I am trying to write or implement my own higher order function my way, but not able to write it.
In below code I tried to write filter.
var array : [String] = ["Sagar","Harshit","Parth","Gunja","Marmik","Sachin","Saurav"]
//Native filter function of Swift
array = array.filter { (name) -> Bool in
return name.prefix(1) == "S"
}
I implement below code, according to method signature of filter, but as I know , we can not write closure with return type(If possible then I don't know).
func filterArray(_ array : [String], completionHandler : (_ name : String) -> ()) -> (){
for (_, value) in array.enumerated(){
completionHandler(value)
}
}
self.filterArray(array) { (name) -> () in
if name.prefix(1) != "S"{
if let index = array.index(of: name){
array.remove(at: index)
}
}
}
My implementation working fine and filtering array. But I want to abstract logic of remove object from array.
Can we write our own higher order functions or not ?
If yes then please help to implement above one.
Thanks in advance.
And you can define a return type to a closure. You can find a working example below, but for this purpose I suggest using the Swift built in filter function which can provide the same solution and much faster.
var array : [String] = ["Sagar","Harshit","Parth","Gunja","Marmik","Sachin","Saurav"]
func filterArray(_ array : inout [String], condition: (_ name : String) -> Bool) -> (){
var filteredArray: [String] = []
for value in array {
if condition(value) {
filteredArray.append(value)
}
}
array = filteredArray
}
filterArray(&array) { (name) -> Bool in
return !name.hasPrefix("S")
}
print(array)
You can define your own higher order functions on collections.
There is a great session about collections where Soroush shows an example of writing your own higher order function extending a collection.
https://academy.realm.io/posts/try-swift-soroush-khanlou-sequence-collection/
// Swit built in filter
let numberOfAdmins = users.filter({ $0.isAdmin }).count // => fine
// Custom "filter"
let numberOfAdmins = users.count({ $0.isAdmin }) // => great
extension Sequence {
func count(_ shouldCount: (Iterator.Element) -> Bool) -> Int {
var count = 0
for element in self {
if shouldCount(element) {
count += 1
}
}
return count
}
}
I'm trying to get used to generics (never used them in objc) and want to write a toy function that takes an object of any type () and returns the first and last element. Hypothetically, I'd only use this on an array or a string - I keep getting an error that has no subscript members. I totally understand that the error message is telling me swift has no clue that T may potentially hold a type that does have subscripts - I just want to know how to get around this.
func firstAndLastFromCollection<T>(a:T?) {
var count: Int = 0
for item in a as! [AnyObject] {
count++
}
if count>1 {
var first = a?[0]
var last = a?[count-1]
return (first, last)
}
return something else here
}
Do I need to typecast somewhere here (which would kind of defeat the purpose here, as I'd need to downcast as either a string or an array, adding code and lessening how generic this func is)?
If you want to return the first and the last element then it's probably safe assuming the input param is an array of some kind of type.
So you can implement your function this way
func firstAndLast<T>(list:[T]) -> (first:T, last:T)? {
guard let first = list.first, last = list.last else { return nil }
return (first, last)
}
The function does return a tuple of 2 element, both have the same type of the generic element of the input array.
The returned tuple is an option because if the array is empty then nil is returned.
Examples
let nums = firstAndLast([1,2,3,4])
let words = firstAndLast(["One", "Two", "Three"])
As you can verify the type of the generic element into the array becomes the type of the elements inside the tuple.
In the example above nums is inferred to be (Int, Int)? and words (Words, Words)?
More examples
let emptyList: [String] = []
firstAndLast(emptyList) // nil
Extension
Finally you can also write this code as an extension of Array.
extension Array {
var firstAndLast: (first:Element, last:Element)? {
guard let first = self.first, last = self.last else { return nil }
return (first, last)
}
}
Now you can write
let aCoupleOfShows = ["Breaking Bad", "Better Call Saul", "Mr Robot"].firstAndLast
Again, if you check the type of the constant aCoupleOfShows you'll see that is a (first: String, last: String)?. Swift automatically did infer the correct type.
