swift - Adding items to Array as a Dictionary Value

Question

I'm trying to figure out the best way in Swift to add values to an Array that is a Value in a Dictionary. I want to build a dictionary of contacts sorted by the first letter of their first name. For example [A : [Aaron, Adam, etc...], B : [Brian, Brittany, ect...], ...]

I found this function:

updateValue(_:forKey:)

And tried using it in a loop:

for contact in self.contacts.sorted() {
    self.contactDictionary.updateValue([contact], forKey: String(describing: contact.characters.first))           
}

But when I tried to use that it replaced the existing array with a new one. I know I can manually check to see if the key in the dictionary exists, if it does, retrieve the array and then append a new value, otherwise add the new key/value pair but I'm not sure if Swift provides an easier/better way to do this.

Any insight would be much appreciated!

Answer 1

You can use reduce(into:) method (Swift4) and as follow:

let contacts = ["Aaron", "Adam", "Brian", "Brittany", ""]
let dictionary = contacts.reduce(into: [String:[String]]()) { result, element in
    // make sure there is at least one letter in your string else return
    guard let first = element.first else { return }
    // create a string with that initial
    let initial = String(first)
    // initialize an array with one element or add another element to the existing value
    result[initial] = (result[initial] ?? []) + [element]
}
print(dictionary)   // ["B": ["Brian", "Brittany"], "A": ["Aaron", "Adam"]]

---

If you are using Swift3 or earlier you would need to create a mutable result dictionary inside the closure:

let contacts = ["Aaron", "Adam", "Brian", "Brittany", ""]
let dictionary = contacts.reduce([String:[String]]()) { result, element in
    var result = result
    guard let first = element.first else { return result }
    let initial = String(first)
    result[initial] = (result[initial] ?? []) + [element]
    return result 
}
print(dictionary)   // ["B": ["Brian", "Brittany"], "A": ["Aaron", "Adam"]]

---

Note that the result is not sorted. A dictionary is an unordered collection. If you need to sort your dictionary and return an array of (key, Value) tuples you can use sorted by key as follow:

let sorted = dictionary.sorted {$0.key < $1.key}
print(sorted)

"[(key: "A", value: ["Aaron", "Adam"]), (key: "B", value: ["Brian", "Brittany"])] "