Dictionaries [String: [String]] in Swift

Question

I have a dictionary that looks like this:

var dict = [String: [String]]()

I want to be able to add multiple arrays for a single key. This works fine:

dict["hello"] = ["item 1"]

But when I assign a new array the previous value is obviously overwritten - we want to avoid that:

dict["hello"] = ["item 2"] // overwrites item 1 – how to avoid overwriting?

So I tried to use the append method, but this returns nil:

dict["hello"]?.append("test") // does nothing? output: ()

How can I append strings to the array (value) of a certain key in Swift?

Solution 1

First of all...

... you don't really want this

I want to be able to add multiple arrays for a single key.

Instead I think you want...

... to add a string to the array associated to a given string

Example

In other words you want to go from this

["hello":["item 1"]]

to this

["hello":["item 1", "item 2"]]]

So, how to do it?

Let's begin with your dictionary

var dict = [String: [String]]()
dict["hello"] = ["item 1"]

Now you need to extract the array associated to the hello key

var list = dict["hello"] ?? []

adding a string to it

list.append("item 2")

and finally adding the updated array back into the dictionary

dict["hello"] = list

That's it

Solution 2

This is what your code does

dict["hello"] = ["item 1"] - This sets hello to ["item 1"]

dict["hello"] = ["item 2"] - This sets hello to ["item 2"]

This is just like a variable, for example:

var hello = Array<String>()

hello = ["item 1"] // prints out ["item 1"]

hello = ["item 2"] // prints out ["item 2"]

This is what is happening with your dictionary. You are overriding any stored data with new data.

---

The problem with appending. This only works if there is already an array at that key.

dict["hello"]?.append("test") This wouldn't work.

But this would.

dict["hello"] = ["test 1"]

dict["hello"]?.append("test")

print(dict) // prints out ["dict":["test 1","test"]]

---

What you need to do

var dict = Dictionary<String,Array<String>>()

func add(string:String,key:String) {

    if var value = dict[key] {

        // if an array exist, append to it

        value.append(string)

        dict[key] = value

    } else {

        // create a new array since there is nothing here

        dict[key] = [string]
    }
}

add(string: "test1", key: "hello")

add(string: "test2", key: "hello")

add(string: "test3", key: "hello")

print(dict) // ["hello": ["test1", "test2", "test3"]]

---

Dictionary Extension

extension Dictionary where Key == String, Value == Array<String> {

    mutating func append(_ string:String, key:String) {

        if var value = self[key] {

            // if an array exist, append to it

            value.append(string)

            self[key] = value

        } else {

            // create a new array since there is nothing here

            self[key] = [string]
        }
    }
}

How to use

override func viewDidLoad() {
    super.viewDidLoad()

    // Do any additional setup after loading the view.

    var dict = Dictionary<String,Array<String>>()

    dict.append("first", key: "hello")

    dict.append("second", key: "hello")

    dict.append("thrid", key: "hello")

    dict.append("one", key: "goodbye")

    dict.append("two", key: "goodbye")

    print(dict) // ["hello": ["first", "second", "thrid"], "goodbye": ["one", "two"]]
}

Solution 3

Other people have the correct solution. Here is a quick shorthand for the same answer.

var dict = [String: [String]]()

dict["hello"] = (dict["hello"] ?? []) + ["item 1"]
dict["hello"] = (dict["hello"] ?? []) + ["item 2"]

In Swift 4, this will be

var dict = [String: [String]]()

dict["hello"] = dict["hello", default: []] + ["item 1"]
dict["hello"] = dict["hello", default: []] + ["item 2"]

Solution 4

Please try this thing and let me know if this is what you require

    import UIKit

    var dict = [String: [String]]()
if var value = dict["hello"]{
    value.append("Hi")
    dict["hello"] = value
}else{
    dict["hello"] = ["item 1"]
}

View more solutions

Comments

[Cesare]

While this works if there is at least a value for a certain key, I want to be able to set the value for a key regardless if the dictionary contains a value for a specific key or not.

[Usama Sadiq]

you can set the new value in else block. check the edited code

[Cesare]

Pretty neat, Tristan! Could this be further developed into an extension?

[Hamish]

In Swift 4, just dict["hello", default: []].append("item 1") :)

[Cesare]

No + candidates produce the expected contextual result type [String]

[Tristan Beaton]

I have added the extension. @Cesare

[Jeffery Thomas]

@Cesare hmm… don't know what to say. This test in Xcode 8.3.1 playground and Xcode 9 Beta playground. It uses the + operator for Arrays.