Last example
In the comments you said you wanted the first and last chars of a String. here it is the code if you use the extension above
if let chars = Array("Hello world".characters).firstAndLast {
print("First char is \(chars.first), last char is \(chars.last) ")
}
//>> First char is H, last char is d
If we are talking about collections, let's use the CollectionType:
func firstAndLastFromCollection<T: CollectionType>(a: T) -> (T.Generator.Element, T.Generator.Element)? {
guard !a.isEmpty else {
return nil
}
return (a.first!, a.lazy.reverse().first!)
}
print(firstAndLastFromCollection(["a", "b", "c"])) // ("a", "c")
print(firstAndLastFromCollection("abc".characters)) // ("a", "c")
print(firstAndLastFromCollection(0..<200)) // (0, 199)
print(firstAndLastFromCollection([] as [String])) // nil
If you specify your generic type to also conform to bidirectional index:
func firstAndLastFromCollection<T: CollectionType where T.Index : BidirectionalIndexType>(...) -> ...
then you can call last directly:
return (a.first!, a.last!)
If we decide to implement it using a category, we don't need generics at all:
extension CollectionType {
func firstAndLast() -> (Generator.Element, Generator.Element)? {
guard !self.isEmpty else {
return nil
}
return (self.first!, self.lazy.reverse().first!)
}
}
extension CollectionType where Index: BidirectionalIndexType {
func firstAndLast() -> (Generator.Element, Generator.Element)? {
guard !self.isEmpty else {
return nil
}
return (self.first!, self.last!)
}
}
print("abc".characters.firstAndLast())
Swift is a protocol oriented language. Usually you will find yourself extend protocols more than extending classes or structs.
In Swift, how can I check if an element exists in an array? Xcode does not have any suggestions for contain, include, or has, and a quick search through the book turned up nothing. Any idea how to check for this? I know that there is a method find that returns the index number, but is there a method that returns a boolean like ruby's #include??
Example of what I need:
var elements = [1,2,3,4,5]
if elements.contains(5) {
//do something
}
Swift 2, 3, 4, 5:
let elements = [1, 2, 3, 4, 5]
if elements.contains(5) {
print("yes")
}
contains() is a protocol extension method of SequenceType (for sequences of Equatable elements) and not a global method as in
earlier releases.
Remarks:
This contains() method requires that the sequence elements
adopt the Equatable protocol, compare e.g. Andrews's answer.
If the sequence elements are instances of a NSObject subclass
then you have to override isEqual:, see NSObject subclass in Swift: hash vs hashValue, isEqual vs ==.
There is another – more general – contains() method which does not require the elements to be equatable and takes a predicate as an
argument, see e.g. Shorthand to test if an object exists in an array for Swift?.
Swift older versions:
let elements = [1,2,3,4,5]
if contains(elements, 5) {
println("yes")
}
For those who came here looking for a find and remove an object from an array:
Swift 1
if let index = find(itemList, item) {
itemList.removeAtIndex(index)
}
Swift 2
if let index = itemList.indexOf(item) {
itemList.removeAtIndex(index)
}
Swift 3, 4
if let index = itemList.index(of: item) {
itemList.remove(at: index)
}
Swift 5.2
if let index = itemList.firstIndex(of: item) {
itemList.remove(at: index)
}
Updated for Swift 2+
Note that as of Swift 3 (or even 2), the extension below is no longer necessary as the global contains function has been made into a pair of extension method on Array, which allow you to do either of:
let a = [ 1, 2, 3, 4 ]
a.contains(2) // => true, only usable if Element : Equatable
a.contains { $0 < 1 } // => false
Historical Answer for Swift 1:
Use this extension: (updated to Swift 5.2)
extension Array {
func contains<T>(obj: T) -> Bool where T: Equatable {
return !self.filter({$0 as? T == obj}).isEmpty
}
}
Use as:
array.contains(1)
If you are checking if an instance of a custom class or struct is contained in an array, you'll need to implement the Equatable protocol before you can use .contains(myObject).
