I have parser in Objc, parser returns NSDictionary. I am using this parser in swift class. But when some value is missing on that dictionary, it shows nil value. e.g. ->
wirlessData = {
"anon" = {
};
"channel" = {
"text" = 1;
};
}
I am checking through
if let wepauthValue = wirlessData["wepauth"] {
if let value = wepauthValue["text"] {
print("\(value)") // nil
}
}
I don't how it satisfy the if let condition. Any one faced this types of problem can help me out.
Thanks,
vikash
You don't need any special code to do this, because it is what a dictionary already does. When you fetch dict[key] you know whether the dictionary contains the key, because the Optional that you get back is not nil (and it contains the value).
So, if you just want to answer the question whether the dictionary contains the key, ask:
let keyExists = dict[key] != nil
If you want the value and you know the dictionary contains the key, say:
let val = dict[key]!
But if, as usually happens, you don't know it contains the key - you want to fetch it and use it, but only if it exists - then use something like if let:
if let val = dict[key] {
// now val is not nil and the Optional has been unwrapped, so use it
}
I have tested it and found that value is still optional.Take a look at screenshot below to understand it better.
"anon" would be an empty dictionary. An empty dictionary is not nil, it is a dictionary. Just an empty one. A JSON parser will never, ever give nil values unless you ask for a key that is not in a dictionary. For example wirlessData ["nonexistingkey"] would give you nil.
If you be more type-strong about it with the if..let's then:
if let anonValue = wirlessData["anon"] {
if let value = anonValue["text"] as? String {
// This won't execute if value isn't converted from `anonvalue["text"]` to String specifically. This includes null been a false match too
print("\(value)") // nil
}else{
print("Value did't match string at all")
}
}
or even more specifically in your case:
if let anonValue = wirlessData["anon"] {
if let value = anonValue["text"] as? Int {
// This won't execute if value isn't converted from `anonvalue["text"]` to String specifically. This includes null been a false match too
print("\(value)") // nil
}else{
print("Value did't match int at all")
}
}
The value your parser is returning not nil, its empty so you need to check on count if inner data type is dictionary or array, I have past 1 sample here
Please use below code and correct your logic accordingly to get it work properly
let wirlessData:[String:AnyObject] = [
"anon" : [],
"channel" : [
"text" : 1
]
]
if wirlessData["anon"]?.count > 0 {
if let value = wirlessData["anon"]!["text"] {
print("\(value)") // nil
}
}
Try this below code using type check operator (is) -
if wirlessData["anon"] is [String:AnyObject]
{
let anon = wirlessData["anon"]!
print(anon)
if anon["random"] is String {
let stringValue = anon["random"]!
print("\(stringValue)")
}
else if anon["random"] is Int
{
let intValue = anon["random"]!
print("\(intValue)") // nil
}
else
{
print(" may be value did't match string & Int or nil ")
}
}
If I have an array in Swift, and try to access an index that is out of bounds, there is an unsurprising runtime error:
var str = ["Apple", "Banana", "Coconut"]
str[0] // "Apple"
str[3] // EXC_BAD_INSTRUCTION
However, I would have thought with all the optional chaining and safety that Swift brings, it would be trivial to do something like:
let theIndex = 3
if let nonexistent = str[theIndex] { // Bounds check + Lookup
print(nonexistent)
...do other things with nonexistent...
}
Instead of:
let theIndex = 3
if (theIndex < str.count) { // Bounds check
let nonexistent = str[theIndex] // Lookup
print(nonexistent)
...do other things with nonexistent...
}
But this is not the case - I have to use the ol' if statement to check and ensure the index is less than str.count.
I tried adding my own subscript() implementation, but I'm not sure how to pass the call to the original implementation, or to access the items (index-based) without using subscript notation:
extension Array {
subscript(var index: Int) -> AnyObject? {
if index >= self.count {
NSLog("Womp!")
return nil
}
return ... // What?
}
}
Alex's answer has good advice and solution for the question, however, I've happened to stumble on a nicer way of implementing this functionality:
extension Collection {
/// Returns the element at the specified index if it is within bounds, otherwise nil.
subscript (safe index: Index) -> Element? {
return indices.contains(index) ? self[index] : nil
}
}
Example
let array = [1, 2, 3]
for index in -20...20 {
if let item = array[safe: index] {
print(item)
}
}
If you really want this behavior, it smells like you want a Dictionary instead of an Array. Dictionaries return nil when accessing missing keys, which makes sense because it's much harder to know if a key is present in a dictionary since those keys can be anything, where in an array the key must in a range of: 0 to count. And it's incredibly common to iterate over this range, where you can be absolutely sure have a real value on each iteration of a loop.
