---
title: "SE-0165: Dictionary & Set Enhancements"
framework: swift-evolution
role: article
path: swift-evolution/0165-dict
---

# SE-0165: Dictionary & Set Enhancements

* Proposal: [SE-0165](0165-dict.md)
* Author: [Nate Cook](https://github.com/natecook1000)
* Review Manager: [Ben Cohen](https://github.com/airspeedswift)
* Status: **Implemented (Swift 4.0)**
* Decision Notes: [Rationale][rationale]

## Introduction

This proposal comprises a variety of commonly (and less commonly) suggested improvements to the standard library's `Dictionary` type, from merging initializers to dictionary-specific `filter` and `mapValues` methods. The proposed additions to `Dictionary`, and the corresponding changes to `Set`, are detailed in the sections below.

- Suggested Improvements:     - [Merging initializers and methods](#1-merging-initializers-and-methods)     - [Key-based subscript with default value](#2-key-based-subscript-with-default-value)     - [Dictionary-specific map and filter](#3-dictionary-specific-map-and-filter)     - [Visible `Dictionary` capacity](#4-visible-dictionary-capacity)     - [Grouping sequence elements](#5-grouping-sequence-elements)     - [Relevant changes to `Set`](#6-apply-relevant-changes-to-set) - [Detailed Design](#detailed-design)     - [Sandbox with Prototype][sandbox] - [Source Compatibility](#source-compatibility)

---

## 1. Merging initializers and methods

The `Dictionary` type should allow initialization from a sequence of `(Key, Value)` tuples and offer methods that merge a sequence of `(Key, Value)` tuples into a new or existing dictionary, using a closure to combine values for duplicate keys.

- [First message of discussion thread](https://forums.swift.org/t/map-like-operation-that-returns-a-dictionary/999) - [Initial proposal draft](https://forums.swift.org/t/map-like-operation-that-returns-a-dictionary/999/18) - [Prior standalone proposal (SE-100)](https://github.com/swiftlang/swift-evolution/blob/master/proposals/0100-add-sequence-based-init-and-merge-to-dictionary.md)

`Array` and `Set` both have initializers that create a new instance from a sequence of elements. The `Array` initializer is useful for converting other sequences and collections to the "standard" collection type, while the `Set` initializer is essential for recovering set operations after performing any functional operations on a set. For example, filtering a set produces a collection without any set operations available.

```swift let numberSet = Set(1 ... 100) let fivesOnly = numberSet.lazy.filter { $0 % 5 == 0 } ```

`fivesOnly` is a `LazyFilterCollection<Set<Int>>` instead of a `Set`—sending that back through the `Set` sequence initializer restores the expected methods.

```swift let fivesOnlySet = Set(numberSet.lazy.filter { $0 % 5 == 0 }) fivesOnlySet.isSubsetOf(numberSet) // true ```

`Dictionary`, on the other hand, has no such initializer, so a similar operation leaves no room to recover dictionary functionality without building a mutable `Dictionary` via iteration or functional methods. These techniques also don't support type inference from the source sequence, increasing verbosity.

```swift let numberDictionary = ["one": 1, "two": 2, "three": 3, "four": 4] let evenOnly = numberDictionary.lazy.filter { (_, value) in     value % 2 == 0 }

var viaIteration: [String: Int] = [:] for (key, value) in evenOnly {     viaIteration[key] = value }

let viaReduce: [String: Int] = evenOnly.reduce([:]) { (cumulative, kv) in     var dict = cumulative     dict[kv.key] = kv.value     return dict } ```

Beyond initialization, `Array` and `Set` both also provide a method to add a new block of elements to an existing collection. `Array` provides this via `append(contentsOf:)` for the common appending case or `replaceSubrange(_:with:)` for general inserting or replacing, while the unordered `Set` type lets you pass any sequence to `unionInPlace(_:)` to add elements to an existing set.

Once again, `Dictionary` has no corresponding API -- looping and adding elements one at a time as shown above is the only way to merge new elements into an existing dictionary.

### Proposed solution

This proposal puts forward two new ways to convert `(Key, Value)` sequences to dictionary form: an initializer and a set of merging APIs that handle input data with duplicate keys.

#### Sequence-based initializer

The proposed solution would add a new initializer to `Dictionary` that accepts any sequence of `(Key, Value)` tuple pairs.