For example:
struct Cup: Equatable {
let filled:Bool
}
static func ==(lhs:Cup, rhs:Cup) -> Bool { // Implement Equatable
return lhs.filled == rhs.filled
}
then you can do:
cupArray.contains(myCup)
Tip: The == override should be at the global level, not within your class/struct
I used filter.
let results = elements.filter { el in el == 5 }
if results.count > 0 {
// any matching items are in results
} else {
// not found
}
If you want, you can compress that to
if elements.filter({ el in el == 5 }).count > 0 {
}
Hope that helps.
Update for Swift 2
Hurray for default implementations!
if elements.contains(5) {
// any matching items are in results
} else {
// not found
}
(Swift 3)
Check if an element exists in an array (fulfilling some criteria), and if so, proceed working with the first such element
If the intent is:
To check whether an element exist in an array (/fulfils some boolean criteria, not necessarily equality testing),
And if so, proceed and work with the first such element,
Then an alternative to contains(_:) as blueprinted Sequence is to first(where:) of Sequence:
let elements = [1, 2, 3, 4, 5]
if let firstSuchElement = elements.first(where: { $0 == 4 }) {
print(firstSuchElement) // 4
// ...
}
In this contrived example, its usage might seem silly, but it's very useful if querying arrays of non-fundamental element types for existence of any elements fulfilling some condition. E.g.
struct Person {
let age: Int
let name: String
init(_ age: Int, _ name: String) {
self.age = age
self.name = name
}
}
let persons = [Person(17, "Fred"), Person(16, "Susan"),
Person(19, "Hannah"), Person(18, "Sarah"),
Person(23, "Sam"), Person(18, "Jane")]
if let eligableDriver = persons.first(where: { $0.age >= 18 }) {
print("\(eligableDriver.name) can possibly drive the rental car in Sweden.")
// ...
} // Hannah can possibly drive the rental car in Sweden.
let daniel = Person(18, "Daniel")
if let sameAgeAsDaniel = persons.first(where: { $0.age == daniel.age }) {
print("\(sameAgeAsDaniel.name) is the same age as \(daniel.name).")
// ...
} // Sarah is the same age as Daniel.
Any chained operations using .filter { ... some condition }.first can favourably be replaced with first(where:). The latter shows intent better, and have performance advantages over possible non-lazy appliances of .filter, as these will pass the full array prior to extracting the (possible) first element passing the filter.
Check if an element exists in an array (fulfilling some criteria), and if so, remove the first such element
A comment below queries:
How can I remove the firstSuchElement from the array?
A similar use case to the one above is to remove the first element that fulfils a given predicate. To do so, the index(where:) method of Collection (which is readily available to array collection) may be used to find the index of the first element fulfilling the predicate, whereafter the index can be used with the remove(at:) method of Array to (possible; given that it exists) remove that element.
var elements = ["a", "b", "c", "d", "e", "a", "b", "c"]
if let indexOfFirstSuchElement = elements.index(where: { $0 == "c" }) {
elements.remove(at: indexOfFirstSuchElement)
print(elements) // ["a", "b", "d", "e", "a", "b", "c"]
}
Or, if you'd like to remove the element from the array and work with, apply Optional:s map(_:) method to conditionally (for .some(...) return from index(where:)) use the result from index(where:) to remove and capture the removed element from the array (within an optional binding clause).
var elements = ["a", "b", "c", "d", "e", "a", "b", "c"]
if let firstSuchElement = elements.index(where: { $0 == "c" })
.map({ elements.remove(at: $0) }) {
// if we enter here, the first such element have now been
// remove from the array
print(elements) // ["a", "b", "d", "e", "a", "b", "c"]
// and we may work with it
print(firstSuchElement) // c
}
Note that in the contrived example above the array members are simple value types (String instances), so using a predicate to find a given member is somewhat over-kill, as we might simply test for equality using the simpler index(of:) method as shown in #DogCoffee's answer. If applying the find-and-remove approach above to the Person example, however, using index(where:) with a predicate is appropriate (since we no longer test for equality but for fulfilling a supplied predicate).