I think the reason it doesn't work this way is a design choice made by the Swift developers. Take your example:
var fruits: [String] = ["Apple", "Banana", "Coconut"]
var str: String = "I ate a \( fruits[0] )"
If you already know the index exists, as you do in most cases where you use an array, this code is great. However, if accessing a subscript could possibly return nil then you have changed the return type of Array's subscript method to be an optional. This changes your code to:
var fruits: [String] = ["Apple", "Banana", "Coconut"]
var str: String = "I ate a \( fruits[0]! )"
// ^ Added
Which means you would need to unwrap an optional every time you iterated through an array, or did anything else with a known index, just because rarely you might access an out of bounds index. The Swift designers opted for less unwrapping of optionals, at the expense of a runtime exception when accessing out of bounds indexes. And a crash is preferable to a logic error caused by a nil you didn't expect in your data somewhere.
And I agree with them. So you won't be changing the default Array implementation because you would break all the code that expects a non-optional values from arrays.
Instead, you could subclass Array, and override subscript to return an optional. Or, more practically, you could extend Array with a non-subscript method that does this.
extension Array {
// Safely lookup an index that might be out of bounds,
// returning nil if it does not exist
func get(index: Int) -> T? {
if 0 <= index && index < count {
return self[index]
} else {
return nil
}
}
}
var fruits: [String] = ["Apple", "Banana", "Coconut"]
if let fruit = fruits.get(1) {
print("I ate a \( fruit )")
// I ate a Banana
}
if let fruit = fruits.get(3) {
print("I ate a \( fruit )")
// never runs, get returned nil
}
Swift 3 Update
func get(index: Int) ->T? needs to be replaced by func get(index: Int) ->Element?
To build on Nikita Kukushkin's answer, sometimes you need to safely assign to array indexes as well as read from them, i.e.
myArray[safe: badIndex] = newValue
So here is an update to Nikita's answer (Swift 3.2) that also allows safely writing to mutable array indexes, by adding the safe: parameter name.
extension Collection {
/// Returns the element at the specified index if it is within bounds, otherwise nil.
subscript(safe index: Index) -> Element? {
return indices.contains(index) ? self[index] : nil
}
}
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
}
}
}
}
extension Array {
subscript (safe index: Index) -> Element? {
0 <= index && index < count ? self[index] : nil
}
}
O(1) performance
type safe
correctly deals with Optionals for [MyType?] (returns MyType??, that can be unwrapped on both levels)
does not lead to problems for Sets
concise code
Here are some tests I ran for you:
let itms: [Int?] = [0, nil]
let a = itms[safe: 0] // 0 : Int??
a ?? 5 // 0 : Int?
let b = itms[safe: 1] // nil : Int??
b ?? 5 // nil : Int? (`b` contains a value and that value is `nil`)
let c = itms[safe: 2] // nil : Int??
c ?? 5 // 5 : Int?
Swift 4
An extension for those who prefer a more traditional syntax:
extension Array {
func item(at index: Int) -> Element? {
return indices.contains(index) ? self[index] : nil
}
}
Valid in Swift 2
Even though this has been answered plenty of times already, I'd like to present an answer more in line in where the fashion of Swift programming is going, which in Crusty's words¹ is: "Think protocols first"
• What do we want to do?
- Get an Element of an Array given an Index only when it's safe, and nil otherwise
• What should this functionality base it's implementation on?
- Array subscripting
• Where does it get this feature from?
- Its definition of struct Array in the Swift module has it
• Nothing more generic/abstract?
- It adopts protocol CollectionType which ensures it as well
• Nothing more generic/abstract?
- It adopts protocol Indexable as well...
• Yup, sounds like the best we can do. Can we then extend it to have this feature we want?
- But we have very limited types (no Int) and properties (no count) to work with now!
• It will be enough. Swift's stdlib is done pretty well ;)
extension Indexable {
public subscript(safe safeIndex: Index) -> _Element? {
return safeIndex.distanceTo(endIndex) > 0 ? self[safeIndex] : nil
}
}
¹: not true, but it gives the idea
Because arrays may store nil values, it does not make sense to return a nil if an array[index] call is out of bounds.
Because we do not know how a user would like to handle out of bounds problems, it does not make sense to use custom operators.
