Dictionaries
A dictionary is like a real dictionary book: you look up a word (the key) to find its meaning (the value).
Syntax
var name = [key1: value1, key2: value2]
name[key] // returns Value? (optional)
name[key] = value // add or update
name[key] = nil // remove
name.removeValue(forKey: key)Creating and Accessing a Dictionary
A dictionary literal lists key: value pairs in square brackets, and values are looked up by key using subscript syntax, which returns an optional.
Example: Creating and Accessing a Dictionary
// Declare a constant `capitals`, set to `["France": "Paris", "Japan": "Tokyo"]`
let capitals = ["France": "Paris", "Japan": "Tokyo"]
// Print `capitals["France"] as Any` to the console
print(capitals["France"] as Any)
// Print `capitals["Germany"] as Any` to the console
print(capitals["Germany"] as Any)Login to try C/C++/Java/PHP code in the editor
Adding and Updating Entries
Assigning to a dictionary's subscript adds a new key-value pair or updates an existing one.
Example: Adding and Updating Entries
// Declare a mutable variable `inventory`, initialized to `["apples": 10, "bananas": 5]`
var inventory = ["apples": 10, "bananas": 5]
inventory["oranges"] = 8
inventory["apples"] = 12
// Print `inventory["apples"]!` to the console
print(inventory["apples"]!)
// Print `inventory["oranges"]!` to the console
print(inventory["oranges"]!)Login to try C/C++/Java/PHP code in the editor
Removing Entries
A key-value pair can be removed either by setting its value to nil or by calling removeValue(forKey:).
Example: Removing Entries
// Declare a mutable variable `scores`, initialized to `["Alice": 90, "Bob": 75]`
var scores = ["Alice": 90, "Bob": 75]
scores["Bob"] = nil
// Print `scores` to the console
print(scores)Login to try C/C++/Java/PHP code in the editor
Related Topics
{# common_mistakes/chapter_summary/browser_support: on Hindi pages the
view already swaps in the hi_ translation fields (or blanks these
out if untranslated), so this renders correctly for both languages
without a lang_code check here. #}
Common Mistakes
- Assuming subscript access like
dict["key"]always returns the value directly; it actually returns an optional since the key might not exist. - Forgetting that dictionaries in Swift have no guaranteed order -- iterating twice may give different orders.
- Overwriting a key's value unintentionally by using the same key twice when building a dictionary literal.
Chapter Summary
- A dictionary maps keys to values, written as
[KeyType: ValueType]. - Subscripting a dictionary (
dict[key]) always returns an optional, since the key might be missing. .updateValue(),removeValue(forKey:), and direct subscript assignment modify a dictionary.- Dictionaries are unordered -- do not rely on insertion order when iterating.
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