如何在 Swift 中使用字典初始化结构

Question

I want to initialize every time a struct with dictionaries. Later, I'm going to use its properties instead a dictionary's keys and values - it seems rather easier. However, when I try the code below, it tells me that "Return from initializer without initializing all stored properties" and "1. 'self.one' not initialized" and "2. 'self.two' not initialized". My question is how to initialize a struct from a dictionary, so that I have basically a struct with the contents of the dictionary? Or how to transform it into struct?

struct Blabla {
    var one: String
    var two: [Int]

    init(three: [String: [Int]]) {
        for i in three {
            self.one = i.key
            self.two = i.value
        }

    } ERROR! - Return from initializer without initializing all stored properties
}

Answer 1

struct Blabla { var one: String var two: [Int]

init(three: [String: [Int]]) {
    one = ""
    two = []
    for i in three {
        self.one = i.key
        self.two = i.value
    }

} ERROR! - Return from initializer without initializing all stored properties

}

Answer 2

for in clause may have zero runs, in which case struct properties will not be initialized. You have to provide default values (or emit fatalError if you really need to).

While I think your example is pure synthetical, there is no need to loop through array, you can set properties to its last entry.

Answer 3

This code should compile, but it feels unsafe to me to initialize a Struct in this way because:

• It assume your dictionary has values in it.
• Your stored properties will always have the last value you looped through.
• In order to pull values out to satisfy the compiler you need to force unwrap them. (With Dávid Pásztor's guard-letting approach, this can be avoided)

struct Blabla {
    var one: String
    var two: [Int]

    init(three: [String: [Int]]) {
        self.one = three.keys.first!
        self.two = three[three.keys.first!]!
    }
}

let input = ["pizza": [1,2]]
let l = Blabla(three: input)

If I were you I would let the memberwise initializer that you get for free do its thing and provide either a specialized initializer to handle your case of taking a Dictionary as input or move that parsing to another function/class/etc....

Answer 4

The issues is that if three is an empty Dictionary , the instance properties one and two don't get initialised. Also, you are overwriting the properties in each iteration of the for loop and the compiler cannot guarantee that there will be any iterations of the loop in compile-time, hence the compiler error.

You could make the initialiser failable to account for this by checking that the dictionary actually contains at least one key-value pair and assigning that first key-value pair to your properties.

struct Blabla {
    var one: String
    var two: [Int]

    init?(three: [String: [Int]]) {
        guard let key = three.keys.first, let value = three[key] else { return nil }
        one = key
        two = value
    }
}

However, you should rethink what it is that you are actually trying to achieve, since with your current setup you have a mismatch between your init input values and the properties of your struct.

Answer 5

The compiler error is clear: If the dictionary is empty the struct members are never initialized. But the code makes no sense anyway as each iteration of the dictionary overwrites the values.

Maybe you mean to map the dictionary to an array of the struct

struct Blabla {
    let one: String
    let two: [Int]
}

let three = ["A":[1,2], "B":[3,4]]
let blabla = three.map{Blabla(one: $0.key, two: $0.value)}

print(blabla) // [Blabla(one: "A", two: [1, 2]), Blabla(one: "B", two: [3, 4])]

Answer 6

i find out that when you are initialising an empty object of some struct then you can use this

var nameofvar = Blabla([:])