Swift: let is uninitialized but var gets initialized. Why?

Question

I'm using playgrounds to learn Swift and I noticed something strange when trying to set optionals on var and let. Below is the code with output:

var num1: Int?
print(num1) //Output: "nil\n"

let num2: Int?
print(num2) //Output: Error: Constant num2 used before initialized

I do not understand why 'var' gets initialized with nil and 'let' is uninitialized when made optional.

My guess is language set `nil` as default value when you create `var` but do not do this when you declare `let` — JuicyFruit, Mar 16 '17 at 08:13
Related: http://stackoverflow.com/questions/34969234/why-doesnt-swift-allow-setting-value-of-an-optional-constant-after-object-initi. — Martin R, Mar 16 '17 at 08:16
@Hamish: That is what I was looking for. How do you find the (correct) duplicates so quickly? — Martin R, Mar 16 '17 at 08:19
@MartinR Combination of keeping them favourited and google searches :) I sometimes remember the name of the person who answered / asked the question – which helps the search. — Hamish, Mar 16 '17 at 08:20

Sagar Chauhan · Answer 1 · 2017-03-16T08:39:36.037

0

var is variable, you can declare it without initialisation. But let is constant, you should initialise it’s value, i.e.

    var num1: Int?
    print(num1) //Output: "nil\n"

    let num2: Int = 20
    print(num2)

edited Mar 16 '17 at 08:39

answered Mar 16 '17 at 08:14

Sagar Chauhan

You can also visit here : http://stackoverflow.com/questions/24002092/what-is-the-difference-between-let-and-var-in-swift – Sagar Chauhan Mar 16 '17 at 08:16
4

That is not quite correct (or misleading). You must assign a value to a constant before it is used, but not necessarily in its declaration. Example: `let num2: Int; num2 = 5 ; print(num2)` – Martin R Mar 16 '17 at 08:18

1 Answers1