What does "??" stand for in the context of: let x = y ?? "Candy bar"

Question

I'm new to Swift. Could someone please tell me what "??" stands for and what is its function in the context of

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

I have included the complete code below:

struct Item{
    var price: Int
    var count: Int
}

class VendingMachine {
    var inventory = [
    "Candy bar": Item(price: 12, count: 7),
    "Chips": Item(price: 10, count: 4),
    "Pretzels": Item(price: 7, count: 11)
    ]
    var coinsDeposited = 0
    func dispenseSnack(snack: String){
        print("dispensing \(snack)")
    }

    func vend(itemNamed name: String) throws {
        guard var item = inventory[name] else {
            throw VendingMachineError.InvalidSelection
        }

        guard item.count > 0 else {
            throw VendingMachineError.OutOfStock
        }

        guard item.price <= coinsDeposited else {
            throw VendingMachineError.InsufficientFunds(coinsNeeded: item.price - coinsDeposited)
        }

        coinsDeposited -= item.price
        --item.count
        inventory[name] = item
        dispenseSnack(name)
    }
}

let favoriteSnacks = [
    "Alice": "Chips",
    "Bob": "Licorice",
    "Eve": "Pretzels",
]

func buyFavoriteSnack(person: String, vendingMachine: VendingMachine) throws {
    let snackName = favoriteSnacks[person] ?? "Candy bar"
    try vendingMachine.vend(itemNamed: snackName)
}

See http://nshipster.com/swift-operators/ - and search for "swift operators"; this search 'trick' works for just about any language. By isolating the type of symbol ("operator"), and then using a localized (in page) search.. — user2864740, Feb 11 '16 at 05:28
Possible duplicate of [?? operator in Swift](https://stackoverflow.com/questions/30772063/operator-in-swift) — Cristik, Feb 06 '19 at 06:02

Hitendra Solanki · Accepted Answer · 2016-02-11T05:47:30.313

Its used to checking nil, if your value will be nil, you will assign default value.

let favoriteSnacks = [
    "Alice": "Chips",
    "Bob": "Licorice",
    "Eve": "Pretzels",
]

from your code,

suppose person = "Eve"

let snackName = favoriteSnacks["Eve"] ?? "Candy bar"

it will first try to find value from dictionary, i.e. favoriteSnacks["Eve"] will give you value "Pretzels".

so it will assign value Pretzels to snackName variable.

suppose person = "Allen"

let snackName = favoriteSnacks["Allen"] ?? "Candy bar"

it will first try to find value from dictionary, i.e. favoriteSnacks["Allen"] will give you value nil.

In this case it will assign defalut value "Candy bar" to your variable snackName.

Thank you so much for the detailed explanation!! Really appreciate your help! — Thor, Feb 11 '16 at 05:43

score 2 · Answer 2 · answered Feb 11 '16 at 05:29

Let me please assume here based on its meaning in other languages:

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

?? checks if the value of favoriteSnacks[person] is null. If it is NOT then the value of favoriteSnacks[person] will be assigned to snackName. If the value of favoriteSnacks[person] is null then "Candy bar" will be assigned to snackName.

It is used for having a default value in case favoriteSnacks[person] comes null.

score 2 · Answer 3 · edited May 23 '17 at 11:46

2

Its the Nil Coalescing Operator basically giving a default for when the value does not exist.

see also Providing a default value for an Optional in Swift?

edited May 23 '17 at 11:46

Community

1
1

answered Feb 11 '16 at 05:30

Timothy Pratley

10,586
3
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don't provide link, instead of this thing you can copy paste the text from that link and paste in answer, because there can be change that webpage will be removed. – Hitendra Solanki Feb 11 '16 at 05:44

score 0 · Answer 4 · answered Feb 06 '19 at 04:32

Ex. if you get only 5 keys and values in dictionary, but some time getting nil key or value, then error will be come for nil value. So, here solution for that,

 var strName = jsonObjectName ?? "Unknown"
 var intAge = jsonObjectAge ?? 25

it means, if jsonObjectName is nil then automatically "Unknown" will be set to strName.

Note: default value provided by "?? value"

score 0 · Answer 5 · answered Jun 20 '19 at 04:35

0

Simply explained,

let value = x ?? y

Is similar to writing,

let value = x != nil ? x! : y

It checks if x contains nil, if it doesn't then it force unwraps it, otherwise returns y in the above case.

answered Jun 20 '19 at 04:35

Mahesh

1,472
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What does "??" stand for in the context of: let x = y ?? "Candy bar"

5 Answers5