run it, you’ll get a question mark (as in, “Yo! What do you want to input?”) and when you type something in (numbers only, for now), it’ll name the input ‘X’. Then you just ask it to display X.

(Input is in the same menu as Disp, if you’re having trouble finding it. Typing in I then N then P…etcetera, won’t work)

Now let’s do something (somewhat) useful:

Input L #L for length

Input W #W for width

Disp L*W

(NOTE: Everything after a # is a comment. Do not type it into the actual program, it’s just me explaining why I wrote a particular line the way it is. So what you should type in is:)

Input L

Input W

Disp L*W

There’ll be a ‘?’. You type in a number (length of rectangle). There’ll be another ‘?’. You type in another number (width of rectangle). It displays the product of the two numbers (area of rectangle).

In terms of function that ends our lesson on Input. However, I find the ‘?’s pretty ugly. Especially when you have several. So here’s the syntax for pretty-fying them.

Input "Length:",L

Input "Width:",W

Disp "Area is:",L*W

Here’s the output:

Length:3

Width:2

Area is:

                6

One more thing: the ‘Input’ function only accepts numbers. This can be lame if you want the person to, say, type in their name. (What’s your name? 1337)

The TI-86 has a different function called “InpSt” (Input String), which lets the user input a string of letters, not numbers. However, I’m pretty sure that the TI-83/84/80 families do not have this. Darn.

That’s all you need to know about Input to succeed in life

Now let’s talk about math. (Don’t worry, it’s a bit more interesting than normal math.) We’ve already touched on it, and the best way of explaining it is giving a sample program.

(Note: I’m going to use ‘->’ for the Store key throughout this entire guide. The Store key has Sto> written on it (the > is filled in though) and is used for assigning values to variables.)(I’ll also use sqrt() for the square root sign)

This program will solve for the hypotenuse of a right triangle.

Input "A:",A
Input "B:",B
A^2 + B^2->D
Disp "C squared is:",D
sqrt(D)->C
Disp "Hypotenuse is:",C

So the moral of the story (erm, program, I mean) is that you can do math with variables in programs.