An array that contains a property that equals to
yourArray.contains(where: {$0.propertyToCheck == value })
Returns boolean.
The simplest way to accomplish this is to use filter on the array.
let result = elements.filter { $0==5 }
result will have the found element if it exists and will be empty if the element does not exist. So simply checking if result is empty will tell you whether the element exists in the array. I would use the following:
if result.isEmpty {
// element does not exist in array
} else {
// element exists
}
Swift 4/5
Another way to achieve this is with the filter function
var elements = [1,2,3,4,5]
if let object = elements.filter({ $0 == 5 }).first {
print("found")
} else {
print("not found")
}
As of Swift 2.1 NSArrays have containsObjectthat can be used like so:
if myArray.containsObject(objectImCheckingFor){
//myArray has the objectImCheckingFor
}
Array
let elements = [1, 2, 3, 4, 5, 5]
Check elements presence
elements.contains(5) // true
Get elements index
elements.firstIndex(of: 5) // 4
elements.firstIndex(of: 10) // nil
Get element count
let results = elements.filter { element in element == 5 }
results.count // 2
Just in case anybody is trying to find if an indexPath is among the selected ones (like in a UICollectionView or UITableView cellForItemAtIndexPath functions):
var isSelectedItem = false
if let selectedIndexPaths = collectionView.indexPathsForSelectedItems() as? [NSIndexPath]{
if contains(selectedIndexPaths, indexPath) {
isSelectedItem = true
}
}
if user find particular array elements then use below code same as integer value.
var arrelemnts = ["sachin", "test", "test1", "test3"]
if arrelemnts.contains("test"){
print("found") }else{
print("not found") }
Here is my little extension I just wrote to check if my delegate array contains a delegate object or not (Swift 2). :) It Also works with value types like a charm.
extension Array
{
func containsObject(object: Any) -> Bool
{
if let anObject: AnyObject = object as? AnyObject
{
for obj in self
{
if let anObj: AnyObject = obj as? AnyObject
{
if anObj === anObject { return true }
}
}
}
return false
}
}
If you have an idea how to optimize this code, than just let me know.
Swift
If you are not using object then you can user this code for contains.
let elements = [ 10, 20, 30, 40, 50]
if elements.contains(50) {
print("true")
}
If you are using NSObject Class in swift. This variables is according to my requirement. you can modify for your requirement.
var cliectScreenList = [ATModelLeadInfo]()
var cliectScreenSelectedObject: ATModelLeadInfo!
This is for a same data type.
{ $0.user_id == cliectScreenSelectedObject.user_id }
If you want to AnyObject type.
{ "\($0.user_id)" == "\(cliectScreenSelectedObject.user_id)" }
Full condition
if cliectScreenSelected.contains( { $0.user_id == cliectScreenSelectedObject.user_id } ) == false {
cliectScreenSelected.append(cliectScreenSelectedObject)
print("Object Added")
} else {
print("Object already exists")
}
what about using a hash table for the job, like this?
first, creating a "hash map" generic function, extending the Sequence protocol.
extension Sequence where Element: Hashable {
func hashMap() -> [Element: Int] {
var dict: [Element: Int] = [:]
for (i, value) in self.enumerated() {
dict[value] = i
}
return dict
}
}
This extension will work as long as the items in the array conform to Hashable, like integers or strings, here is the usage...
let numbers = Array(0...50)
let hashMappedNumbers = numbers.hashMap()
let numToDetect = 35
let indexOfnumToDetect = hashMappedNumbers[numToDetect] // returns the index of the item and if all the elements in the array are different, it will work to get the index of the object!
print(indexOfnumToDetect) // prints 35
But for now, let's just focus in check if the element is in the array.
let numExists = indexOfnumToDetect != nil // if the key does not exist
means the number is not contained in the collection.
print(numExists) // prints true
Swift 4.2 +
You can easily verify your instance is an array or not by the following function.
