?? operator in Swift

Question

In the "The Swift Programming Language" book (page 599), I came across this code snippet that kind of confused me. It went like this:

func buyFavoriteSnack(person:String) throws {
    let snackName = favoriteSnacks[person] ?? "Candy Bar"
    try vend(itemName:snackName)
}

Its explanation was:

The buyFavoriteSnack(_:) function looks up the given person's favorite snack and tries to buy it for them. If they don't have a favorite snack listed, it tries to buy a candy bar. If they...

How can this explanation map to the "??" operator in the code given. When should/can we use this syntax in our own code?

There is a "The Swift Programming Language" iBook, and you *can* search for "??" in iBooks. The first hit is the "Nil Coalescing Operator" chapter ... — Martin R, Jun 11 '15 at 05:21
It might help to pronounce `??` as "otherwise" when you're reading code. — Luke, Mar 16 '18 at 20:58

score 92 · Accepted Answer · edited Apr 02 '22 at 13:35

92

It is "nil coalescing operator" (also called "default operator"). a ?? b is value of a (i.e. a!), unless a is nil, in which case it yields b. I.e. if favouriteSnacks[person] is missing, return assign "Candy Bar" in its stead.

edited Apr 02 '22 at 13:35

Vega

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answered Jun 11 '15 at 05:15

Amadan

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Yup, Google wasn't helpful with the question marks. Somehow I missed this in my studies. The nil coalescing operator did it for me. – avismara Jun 11 '15 at 05:18
If the value of `a` is non-nil, the value of `b` is not evaluated. This is known as short-circuit evaluation. – iDhaval Jun 11 '15 at 05:58
The big problem with the ?? operator in Swift is that `a` is not unwrapped as a part of it. But `b` has to be a none optional or implicit unwrapped optional variable. – Morten Holmgaard Jan 07 '16 at 07:31
1

@MortenHolmgaard: What do you mean? ["The *nil coalescing operator* (`a ?? b`) unwraps an optional `a` if it contains a value, or returns a default value `b` if `a` is `nil`"](https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/BasicOperators.html#//apple_ref/doc/uid/TP40014097-CH6-ID72). – Amadan Jan 07 '16 at 07:47
@Amadan Strange - your right it seems, I just testede it in Playground, but I have multiple places in my apps where I have had to handle this case manually not to get optional(somevalue) when formatting a string. – Morten Holmgaard Jan 08 '16 at 07:23

score 29 · Answer 2 · answered Mar 31 '16 at 20:48

29

let something = a ?? b

means

let something = a != nil ? a! : b

answered Mar 31 '16 at 20:48

Bader Al-Rasheed

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I like this the best because it reminds me of my C/C++/PHP days! – Neal Davis Oct 28 '18 at 15:40

score 26 · Answer 3 · edited Nov 29 '15 at 01:39

26

This:

let snackName = favoriteSnacks[person] ?? "Candy Bar"

Is equals this:

if favoriteSnacks[person] != nil {
    let snackName = favoriteSnacks[person]    
} else {
    let snackName = "Candy Bar"
}

Explaining in words, if the let statement fail to grab person from favoriteSnacks it will assigned Candy Bar to the snackName

edited Nov 29 '15 at 01:39

Ray Toal

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answered Jun 11 '15 at 05:18

Icaro

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Seems Candy Bars are everyones favourite :) – BangOperator Apr 08 '16 at 16:48

score 5 · Answer 4 · edited Mar 16 '18 at 20:28

5

The nil-coalescing operator a ?? b is a shortcut for a != nil ? a! : b

edited Mar 16 '18 at 20:28

Floern

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answered Mar 16 '18 at 20:02

Muhammad Naeem Paracha

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score 0 · Answer 5 · answered Jun 11 '15 at 07:38

One addition to @Icaro's answer you can declare values without initialize them. In my opinion this is better:

func buyFavoriteSnack(person:String) throws {
    // let snackName = favoriteSnacks[person] ?? "Candy Bar"
    let snackName: String

    if let favoriteSnackName = favoriteSnacks[person] {
        snackName = favoriteSnackName
    } else {
        snackName = "Candy Bar"
    }

    try vend(itemName:snackName)
}

?? operator in Swift

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