Dictionary Access

To get the value for a specific key from a dictionary, the access syntax can be used: The dictionary is followed by an opening square bracket [, the key, and ends with a closing square bracket ].

Accessing a key returns an optional: If the key is found in the dictionary, the value for the given key is returned, and if the key is not found, nil is returned.

// Declare a constant that has type `{Bool: Int}`,
// a dictionary mapping integers to booleans.
//
let booleans = {
    1: true,
    0: false
}

// The result of accessing a key has type `Bool?`.
//
booleans[1]  // is `true`
booleans[0]  // is `false`
booleans[2]  // is `nil`

// Invalid: Accessing a key which does not have type `Int`.
//
booleans["1"]

// Declare a constant that has type `{Bool: Int}`,
// a dictionary mapping booleans to integers.
//
let integers = {
    true: 1,
    false: 0
}

// The result of accessing a key has type `Int?`
//
integers[true] // is `1`
integers[false] // is `0`

To set the value for a key of a dictionary, the access syntax can be used as well.

// Declare a constant that has type `{Int: Bool}`,
// a dictionary mapping booleans to integers.
//
let booleans = {
    1: true,
    0: false
}

// Assign new values for the keys `1` and `0`.
//
booleans[1] = false
booleans[0] = true
// `booleans` is `{1: false, 0: true}`