How to "Append" to an Immutable Dictionary in Swift

How do I append to an immutable dictionary in Swift?

Unfortunately, this is a good question because the answer is "you can't". Not yet, anyway--others agree this should be added, because there's a Swift Evolution proposal for this (and some other missing Dictionary features). It's currently "awaiting review", so you may see a merged() method that's basically your + operator in a future version of Swift!

In the meantime, you can use your solution to append entire dictionaries, or for one value at a time:

extension Dictionary {
    func appending(_ key: Key, _ value: Value) -> [Key: Value] {
        var result = self
        result[key] = value
        return result
    }
}

How to add another dictionary entry to an existing dictionary to form a new dictionary (i.e. not append)

There is a built-in merging(_:uniquingKeysWith:) function on Dictionary that does exactly what you need.

let dictionary = [1:1]
let otherDictionary = [1:2, 2:3]
// This will take the values from `otherDictionary` if the same key exists in both
// You can use `$0` if you want to take the value from `dictionary` instead
let mergedDict = dictionary.merging(otherDictionary, uniquingKeysWith: { $1 })

If you want, you can easily define a + operator for Dictionary that uses the above function.

extension Dictionary {
    static func + (lhs: Self, rhs: Self) -> Self {
        lhs.merging(rhs, uniquingKeysWith: { $1 })
    }
}

let addedDict = dictionary + otherDictionary

How do I append to an immutable dictionary in Swift?

Unfortunately, this is a good question because the answer is "you can't". Not yet, anyway--others agree this should be added, because there's a Swift Evolution proposal for this (and some other missing Dictionary features). It's currently "awaiting review", so you may see a merged() method that's basically your + operator in a future version of Swift!

In the meantime, you can use your solution to append entire dictionaries, or for one value at a time:

extension Dictionary {
    func appending(_ key: Key, _ value: Value) -> [Key: Value] {
        var result = self
        result[key] = value
        return result
    }
}

Swift - Append to NSMutableDictionary

Yes you are actually replacing the RSVPDirectory[firstLetter], overriding it every time with new tmp.

What you are looking for is this:

//RSVPDirectory[firstLetter] = tmp //Replace this line with below code
let tempArray = RSVPDirectory[firstLetter] as? [AnyHashable]
tempArray?.append(tmp)
RSVPDirectory[firstLetter] = tmpArray

Here I have used a tempArray because we want to mutate the array. Accessing it directly and trying to append new value will in-turn try to mutate an immutable value. So first I have got the array in the tempArray and then after mutating the array I swapped it back in the dictionary with updated values.

Append to array in [String: Any] dictionary structure

The type of data[Items] isn't Array but actually Array<[String: Any]>.

You could probably squeeze this into fewer steps, but I prefer the clarity of multiple steps:

var data: [String: Any] = [
    "key1": "example value 1",
    "key2": "example value 2",
    "items": []
]

for index in 1...3 {

    let item: [String: Any] = [
        "key": "new value"
    ]

    // get existing items, or create new array if doesn't exist
    var existingItems = data["items"] as? [[String: Any]] ?? [[String: Any]]()

    // append the item
    existingItems.append(item)

    // replace back into `data`
    data["items"] = existingItems
}

How do I append an element to an array in a nested dictionary

You do it the same way Superman gets into his pants — one leg at a time. Pull out the array, append to it, put it back again:

var B = [ "EA" : [ "status": [["000": "OK"]]]]
B["EA"]?["status"] = [["000": "NOT OK"]]

print(B) // ["EA": ["status": [["000": "NOT OK"]]]]

if var arr = B["EA"]?["status"] {
    arr.append(["001":"Good"])
    B["EA"]?["status"] = arr
}

print(B) // ["EA": ["status": [["000": "NOT OK"], ["001": "Good"]]]]

How do you add a Dictionary of items into another Dictionary

You can define += operator for Dictionary, e.g.,

func += <K, V> (left: inout [K:V], right: [K:V]) { 
    for (k, v) in right { 
        left[k] = v
    } 
}

How to add field to NSMutableDictionary Swift

Use Dictionary in swift instead of NSDictionary. Also, you need to give the type while declaring the Dictionary variable.

Try this:

var villages = [[String : Any]]()
var bars = [[String : Any]]()
var allBars = [[String : Any]]()

//And this
bars[2]["Distance"] = distanceInMiles

bars[2] in the above code will only work if the array bars has atleast 3 elements in it. Otherwise it will give "Array index out of bounds" exception.

Swift - Adding value to an array inside a dictionary

EDIT: Thanks to Martin's comment. The snippet below is the the most succinct answer I can think of. I was initially coming at it from a wrong direction. and I was getting an error. See comments

struct Student { 
    let id: Int
    let subject : String
}

var studentArray = [Student(id: 1, subject: "History"), Student(id: 2, subject: "History"), Student(id:1, subject: "Maths")]

typealias Subject = String
var dict : [Int: [Subject]] = [:]

for student in studentArray {

    (dict[student.id, default: []]).append(student.subject)
}

print(dict)

---

Previous answers:

struct Student { 
    let id: Int
    let subject : String
}

var studentArray = [Student(id: 1, subject: "History"), Student(id: 2, subject: "History"), Student(id:1, subject: "Maths")]

typealias Subject = String
var dict : [Int: [Subject]] = [:]

for student in studentArray {
    var subjects = dict[student.id] ?? [String]()
    subjects.append(student.subject)
    dict[student.id] = subjects
}

print(dict)

Or you can do it this way:

struct Student { 
    let id: Int
    let subject : String
}

var studentArray = [Student(id: 1, subject: "History"), Student(id: 2, subject: "History"), Student(id:1, subject: "Maths")]

typealias Subject = String
var dict : [Int: [Subject]] = [:]

for student in studentArray {
    if let _ = dict[student.id]{
        dict[student.id]!.append(student.subject)
    }else{
        dict[student.id] = [student.subject]
    }
}

print(dict)

whichever you like

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