```swift init<S: Sequence where S.Iterator.Element == (key: Key, value: Value)>(     uniqueKeysWithValues: S) ```

With the proposed initializer, creating a `Dictionary` instance from a sequence of key/value pairs is as easy as creating an `Array` or `Set`:

```swift let viaProposed = Dictionary(uniqueKeysWithValues: evenOnly) ```

Like `Array.init(_:)` and `Set.init(_:)`, this is a full-width initializer. To ensure this, the initializer has a precondition that each key in the supplied sequence is unique, and traps whenever that condition isn't met. When duplicate keys are possible in the input, the merging initializer described in the next section must be used instead.

The new initializer allows for some convenient uses that aren't currently possible.

- Initializing from a `DictionaryLiteral` (the type, not an actual literal)

```swift     let literal: DictionaryLiteral = ["a": 1, "b": 2, "c": 3, "d": 4]     let dictFromDL = Dictionary(uniqueKeysWithValues: literal)     ```

- Converting an array to an indexed dictionary (popular on the thread)

```swift     let names = ["Cagney", "Lacey", "Bensen"]     let dict = Dictionary(uniqueKeysWithValues: names.enumerated().map { (i, val) in (i + 1, val) })     // [2: "Lacey", 3: "Bensen", 1: "Cagney"]     ```

- Initializing from a pair of zipped sequences (examples abound)

```swift     let letters = "abcdef".characters.lazy.map(String.init)     let dictFromZip = Dictionary(uniqueKeysWithValues: zip(letters, 1...10))     // ["b": 2, "e": 5, "a": 1, "f": 6, "d": 4, "c": 3]     ```

> This particular use is currently blocked by [SR-922](https://bugs.swift.org/browse/SR-922). As a workaround, add `.map {(key: $0, value: $1)}`.

#### Merging initializer and methods

Creating a `Dictionary` from a dictional literal checks the keys for uniqueness, trapping on a duplicate. The sequence-based initializer shown above has the same requirement.

```swift let duplicates: DictionaryLiteral = ["a": 1, "b": 2, "a": 3, "b": 4] let letterDict = Dictionary(uniqueKeysWithValues: duplicates) // error: Duplicate key found: "a" ```

Because some use cases can be forgiving of duplicate keys, this proposal includes a second new initializer. This initializer allows the caller to supply, along with the sequence, a combining closure that's called with the old and new values for any duplicate keys.

```swift init<S: Sequence>(     _ keysAndValues: S,     uniquingKeysWith combine: (Value, Value) throws -> Value ) rethrows where S.Iterator.Element == (key: Key, value: Value) ```

This example shows how one could keep the first value of all those supplied for a duplicate key.

```swift let letterDict2 = Dictionary(duplicates, uniquingKeysWith: { (first, _) in first }) // ["b": 2, "a": 1] ```

Or the largest value for any duplicate keys.

```swift let letterDict3 = Dictionary(duplicates, uniquingKeysWith: max) // ["b": 4, "a": 3] ```

At other times the merging initializer could be used to combine values for duplicate keys. Donnacha Oisín Kidney wrote a neat `frequencies()` method for sequences as an example of such a use in the thread.

```swift extension Sequence where Iterator.Element: Hashable {     func frequencies() -> [Iterator.Element: Int] {         return Dictionary(self.lazy.map { v in (v, 1) }, uniquingKeysWith: +)     } } [1, 2, 2, 3, 1, 2, 4, 5, 3, 2, 3, 1].frequencies() // [2: 4, 4: 1, 5: 1, 3: 3, 1: 3] ```

This proposal also includes new mutating and non-mutating methods for `Dictionary` that merge the contents of a sequence of `(Key, Value)` tuples into an existing dictionary, `merge(_:uniquingKeysWith:)` and `merging(_:uniquingKeysWith:)`.

```swift mutating func merge<S: Sequence>(     _ other: S,     uniquingKeysWith combine: (Value, Value) throws -> Value ) rethrows where S.Iterator.Element == (key: Key, value: Value)

func merging<S: Sequence>(     _ other: S,     uniquingKeysWith combine: (Value, Value) throws -> Value ) rethrows -> [Key: Value] where S.Iterator.Element == (key: Key, value: Value) ```

As above, there are a wide variety of uses for the merge.

```swift // Adding default values let defaults: [String: Bool] = ["foo": false, "bar": false, "baz": false] var options: [String: Bool] = ["foo": true, "bar": false] options.merge(defaults) { (old, _) in old } // options is now ["foo": true, "bar": false, "baz": false]

// Summing counts repeatedly var bugCounts: [String: Int] = ["bees": 9, "ants": 112, ...] while bugCountingSource.hasMoreData() {     bugCounts.merge(bugCountingSource.countMoreBugs(), uniquingKeysWith: +) } ```

---

## 2. Key-based subscript with default value

Another common challenge with dictionaries is iteratively making changes to key/value pairs that may or may not already be present. For example, to iteratively add count the frequencies of letters in a string, one might write something like the following:

```swift let source = "how now brown cow" var frequencies: [Character: Int] = [:] for c in source.characters {     if frequencies[c] == nil {         frequencies[c] = 1     } else {         frequencies[c]! += 1     } } ```

Testing for `nil` and assigning through the force unwrapping operator are awkward at best for such a common operation. Furthermore, the `Optional<Value>` return type of the current keyed subscript complicates efficiencies that could be gained for this type of `modify` action under a future ownership model.

