The way Optional described makes me think that aren't all data optionals ?

In the end , who can always been 100% sure the specific piece of data we are trying to access is there ?

Right ?

Then, do we need to always mark data with ? or using default value or using nil-coalescing ?

Thank you in advance,

Boat

Vince has the right answer! Here are additional examples.

Example

Get a box and write "Boat's Height" on it. Make a declaration that only Integers can go into the box. Now try and put the box on your desk. Your desk will complain that you cannot put the box on the desk until you put a value in the box. (You must initialize it.)

Once you initialize the box with a value and you declared the box as a var, you can change the value inside the box, but the box must always have an integer in it.

If you try to take the value out and put nothing in the box (ie give the box a nil value, your desk will complain again.)

var boatsHeight : Int  // Box can only hold Integers
print("Boats' height: \(boatsHeight)") // Swift will complain about this!

boatsHeight = 103  // Must have a value!
print("Boats' height: \(boatsHeight)")  // get the value in the box.

// Try!  Try to put nothing in the box. Swift complains!
boatsHeight = nil // error: 'nil' cannot be assigned to type 'Int'

Ask yourself, what's a good value for boatsHeight if you don't know it?
Zero? A negative one? Any negative number? Using a convention like this will add additional if-else rules to your business logic. Not best practice.

Optional Example

Now make a declaration that the box is optional Integer. You can put the box on your desk with or without a value in it. If a friend calls you up and asks "What's the value in the box?" You can have a look and report back the box is empty! Or you can tell them there's an integer in the box, and give them the value.

To use the value, you first must determine if you HAVE A VALUE IN THE BOX. If the box has a value, then you can use it.

// Paste into Playgrounds
var boatsHeight : Int? = 103

boatsHeight // Ask playgrounds to tell you what's in the box.

// You always have this box on your desk.
// But, it might not have anything inside it.
boatsHeight = nil // Put nothing in the box
let height = boatsHeight == nil ?  "unknown" : "\(boatsHeight)"  // check first before using
print("Boat's height is: \(height)")

What if you don't know a person's height?
You could set height to 0, or to a negative number. But in your program, you'll have to deal with this extra business rule. Avoid that issue by using an optional when ever you may or may NOT know the value.

boatsHeight = 105  // Now the box has a value.
if let height = boatsHeight { // Still, please check to see if it has a value.
    print("Boat’s height is: \(height)" )
}

HI Vince, @vtabmow

you said:

When you create a value by saying Int, Date, String,... you're saying it has to contain something. So by creating a non-optional value you're saying it absolutely has to have something in there or your program is not going to compile. You create it as non-optional and you assign it something, so you're 100% sure it's there.

can I understand it that:

By creating an non-optional value , I tell Swift that "Don't compile if there is not value , because it is supposed to have a value for sure"

By creating an optional value, I tell Swift that "You may still compile if there is no value, because I am telling you so. "

Is my understanding above correct ?

Boat

Almost.

If you declare a non-optional value the compiler don't let you compile your program if there is no value before accessing it the first time.

On the other hand, if you declare an optional value the compiler makes sure you unwrap the value before accessing it (either with if let, guard or !)

So in both cases the compiler helps you to avoid a fatal error at runtime.

Thank you @Hatsushira

Have a nice day

Boat