How to append to an array that is a value in a Swift dictionary

Question

Let\'s assume I have a dictionary that accepts a string as the key and an array as value:

var d = [String: [Int]]()
d[\"k\"] = [Int]()

Now I wo

Answer 1

Like this:

var d = [String: [Int]]()
d["k"] = [Int]()
if var items = d["k"] {
    items.append(1)
    d["k"] = items
}

You have to:

1. pull it out
2. unwrap the Optional
3. assign it to a var reference
4. append, and
5. put it back again

The if var line does the first three steps all in one.

Answer 2

All other answers are correct, but If you feel annoying to check nil, unwrap ...

I made a custom operator.

With this, you can unconditionally expand Optional arrays

infix operator +=! {
    associativity right
    precedence 90
    assignment
}

func +=!<T, S:SequenceType where S.Generator.Element == T>(inout lhs:[T]?, rhs:S) {
    if lhs == nil {
        // the target is `nil`, create a new array
        lhs = Array(rhs)
    }
    else {
        // the target is not `nil`, just expand it
        lhs! += rhs
    }
}

Usage:

var d = [String: [Int]]()

// ... d["test"] may or may not exists ...

d["test"] +=! [1]

Answer 3

Because Dictionary and Array are both struct types, you can't modify the Array in place, but instead have to extract and restore it:

if var items = d["k"] {
    items.append(1)
    d["k"] = items
}
else {
    d["k"] = [1]
}

Answer 4

If the element for that key exists, you can simply do:

d["k"]?.append(1)

Note that if no element exists for the "k" key, then nothing happens. If you want to ensure there's actually an array for that key, just add this before appending:

if d["k"] == nil {
    d["k"] = []
}