### Proposed solution

A keyed subscript with a default value neatly simplifies this usage. Instead of checking for `nil`, one can pass the default value along with the key as a `default` subscript parameter.

```swift let source = "how now brown cow" var frequencies: [Character: Int] = [:] for c in source.characters {     frequencies[c, default: 0] += 1 } ```

The return type of this subscript is a non-optional `Value`. Note that accessing the subscript as a getter does not store the default value in the dictionary—the following two lines are equivalent:

```swift let x = frequencies["a", default: 0] let y = frequencies["a"] ?? 0 ```

---

## 3. Dictionary-specific map and filter

The standard `map` and `filter` methods, while always useful and beloved, aren't ideal when applied to dictionaries. In both cases, the desired end-result is frequently another dictionary instead of an array of key-value pairs—even with the sequence-based initializer proposed above this is an inefficient way of doing things.

Additionally, the standard `map` method doesn't gracefully support passing a function when transforming only the *values* of a dictionary. The transform function must accept and return key/value pairs, necessitating a custom closure in nearly every case.

Assuming the addition of a sequence-based initializer, the current `filter` and `map` look like the following:

```swift let numbers = ["one": 1, "two": 2, "three": 3, "four": 4] let evens = Dictionary(numbers.lazy.filter { $0.value % 2 == 0 })! // ["four": 4, "two": 2] let strings = Dictionary(numbers.lazy.map { (k, v) in (k, String(v)) })! // ["three": "3", "four": "4", "one": "1", "two": "2"] ```

### Proposed solution

This proposal adds two new methods for `Dictionary`:

1. A `mapValues` method that keeps a dictionary's keys intact while transforming the values. Mapping a dictionary's key/value pairs can't always produce a new dictionary, due to the possibility of key collisions, but mapping only the values can produce a new dictionary with the same underlying layout as the original.

```swift     let strings = numbers.mapValues(String.init)     // ["three": "3", "four": "4", "one": "1", "two": "2"]     ```

2. A `Dictionary`-returning `filter` method. While transforming the keys and values of a dictionary can result in key collisions, filtering the elements of a dictionary can at worst replicate the entire dictionary.

```swift     let evens = numbers.filter { $0.value % 2 == 0 }     // ["four": 4, "two": 2]     ```

Both of these can be made significantly more efficient than their `Sequence`-sourced counterparts. For example, the `mapValues` method can simply copy the portion of the storage that holds the keys to the new dictionary before transforming the values.

---

## 4. Visible dictionary capacity

As you add elements to a dictionary, it automatically grows its backing storage as necessary. This reallocation is a significant operation—unlike arrays, where the existing elements can be copied to a new block of storage en masse, every key/value pair must be moved over individually, recalculating the hash value for the key to find its position in the larger backing buffer.

While dictionaries uses an exponential growth strategy to make this as efficient as possible, beyond the `init(minimumCapacity:)` initializer they do not expose a way to reserve a specific capacity. In addition, adding a key/value pair to a dictionary is guaranteed not to invalidate existing indices as long as the capacity doesn't change, yet we don't provide any way of seeing a dictionary's current or post-addition capacity.

### Proposed solution

`Dictionary` should add a `capacity` property and a `reserveCapacity(_:)` method, like those used in range-replaceable collections. The `capacity` of a dictionary is the number of elements it can hold without reallocating a larger backing storage, while calling `reserveCapacity(_:)` allocates a large enough buffer to hold the requested number of elements without reallocating.

```swift var numbers = ["one": 1, "two": 2, "three": 3, "four": 4] numbers.capacity   // 6 numbers.reserveCapacity(20) numbers.capacity   // 24 ```

Because hashed collections use extra storage capacity to reduce the likelihood and cost of collisions, the value of the `capacity` property won't be equal to the actual size of the backing storage. Likewise, the capacity after calling `reserveCapacity(_:)` will be at least as large as the argument, but usually larger. (In its current implementation, `Dictionary` always has a power of 2-sized backing storage.)

