Swift Dictionary ExamplesUse a dictionary to look up values from keys. Add elements and loop over tuples with for-in.
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Dictionary. In Swift, a dictionary is a primary collection type. We use special syntax to specify key and value types. We add pairs with assignment.

Dictionary info. We can use if-let to safely get a value from the dictionary—this syntax is called optional binding. And we can loop over the dictionary with a for-loop.

An example. This program creates an empty dictionary. It has String keys and Int values—this is specified in the type. It adds 3 keys and values with assignment.

Lookup It accesses a value from the dictionary with key "cat." The result is not nil—it exists.

Value We access the value of the optional Int returned by the dictionary lookup by placing an exclamation mark at its end.

Swift program that creates dictionary
// Create a dictionary with String keys and Int values. var weights = [String: Int]() // Add three pairs to the dictionary. weights["cat"] = 8 weights["dog"] = 30 weights["horse"] = 1100 // Look up value in dictionary and see if it exists. let result = weights["cat"] if result != nil { print("Weight is \(result!)") }
Weight is 8

Initializer. A dictionary can be filled through assignment statements. But an initializer expression requires less code. Here we create another String key, Int value dictionary.

If let With the "if let" syntax, we look up a value in the dictionary. Optional binding means we can directly access the Int.

Swift program that uses dictionary initializer
// Create a constant dictionary. // ... It has String keys and Int values. // ... Initialize it with three pairs. let lookup: [String: Int] = ["Swift": 10, "Python": 5, "Java": 3] // Look up the value for Swift key. if let value = lookup["Swift"] { print(value) }

Strings. We vary the types of keys and values. This dictionary uses String keys and String values—a common pattern in programs. It caches capitalized strings.

Swift program that shows String keys and String values
// This dictionary uses String keys and String values. var capitalized = [String: String]() // Add some data to the dictionary. capitalized["dog"] = "DOG" capitalized["bird"] = "BIRD" // Look up a String value. if let result = capitalized["dog"] { print(result) }

For-in loop. Sometimes we want to loop over all the pairs (tuples) in a dictionary. We use a for-in loop here. In each tuple, we have the key and the value.


Swift program that uses for-in loop on dictionary
// Create a String to Int dictionary. var colors = [String: Int]() colors["blue"] = 10 colors["red"] = 40 colors["magenta"] = 20 // Loop over all pairs in the Dictionary. // ... Order is not maintained. for (color, code) in colors { print("Color is \(color), code is \(code)") }
Color is red, code is 40 Color is magenta, code is 20 Color is blue, code is 10

Keys. A dictionary has a list of keys. Often in programs we use the keys in a dictionary—the values are not always relevant. We loop over the result of the keys property.

Note The keys and values properties are not arrays. But they can easily be converted into arrays.

Int Arrays

Swift program that uses keys
let multipliers: [Int: Int] = [10: 20, 30: 60, 90: 180] // Loop over and display all keys. for key in multipliers.keys { print(key) }
30 10 90

Values, convert to array. Here we initialize another dictionary. We then create a String array based on the dictionary's values property.

And We can use the array (sizesArray) as any other array. We append another element to it.

Swift program that uses values, converts to array
let sizes: [Int: String] = [1: "Small", 10: "Medium", 50: "Large"] // Convert values of dictionary into a String array. var sizesArray = [String](sizes.values) // Add another String. sizesArray.append("Huge") print(sizesArray)
["Large", "Small", "Medium", "Huge"]

Count. A dictionary has a number of key and value pairs. The count property returns this number. It provides a count of pairs—which is the same as key count or value count.

Swift program that uses count
let pages: [String: Int] = ["Index": 0, "About": 10] // Use count property to get number of pairs in dictionary. if pages.count == 2 { print(true) }

UpdateValue. This func changes the value for a key in a dictionary. If the key does not exist, a new value is added. If the key is present, the value is altered at that key.

Swift program that uses updateValue
var pages: [String: Int] = ["Index": 0, "About": 10] // Add or update values in the dictionary. pages.updateValue(200, forKey: "Index") pages.updateValue(300, forKey: "Changes") // Display contents. for (key, value) in pages { print(key, value) }
About 10 Changes 300 Index 200

RemoveValue. This func eliminates a pair in a dictionary. With forKey we specify the key we want to remove. And the value and the key are both erased.

Here We create a dictionary with a key "About." We call removeValue with the forKey argument. The "About" key no longer exists.

Swift program that uses removeValue, forKey
var pages: [String: Int] = ["Index": 0, "About": 10, "Updates": 20] // Remove this key. pages.removeValue(forKey: "About") // Display dictionary. print(pages)
["Updates": 20, "Index": 0]

Contains key. In Swift, a nil value in a dictionary means there is no value. So to determine if a key exists, we just test its value for nil. If nil, the key does not exist.

Here We test for the key "blue." This is set to value 1, so it exists and the first message is printed.

Nil We then determine that "magenta" and "orange" are not keys in the dictionary. We removed "orange" by assigning its value to nil.

Swift program that determines if dictionary contains keys
// Create a dictionary with strings and Ints. // ... Nil values mean "does not exist." var colorIds = [String: Int]() colorIds["blue"] = 1 colorIds["red"] = 2 colorIds["yellow"] = 3 colorIds["orange"] = nil // Detect whether the dictionary contains this string. if colorIds["blue"] != nil { print("Dictionary contains value") } // A nil value means the key does not exist. if colorIds["magenta"] == nil { print("Not found 1") } // This key was assigned nil. // ... This means it does not exist. if colorIds["orange"] == nil { print("Not found 2") }
Dictionary contains value Not found 1 Not found 2

Increment, decrement. It is possible to increment and decrement the values in a dictionary. We must use the question mark to access the optional value (which may not exist).

Note If we increment an optional value that is nil (on a key that does not exist) nothing will happen. No key will be added.

Syntax We must use the "+=" operator to increment by 1. Only Swift 1 and 2 support the "++" operator.

Swift program that increments values with optional syntax
var freqs = ["cat": 10, "dog": 20] // Use optional syntax to increment value. freqs["cat"]? += 1 // Decrement value. freqs["dog"]? -= 1 // A nonexistent key will not cause an error. // ... No value will be added. freqs["bird"]? += 1 freqs["bird"]? += 2 // Cat and dog were modified. print(freqs)
["cat": 11, "dog": 19]

Argument. A dictionary can be passed as an argument to a func. We specify the type of the keys and the values. In Swift we use a ":" to begin a type description.

Here We pass the stocks dictionary of String keys and Double values to a validate func. A guard statement ensures the dictionary has data.


Swift program that uses dictionary as func argument
func validate(stocks: [String: Double]) { // Ensure at least one pair in dictionary. guard stocks.count >= 1 else { print("Error") return } // Dictionary is valid. print("OK") } // Create a String, Double dictionary. var stocks = ["ABC": 10.99, "XYZA": 9.24] validate(stocks: stocks) // ... This will not print OK. validate(stocks: [String: Double]())
OK Error

Nested dictionary. A dictionary can be placed inside another collection like an array. With an array of dictionaries, we can store more complex data in a simple way.

Dictionary, nested

Sort. The keys and values in a dictionary cannot be directly sorted. But we can take the keys (or values) from the collection and place them in a list, sorting that.

Sort: dictionary keys

Memoization. Suppose we need to lowercase many strings, and many values will be repeated. We can use a dictionary to memoize (cache) the result. This can improve performance.


A review. This generic collection is powerful. With dictionaries, we can make lookups faster. Often for optimization, changing an array to a dictionary is effective.

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