func verifyIsObjectOfAnArray<T>(_ object: T) -> Bool {
if let _ = object as? [T] {
return true
}
return false
}
Even you can access it as follows. You will receive nil if the object wouldn't be an array.
func verifyIsObjectOfAnArray<T>(_ object: T) -> [T]? {
if let array = object as? [T] {
return array
}
return nil
}
You can add an extension for Array as such:
extension Array {
func contains<T>(_ object: T) -> Bool where T: Equatable {
!self.filter {$0 as? T == object }.isEmpty
}
}
This can be used as:
if myArray.contains(myItem) {
// code here
}
I am using Swift 1.2 in Xcode 6.3.1
Following is my Person struct
struct Person {
let age: Int
init?(age: Int) { //Failable init
if age > 100 { return nil }
self.age = age
}
}
I am having a list of ages against which I have to make Person Objects.
I have made playground file.
let arr = Array(1...150) //Sample set of ages against which Person is created
var personList: [Person]!
and
personList = arr.map({ (val: Int) -> Person? in
return Person(age: val) //Makes object of type Person?
}).filter {
$0 != nil
}.map {
return $0!
}
Here I have uses map - filter - map because the first map invokes failable intializer, (hence it returns Person?) and personList is of type [Person].
Hence second function filters all the non nil objects and third map forcefully opens to optional therby making Person? to Person.
Is there a more easy/readable way out ? Chaining map-filter-map definitely seems to be an overkill for this
You can use flatMap to get rid of any nils in the array, this tutorial discusses the method in length, but the following will work best:
let personList = arr.flatMap { Person(age: $0) }
Note: This answer was given for Swift 1.2, the current
version at the time the question was posted. Since Swift 2 there is a better solution, see #Jeremie's answer.
I don't know of a built-in function that combines filter()
and map(). You can write the code slightly more compact using
the shorthand argument $0 in all closures:
let personList = arr.map { Person(age: $0) }
.filter { $0 != nil }
.map { $0! }
Of course you can define your own extension method which maps the
array elements and keeps only the non-nil results:
extension Array {
func optmap<U>(transform: T -> U?) -> [U] {
var result : [U] = []
for elem in self {
if let mapped = transform(elem) {
result.append(mapped)
}
}
return result
}
}
and then use it as
let personList = arr.optmap { Person(age: $0) }
You can use compactMap which is better that flatMap in this case to remove any nils in the array:
let personList = arr.compactMap { Person(age: $0) }
The Swift document declared:
Returns an array containing the non-nil results of calling the given
transformation with each element of this sequence.
In Swift, is there any way to check if an index exists in an array without a fatal error being thrown?
I was hoping I could do something like this:
let arr: [String] = ["foo", "bar"]
let str: String? = arr[1]
if let str2 = arr[2] as String? {
// this wouldn't run
println(str2)
} else {
// this would be run
}
But I get
fatal error: Array index out of range
An elegant way in Swift:
let isIndexValid = array.indices.contains(index)
Type extension:
extension Collection {
subscript(optional i: Index) -> Iterator.Element? {
return self.indices.contains(i) ? self[i] : nil
}
}
Using this you get an optional value back when adding the keyword optional to your index which means your program doesn't crash even if the index is out of range. In your example:
let arr = ["foo", "bar"]
let str1 = arr[optional: 1] // --> str1 is now Optional("bar")
if let str2 = arr[optional: 2] {
print(str2) // --> this still wouldn't run
} else {
print("No string found at that index") // --> this would be printed
}
Just check if the index is less than the array size:
if 2 < arr.count {
...
} else {
...
}
Add some extension sugar:
extension Collection {
subscript(safe index: Index) -> Iterator.Element? {
guard indices.contains(index) else { return nil }
return self[index]
}
}
if let item = ["a", "b", "c", "d"][safe: 3] { print(item) } // Output: "d"
// or with guard:
guard let anotherItem = ["a", "b", "c", "d"][safe: 3] else { return }
print(anotherItem) // "d"
Enhances readability when doing if let style coding in conjunction with arrays
the best way.
let reqIndex = array.indices.contains(index)
print(reqIndex)
Swift 4 extension:
For me i prefer like method.