---

## 5. Grouping sequence elements

As a final `Dictionary`-related issue, grouping elements in a sequence by some computed key is a commonly requested addition that we can add as part of this omnibus proposal. Call a new `Dictionary(grouping:by:)` initializer with a closure that converts each value in a sequence to a hashable type `T`; the result is a dictionary with keys of type `T` and values of type `[Iterator.Element]`.

```swift let names = ["Patti", "Aretha", "Anita", "Gladys"]

// By first letter Dictionary(grouping: names, by: { $0.characters.first! }) // ["P": ["Patti"], "G": ["Gladys"], "A": ["Aretha", "Anita"]]

// By name length Dictionary(grouping: names) { $0.utf16.count } // [5: ["Patti", "Anita"], 6: ["Aretha", "Gladys"]] ```

---

## 6. Apply relevant changes to `Set`

As the `Set` and `Dictionary` types are similar enough to share large chunks of their implementations, it makes sense to look at which of these `Dictionary` enhancements can also be applied to `Set`. Of the symbols proposed above, the following additions would also be useful and appropriate for the `Set` type:

- Add a `Set`-specific `filter` method that returns a new set—this would function essentially as a predicate-based `intersection` method. - Add a `capacity` property and `reserveCapacity()` method, for the reasons listed above.

## Detailed design

With the exception of the proposed capacity property and method, the proposed additions to `Dictionary`, `Set`, and `Sequence` are available in [this Swift Sandbox][sandbox]. Note that this prototype is not a proposed implementation; rather a way to try out the behavior of the proposed changes.

Collected in one place, these are the new APIs for `Dictionary`, `Set`, and `Sequence`:

```swift struct Dictionary<Key: Hashable, Value> {     typealias Element = (key: Key, value: Value)

// existing declarations

/// Creates a new dictionary using the key/value pairs in the given sequence.     /// If the given sequence has any duplicate keys, the result is `nil`.     init<S: Sequence>(uniqueKeysWithValues: S) where S.Iterator.Element == Element

/// Creates a new dictionary using the key/value pairs in the given sequence,     /// using a combining closure to determine the value for any duplicate keys.     init<S: Sequence>(         _ keysAndValues: S,         uniquingKeysWith combine: (Value, Value) throws -> Value     ) rethrows where S.Iterator.Element == Element

/// Creates a new dictionary where the keys are the groupings returned by     /// the given closure and the values are arrays of the elements that     /// returned each specific key.     init<S: Sequence>(     	grouping values: S, 	by keyForValue: (S.Iterator.Element) throws -> Key     ) rethrows where Value == [S.Iterator.Element]

/// Merges the key/value pairs in the given sequence into the dictionary,     /// using a combining closure to determine the value for any duplicate keys.     mutating func merge<S: Sequence>(         _ other: S,         uniquingKeysWith combine: (Value, Value) throws -> Value     ) rethrows where S.Iterator.Element == Element

/// Returns a new dictionary created by merging the key/value pairs in the     /// given sequence into the dictionary, using a combining closure to determine     /// the value for any duplicate keys.     func merging<S: Sequence>(         _ other: S,         uniquingKeysWith combine: (Value, Value) throws -> Value     ) rethrows -> [Key: Value] where S.Iterator.Element == Element

/// Accesses the element with the given key, or the specified default value,     /// if the dictionary doesn't contain the given key.     subscript(key: Key, default defaultValue: Value) -> Value { get set }

/// Returns a new dictionary containing the key/value pairs that satisfy     /// the given predicate.     func filter(_ isIncluded: (Key, Value) throws -> Bool) rethrows -> [Key: Value]

/// Returns a new dictionary containing the existing keys and the results of     /// mapping the given closure over the dictionary's values.     func mapValues<T>(_ transform: (Value) throws -> T) rethrows -> [Key: T]

/// The number of key/value pairs that can be stored by the dictionary without     /// reallocating storage.     var capacity: Int { get }

/// Ensures that the dictionary has enough storage for `capacity` key/value     /// pairs.     var reserveCapacity(_ capacity: Int) }

struct Set<Element: Hashable> {     // existing declarations

/// Returns a new set containing the elements that satisfy the given predicate.     func filter(_ isIncluded: (Element) throws -> Bool) rethrows -> Set

/// The number of elements that can be stored by the set without     /// reallocating storage.     var capacity: Int { get }

/// Ensures that the set has enough storage for `capacity` elements.     var reserveCapacity(_ capacity: Int) } ```

## Alternatives Considered

- An earlier version of this proposal declared the first merging initializer as failable, returning `nil` when a sequence with duplicate keys was passed. That initializer is really only appropriate when working with known unique keys, however, as there isn't a feasible recovery path when there can be duplicate keys. That leaves two possibilities: 	1. The source input can *never* have duplicate keys, and the programmer has to write `!` or unwrap in some other way, or 	2. The source input *can* have duplicate keys, in which case the programmer should be using `init(merging:mergingValues:)` instead.