In contrast, use traditional control flow for unwrapping objects and ensure type safety.
if let index = array.checkIndexForSafety(index:Int)
let item = array[safeIndex: index]
if let index = array.checkIndexForSafety(index:Int)
array[safeIndex: safeIndex] = myObject
extension Array {
#warn_unused_result public func checkIndexForSafety(index: Int) -> SafeIndex? {
if indices.contains(index) {
// wrap index number in object, so can ensure type safety
return SafeIndex(indexNumber: index)
} else {
return nil
}
}
subscript(index:SafeIndex) -> Element {
get {
return self[index.indexNumber]
}
set {
self[index.indexNumber] = newValue
}
}
// second version of same subscript, but with different method signature, allowing user to highlight using safe index
subscript(safeIndex index:SafeIndex) -> Element {
get {
return self[index.indexNumber]
}
set {
self[index.indexNumber] = newValue
}
}
}
public class SafeIndex {
var indexNumber:Int
init(indexNumber:Int){
self.indexNumber = indexNumber
}
}
I realize this is an old question. I'm using Swift5.1 at this point, the OP was for Swift 1 or 2?
I needed something like this today, but I didn't want to add a full scale extension for just the one place and wanted something more functional (more thread safe?). I also didn't need to protect against negative indices, just those that might be past the end of an array:
let fruit = ["Apple", "Banana", "Coconut"]
let a = fruit.dropFirst(2).first // -> "Coconut"
let b = fruit.dropFirst(0).first // -> "Apple"
let c = fruit.dropFirst(10).first // -> nil
For those arguing about Sequences with nil's, what do you do about the first and last properties that return nil for empty collections?
I liked this because I could just grab at existing stuff and use it to get the result I wanted. I also know that dropFirst(n) is not a whole collection copy, just a slice. And then the already existent behavior of first takes over for me.
I found safe array get, set, insert, remove very useful. I prefer to log and ignore the errors as all else soon gets hard to manage. Full code bellow
/**
Safe array get, set, insert and delete.
All action that would cause an error are ignored.
*/
extension Array {
/**
Removes element at index.
Action that would cause an error are ignored.
*/
mutating func remove(safeAt index: Index) {
guard index >= 0 && index < count else {
print("Index out of bounds while deleting item at index \(index) in \(self). This action is ignored.")
return
}
remove(at: index)
}
/**
Inserts element at index.
Action that would cause an error are ignored.
*/
mutating func insert(_ element: Element, safeAt index: Index) {
guard index >= 0 && index <= count else {
print("Index out of bounds while inserting item at index \(index) in \(self). This action is ignored")
return
}
insert(element, at: index)
}
/**
Safe get set subscript.
Action that would cause an error are ignored.
*/
subscript (safe index: Index) -> Element? {
get {
return indices.contains(index) ? self[index] : nil
}
set {
remove(safeAt: index)
if let element = newValue {
insert(element, safeAt: index)
}
}
}
}
Tests
import XCTest
class SafeArrayTest: XCTestCase {
func testRemove_Successful() {
var array = [1, 2, 3]
array.remove(safeAt: 1)
XCTAssert(array == [1, 3])
}
func testRemove_Failure() {
var array = [1, 2, 3]
array.remove(safeAt: 3)
XCTAssert(array == [1, 2, 3])
}
func testInsert_Successful() {
var array = [1, 2, 3]
array.insert(4, safeAt: 1)
XCTAssert(array == [1, 4, 2, 3])
}
func testInsert_Successful_AtEnd() {
var array = [1, 2, 3]
array.insert(4, safeAt: 3)
XCTAssert(array == [1, 2, 3, 4])
}
func testInsert_Failure() {
var array = [1, 2, 3]
array.insert(4, safeAt: 5)
XCTAssert(array == [1, 2, 3])
}
func testGet_Successful() {
var array = [1, 2, 3]
let element = array[safe: 1]
XCTAssert(element == 2)
}
func testGet_Failure() {
var array = [1, 2, 3]
let element = array[safe: 4]
XCTAssert(element == nil)
}
func testSet_Successful() {
var array = [1, 2, 3]
array[safe: 1] = 4
XCTAssert(array == [1, 4, 3])
}
func testSet_Successful_AtEnd() {
var array = [1, 2, 3]
array[safe: 3] = 4
XCTAssert(array == [1, 2, 3, 4])
}
func testSet_Failure() {
var array = [1, 2, 3]
array[safe: 4] = 4
XCTAssert(array == [1, 2, 3])
}
}
Swift 5.x
An extension on RandomAccessCollection means that this can also work for ArraySlice from a single implementation. We use startIndex and endIndex as array slices use the indexes from the underlying parent Array.
public extension RandomAccessCollection {
/// Returns the element at the specified index if it is within bounds, otherwise nil.