// MARK: - Extension Collection
extension Collection {
/// Get at index object
///
/// - Parameter index: Index of object
/// - Returns: Element at index or nil
func get(at index: Index) -> Iterator.Element? {
return self.indices.contains(index) ? self[index] : nil
}
}
Thanks to #Benno Kress
You can rewrite this in a safer way to check the size of the array, and use a ternary conditional:
if let str2 = (arr.count > 2 ? arr[2] : nil) as String?
Asserting if an array index exist:
This methodology is great if you don't want to add extension sugar:
let arr = [1,2,3]
if let fourthItem = (3 < arr.count ? arr[3] : nil ) {
Swift.print("fourthItem: \(fourthItem)")
}else if let thirdItem = (2 < arr.count ? arr[2] : nil) {
Swift.print("thirdItem: \(thirdItem)")
}
//Output: thirdItem: 3
extension Array {
func isValidIndex(_ index : Int) -> Bool {
return index < self.count
}
}
let array = ["a","b","c","d"]
func testArrayIndex(_ index : Int) {
guard array.isValidIndex(index) else {
print("Handle array index Out of bounds here")
return
}
}
It's work for me to handle indexOutOfBounds.
Swift 4 and 5 extension:
As for me I think this is the safest solution:
public extension MutableCollection {
subscript(safe index: Index) -> Element? {
get {
return indices.contains(index) ? self[index] : nil
}
set(newValue) {
if let newValue = newValue, indices.contains(index) {
self[index] = newValue
}
}
}
}
Example:
let array = ["foo", "bar"]
if let str = array[safe: 1] {
print(str) // "bar"
} else {
print("index out of range")
}
I believe the existing answers could be improved further because this function could be needed in multiple places within a codebase (code smell when repeating common operations). So thought of adding my own implementation, with reasoning for why I considered this approach (efficiency is an important part of good API design, and should be preferred where possible as long as readability is not too badly affected). In addition to enforcing good Object-Oriented design with a method on the type itself, I think Protocol Extensions are great and we can make the existing answers even more Swifty. Limiting the extension is great because you don’t create code you don’t use. Making the code cleaner and extensible can often make maintenance easier, but there are trade-offs (succinctness being the one I thought of first).
So, you can note that if you'd ONLY like to use the extension idea for reusability but prefer the contains method referenced above you can rework this answer. I have tried to make this answer flexible for different uses.
TL;DR
You can use a more efficient algorithm (Space and Time) and make it extensible using a protocol extension with generic constraints:
extension Collection where Element: Numeric { // Constrain only to numerical collections i.e Int, CGFloat, Double and NSNumber
func isIndexValid(index: Index) -> Bool {
return self.endIndex > index && self.startIndex <= index
}
}
// Usage
let checkOne = digits.isIndexValid(index: index)
let checkTwo = [1,2,3].isIndexValid(index: 2)
Deep Dive
Efficiency
#Manuel's answer is indeed very elegant but it uses an additional layer of indirection (see here). The indices property acts like a CountableRange<Int> under the hood created from the startIndex and endIndex without reason for this problem (marginally higher Space Complexity, especially if the String is long). That being said, the Time Complexity should be around the same as a direct comparison between the endIndex and startIndex properties because N = 2 even though contains(_:) is O(N) for Collections (Ranges only have two properties for the start and end indices).
For the best Space and Time Complexity, more extensibility and only marginally longer code, I would recommend using the following:
extension Collection {
func isIndexValid(index: Index) -> Bool {
return self.endIndex > index && self.startIndex <= index
}
}
Note here how I've used startIndex instead of 0 - this is to support ArraySlices and other SubSequence types. This was another motivation to post a solution.
Example usage:
let check = digits.isIndexValid(index: index)
For Collections in general, it's pretty hard to create an invalid Index by design in Swift because Apple has restricted the initializers for associatedtype Index on Collection - ones can only be created from an existing valid Collection.Index (like startIndex).