- An earlier version of this proposal included the addition of two new top-level functions to the standard library: `first(_:_:)` and `last(_:_:)`, which return their first and last arguments, respectively. These new functions would be passed instead of a custom closure to a merging method or initializer:

```swift 	let firstWins = Dictionary(merging: duplicates, mergingValues: first) 	// ["b": 2, "a": 1]

let lastWins = Dictionary(merging: duplicates, mergingValues: last) 	// ["b": 4, "a": 3] 	```

As an alternative to the `first(_:_:)` and `last(_:_:)` functions, at the cost of three additional overloads for the merging initializer and methods, we could allow users to specify the `useFirst` and `useLast` cases of a `MergeCollisionStrategy` enumeration.

```swift 	extension Dictionary { 	    /// The strategy to use when merging a sequence of key-value pairs into a dictionary. 	    enum MergeCollisionStrategy { 	        /// If there is more than one instance of a key in the sequence to merge, use 	        /// only the first value for the dictionary. 	        case useFirst 	        /// If there is more than one instance of a key in the sequence to merge, use 	        /// only the last value for the dictionary. 	        case useLast 	    }

init<S: Sequence>( 	        merging keysAndValues: S, 	        mergingValues strategy: MergeCollisionStrategy 	    )

// other merging overloads 	} 	```

In use, this overload would look similar to the functional version, but may aid in discoverability:

```swift 	let firstWins = Dictionary(merging: duplicates, mergingValues: .useFirst) 	// ["b": 2, "a": 1]

let lastWins = Dictionary(merging: duplicates, mergingValues: .useLast) 	// ["b": 4, "a": 3] 	```

- An earlier version of this proposal included a change to both collections' `remove(at:)` method, such that it would return both the removed element and the next valid index in the iteration sequence. This change lets a programmer write code that would remove elements of a dictionary or a set in place, which can't currently be done with as much efficiency due to index invalidation:

```swift     var i = dict.startIndex     while i != dict.endIndex {         if shouldRemove(dict[i]) {             (_, i) = dict.remove(at: i)         } else {             dict.formIndex(after: &i)         }     }     ```

This change to `remove(at:)` has been deferred to a later proposal that can better deal with the impact on existing code.

- The reviewed version of this proposal had different spelling of the merging initializers and methods:

```swift     init<S: Sequence where S.Iterator.Element == (key: Key, value: Value)>(         _ keysAndValues: S)

init<S: Sequence>(         merging keysAndValues: S,         mergingValues combine: (Value, Value) throws -> Value     ) rethrows where S.Iterator.Element == (key: Key, value: Value)

mutating func merge<S: Sequence>(         _ other: S,         mergingValues combine: (Value, Value) throws -> Value     ) rethrows where S.Iterator.Element == (key: Key, value: Value)

func merged<S: Sequence>(         with other: S,         mergingValues combine: (Value, Value) throws -> Value     ) rethrows -> [Key: Value] where S.Iterator.Element == (key: Key, value: Value)     ```

In addition, the `Dictionary(grouping:by:)` initializer was proposed as a `grouped(by:)` method on the `Sequence` protocol. The [rationale][] for accepting the proposal explains the reasons for these changes.

## Source compatibility

All the proposed additions are purely additive and should impose only a minimal source compatibility burden. The addition of same-type returning `filter(_:)` methods to `Dictionary` and `Set` could cause problems if code is relying on the previous behavior of returning an array. For example, the following code would break after this change:

```swift let digits = Set(0..<10) let evens = digits.filter { $0 % 2 == 0 } functionThatTakesAnArray(evens) ```

To mitigate the impact of this on code compiled in Swift 3 mode, `Dictionary` and `Set` will have an additional overload of the `Array`-returning `filter(_:)` that is marked as obsoleted in Swift 4. This will allow Swift 3 code to continue to compile and work as expected:

```swift extension Set {     @available(swift, obsoleted: 4)     func filter(_ isIncluded: (Element) throws -> Bool) rethrows -> [Element]

@available(swift, introduced: 4)     func filter(_ isIncluded: (Element) throws -> Bool) rethrows -> Set<Element> } ```

[sandbox]: http://swift.sandbox.bluemix.net/#/repl/58f8e3aee785f75678f428a8 [rationale]: https://forums.swift.org/t/accepted-se-165-dictionary-set-enhancements/5727