Instance Variables

Quick reference

• 00:04 In this video, I want to continue on talking about classes,
• 00:07 specifically we want to look at the instance variable right here.
• 00:11 So an instance variable always starts with the @ sign, that's what designates it,
• 00:15 that's what makes it an instance variable.
• 00:17 And our initialize method, I think pretty much all the time,
• 00:21 this is going to be the totality of all your initialized methods.
• 00:25 So they're just going to pass in whatever parameters you want to let into your class
• 00:29 and assign it.
• 00:30 Why?
• 00:31 Why do we do that?
• 00:32 Well, there's a concept called scope, and
• 00:34 in scope where you can use your variables, it matters.
• 00:38 So a regular variable, you can use it outside of wherever,
• 00:42 an instance variable, you can only use it inside of the class.
• 00:47 It doesn't work anywhere else, the scope of it is inside of its class.
• 00:52 Now, we can use this not just in this method,
• 00:55 we can use this in every method inside of our class.
• 00:58 Now, if we had some other variable and it was just a regular variable,
• 01:02 we couldn't use it in an another method, right?
• 01:04 But these instance variables, we can, so it's very important.
• 01:08 So let's create a new method called blueprint for now.
• 01:13 We can use our instance variable of side_length inside this method,
• 01:21 so we might say puts Your square as a side length of, and
• 01:27 let's interpolate this and go @side_length.
• 01:34 So if we save this guy, and let's come down here.
• 01:38 Now, lets say we want to call this method, we could just go puts my_square.blueprint.
• 01:46 Remember, earlier when we wanted a random number we called Random.rand,
• 01:50 same thing here, we're calling my_square.blueprint
• 01:54 Calling the blueprint method in our my_square class,
• 01:58 which is actually this square class, so if I save this and
• 02:03 run it you get, Your square has a side length of 10.
• 02:07 Ain't that cool?
• 02:08 So if we change this to 20, save it, run it again,
• 02:12 Your square has a side length of 20, I think that's really cool.
• 02:15 So puts my_square.inspect,
• 02:20 now, if we look at our class, nothing has changed,
• 02:22 we still have our side_length=20, this is where it exists in memory.
• 02:26 But now, we can use this @side_length anywhere we want inside of our class.
• 02:32 Now, like I've said in the past,
• 02:33 it's not a good idea to use puts inside of a method.
• 02:37 If we save this and run it again, same thing, we're just returning it.
• 02:41 Because we're puts-ing it out onto the screen.
• 02:44 One kind of neat thing, there's a internal Ruby method called to_s.
• 02:49 So if we change the name of this to to_s, and we don't have that return.
• 02:55 I don't know if we save this, and if we just puts out now our class itself,
• 03:02 we get the same, Your square has a side length of 20 because
• 03:07 the to_s replaces that gobbledygook that we had earlier.
• 03:12 We can do an inspect, and we still get the gobbledygook, but if we do it without
• 03:18 the inspect, save it and run it, we get the, Your square has a side length of 20.
• 03:24 That's sort of interesting trivia, we don't need to know that right now.
• 03:27 But anyway, those are instance variables, very important to class.
• 03:31 The scope of an instance variable is within its class,
• 03:34 you can't use it anywhere else.
• 03:35 In the next video, we're going to move into something called class getters.

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