That being said, it's common to use raw Int indices for Arrays because there are many instances when you need to check random Array indices. So you may want to limit the method to fewer structs...
Limit Method Scope
You will notice that this solution works across all Collection types (extensibility), but you can restrict this to Arrays only if you want to limit the scope for your particular app (for example, if you don't want the added String method because you don't need it).
extension Array {
func isIndexValid(index: Index) -> Bool {
return self.endIndex > index && self.startIndex <= index
}
}
For Arrays, you don't need to use an Index type explicitly:
let check = [1,2,3].isIndexValid(index: 2)
Feel free to adapt the code here for your own use cases, there are many types of other Collections e.g. LazyCollections. You can also use generic constraints, for example:
extension Collection where Element: Numeric {
func isIndexValid(index: Index) -> Bool {
return self.endIndex > index && self.startIndex <= index
}
}
This limits the scope to Numeric Collections, but you can use String explicitly as well conversely. Again it's better to limit the function to what you specifically use to avoid code creep.
Referencing the method across different modules
The compiler already applies multiple optimizations to prevent generics from being a problem in general, but these don't apply when the code is being called from a separate module. For cases like that, using #inlinable can give you interesting performance boosts at the cost of an increased framework binary size. In general, if you're really into improving performance and want to encapsulate the function in a separate Xcode target for good SOC, you can try:
extension Collection where Element: Numeric {
// Add this signature to the public header of the extensions module as well.
#inlinable public func isIndexValid(index: Index) -> Bool {
return self.endIndex > index && self.startIndex <= index
}
}
I can recommend trying out a modular codebase structure, I think it helps to ensure Single Responsibility (and SOLID) in projects for common operations. We can try following the steps here and that is where we can use this optimisation (sparingly though). It's OK to use the attribute for this function because the compiler operation only adds one extra line of code per call site but it can improve performance further since a method is not added to the call stack (so doesn’t need to be tracked). This is useful if you need bleeding-edge speed, and you don’t mind small binary size increases. (-: Or maybe try out the new XCFrameworks (but be careful with the ObjC runtime compatibility for < iOS 13).
I think we should add this extension to every project in Swift
extension Collection {
#inlinable func isValid(position: Self.Index) -> Bool {
return (startIndex..<endIndex) ~= position
}
#inlinable func isValid(bounds: Range<Self.Index>) -> Bool {
return (startIndex..<endIndex) ~= bounds.upperBound
}
#inlinable subscript(safe position: Self.Index) -> Self.Element? {
guard isValid(position: position) else { return nil }
return self[position]
}
#inlinable subscript(safe bounds: Range<Self.Index>) -> Self.SubSequence? {
guard isValid(bounds: bounds) else { return nil }
return self[bounds]
}
}
extension MutableCollection {
#inlinable subscript(safe position: Self.Index) -> Self.Element? {
get {
guard isValid(position: position) else { return nil }
return self[position]
}
set {
guard isValid(position: position), let newValue = newValue else { return }
self[position] = newValue
}
}
#inlinable subscript(safe bounds: Range<Self.Index>) -> Self.SubSequence? {
get {
guard isValid(bounds: bounds) else { return nil }
return self[bounds]
}
set {
guard isValid(bounds: bounds), let newValue = newValue else { return }
self[bounds] = newValue
}
}
}
note that my isValid(position:) and isValid(bounds:) functions is of a complexity O(1), unlike most of the answers below, which uses the contains(_:) method which is of a complexity O(n)
Example usage:
let arr = ["a","b"]
print(arr[safe: 2] ?? "nil") // output: nil
print(arr[safe: 1..<2] ?? "nil") // output: nil
var arr2 = ["a", "b"]
arr2[safe: 2] = "c"
print(arr2[safe: 2] ?? "nil") // output: nil
arr2[safe: 1..<2] = ["c","d"]
print(arr[safe: 1..<2] ?? "nil") // output: nil