Exercise 38: Doing Things to Arrays

You have learned about arrays. When you learned about while-loops you "appended" numbers to the end of an array and printed them out. There were also Study Drills where you were supposed to find all the other things you can do to arrays in the Ruby documentation. That was a while back, so review those topics if you do not know what I'm talking about.

Found it? Remember it? Good. When you did this you had an array, and you "called" the function push on it. However, you may not really understand what's going on so let's see what we can do to arrays.

When you write mystuff.push('hello') you are actually setting off a chain of events inside Ruby to cause something to happen to the mystuff array. Here's how it works:

  1. Ruby sees you mentioned mystuff and looks up that variable. It might have to look backward to see if you created with =, if it is a function argument, or if it's a global variable. Either way it has to find the mystuff first.
  2. Once it finds mystuff it reads the . (period) operator and starts to look at variables that are a part of mystuff. Since mystuff is an array, it knows that mystuff has a bunch of functions.
  3. It then hits push and compares the name to all the names that mystuff says it owns. If push is in there (it is) then Ruby grabs that to use.
  4. Next Ruby sees the ( (parenthesis) and realizes, "Oh hey, this should be a function." At this point it calls (runs, executes) the function just like normally, but instead it calls the function with an extra argument.
  5. That extra argument is ... mystuff! I know, weird, right? But that's how Ruby works so it's best to just remember it and assume that's the result. What happens, at the end of all this, is a function call that looks like: push(mystuff, 'hello') instead of what you read which is mystuff.append('hello').

This might be a lot to take in, but we're going to spend a few exercises getting this concept firm in your brain. To kick things off, here's an exercise that mixes strings and arrays for all kinds of fun.

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ten_things = "Apples Oranges Crows Telephone Light Sugar"

puts "Wait there are not 10 things in that list. Let's fix that."

stuff = ten_things.split(' ')
more_stuff = ["Day", "Night", "Song", "Frisbee", "Corn", "Banana", "Girl", "Boy"]

# using math to make sure there's 10 items

while stuff.length != 10
  next_one = more_stuff.pop
  puts "Adding: #{next_one}"
  stuff.push(next_one)
  puts "There are #{stuff.length} items now."
end

puts "There we go: #{stuff}"

puts "Let's do some things with stuff."

puts stuff[1]
puts stuff[-1] # whoa! fancy
puts stuff.pop()
puts stuff.join(' ')
puts stuff[3..5].join("#")

What You Should See

$ ruby ex38.rb
Wait there are not 10 things in that list. Let's fix that.
Adding: Boy
There are 7 items now.
Adding: Girl
There are 8 items now.
Adding: Banana
There are 9 items now.
Adding: Corn
There are 10 items now.
There we go: ["Apples", "Oranges", "Crows", "Telephone", "Light", "Sugar", "Boy", "Girl", "Banana", "Corn"]
Let's do some things with stuff.
Oranges
Corn
Corn
Apples Oranges Crows Telephone Light Sugar Boy Girl Banana
Telephone#Light#Sugar

What Arrays Can Do

Let's say you want to create a computer game based on Go Fish. If you don't know what Go Fish is, take the time now to go read up on it on the internet. To do this you would need to have some way of taking the concept of a "deck of cards" and put it into your Ruby program. You then have to write Ruby code that knows how to work this imaginary version of a deck of cards so that a person playing your game thinks that it's real, even if it isn't. What you need is a "deck of cards" structure, and programmers call this kind of thing a "data structure".

What's a data structure? If you think about it, a "data structure" is just a formal way to structure (organize) some data (facts). It really is that simple. Even though some data structures can get insanely complex, all they are is just a way to store facts inside a program so you can access them in different ways. They structure data.

I'll be getting into this more in the next exercise, but arrays are one of the most common data structures programmers use. They are simply ordered lists of facts you want to store and access randomly or linearly by an index. What?! Remember what I said though, just because a programmer said "array is a list" doesn't mean that it's any more complex than what an array already is in the real world. Let's look at the deck of cards as an example of an array:

  1. You have a bunch of cards with values.
  2. Those cards are in a stack, list, or array from the top card to the bottom card.
  3. You can take cards off the top, the bottom, the middle at random.
  4. If you want to find a specific card, you have to grab the deck and go through it one at a time.

Let's look at what I said:

"An ordered array"
Yes, deck of cards is in order with a first, and a last.
"of things you want to store"
Yes, cards are things I want to store.
"and access randomly"
Yes, I can grab a card from anywhere in the deck.
"or linearly"
Yes, if I want to find a specific card I can start at the beginning and go in order.
"by an index"
Almost, since with a deck of cards if I told you to get the card at index 19 you'd have to count until you found that one. In our Ruby arrays the computer can just jump right to any index you give it.

That is all an array does, and this should give you a way to figure out concepts in programming. Every concept in programming usually has some relationship to the real world. At least the useful ones do. If you can figure out what the analog in the real world is, then you can use that to figure out what the data structure should be able to do.

When to Use Arrays

You use an array whenever you have something that matches the array data structure's useful features:

  1. If you need to maintain order. Remember, this is listed order, not sorted order. Arrays do not sort for you.
  2. If you need to access the contents randomly by a number. Remember, this is using cardinal numbers starting at 0.
  3. If you need to go through the contents linearly (first to last). Remember, that's what for-loops are for.

Then that's when you use an array.

Study Drills

  1. Take each function that is called, and go through the steps for function calls to translate them to what Ruby does. For example, more_stuff.pop() is pop(more_stuff).
  2. Translate these two ways to view the function calls in English. For example, more_stuff.pop() reads as, "Call pop on more_stuff." Meanwhile, pop(more_stuff) means, "Call pop with argument more_stuff." Understand how they are really the same thing.
  3. Go read about "object-oriented programming" online. Confused? I was too. Do not worry. You will learn enough to be dangerous, and you can slowly learn more later.
  4. Read up on what a "class" is in Ruby. Do not read about how other languages use the word "class." That will only mess you up.
  5. Do not worry If you do not have any idea what I'm talking about. Programmers like to feel smart, so they invented object-oriented programming, named it OOP, and then used it way too much. If you think that's hard, you should try to use "functional programming."
  6. Find 10 examples of things in the real world that would fit in an array. Try writing some scripts to work with them.

Common Student Questions

Why did you use a while-loop?
Try rewriting it with a for-loop and see if that's easier.
What does stuff[3...5] do?
That extracts a "slice" from the stuff array that is from element 3 to element 4, meaning it does not include element 5. It's similar to how (3...5) would work.

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