/// - complexity: O(1)
subscript (safe index: Index) -> Element? {
guard index >= startIndex, index < endIndex else {
return nil
}
return self[index]
}
}
extension Array {
subscript (safe index: UInt) -> Element? {
return Int(index) < count ? self[Int(index)] : nil
}
}
Using Above mention extension return nil if anytime index goes out of bound.
let fruits = ["apple","banana"]
print("result-\(fruits[safe : 2])")
result - nil
I have padded the array with nils in my use case:
let components = [1, 2]
var nilComponents = components.map { $0 as Int? }
nilComponents += [nil, nil, nil]
switch (nilComponents[0], nilComponents[1], nilComponents[2]) {
case (_, _, .Some(5)):
// process last component with 5
default:
break
}
Also check the subscript extension with safe: label by Erica Sadun / Mike Ash: http://ericasadun.com/2015/06/01/swift-safe-array-indexing-my-favorite-thing-of-the-new-week/
The "Commonly Rejected Changes" for Swift list contains a mention of changing Array subscript access to return an optional rather than crashing:
Make Array<T> subscript access return T? or T! instead of T: The current array behavior is intentional, as it accurately reflects the fact that out-of-bounds array access is a logic error. Changing the current behavior would slow Array accesses to an unacceptable degree. This topic has come up multiple times before but is very unlikely to be accepted.
https://github.com/apple/swift-evolution/blob/master/commonly_proposed.md#strings-characters-and-collection-types
So the basic subscript access will not be changing to return an optional.
However, the Swift team/community does seem open to adding a new optional-returning access pattern to Arrays, either via a function or subscript.
This has been proposed and discussed on the Swift Evolution forum here:
https://forums.swift.org/t/add-accessor-with-bounds-check-to-array/16871
Notably, Chris Lattner gave the idea a "+1":
Agreed, the most frequently suggested spelling for this is: yourArray[safe: idx], which seems great to me. I am very +1 for adding this.
https://forums.swift.org/t/add-accessor-with-bounds-check-to-array/16871/13
So this may be possible out of the box in some future version of Swift. I'd encourage anyone who wants it to contribute to that Swift Evolution thread.
Not sure why no one, has put up an extension that also has a setter to automatically grow the array
extension Array where Element: ExpressibleByNilLiteral {
public subscript(safe index: Int) -> Element? {
get {
guard index >= 0, index < endIndex else {
return nil
}
return self[index]
}
set(newValue) {
if index >= endIndex {
self.append(contentsOf: Array(repeating: nil, count: index - endIndex + 1))
}
self[index] = newValue ?? nil
}
}
}
Usage is easy and works as of Swift 5.1
var arr:[String?] = ["A","B","C"]
print(arr) // Output: [Optional("A"), Optional("B"), Optional("C")]
arr[safe:10] = "Z"
print(arr) // [Optional("A"), Optional("B"), Optional("C"), nil, nil, nil, nil, nil, nil, nil, Optional("Z")]
Note: You should understand the performance cost (both in time/space) when growing an array in swift - but for small problems sometimes you just need to get Swift to stop Swifting itself in the foot
To propagate why operations fail, errors are better than optionals.
public extension Collection {
/// Ensure an index is valid before accessing an element of the collection.
/// - Returns: The same as the unlabeled subscript, if an error is not thrown.
/// - Throws: `AnyCollection<Element>.IndexingError`
/// if `indices` does not contain `index`.
subscript(validating index: Index) -> Element {
get throws {
guard indices.contains(index)
else { throw AnyCollection<Element>.IndexingError() }
return self[index]
}
}
}
public extension AnyCollection {
/// Thrown when `[validating:]` is called with an invalid index.
struct IndexingError: Error { }
}
XCTAssertThrowsError(try ["🐾", "🥝"][validating: 2])
let collection = Array(1...10)
XCTAssertEqual(try collection[validating: 0], 1)
XCTAssertThrowsError(try collection[validating: collection.endIndex]) {
XCTAssert($0 is AnyCollection<Int>.IndexingError)
}
I think this is not a good idea. It seems preferable to build solid code that does not result in trying to apply out-of-bounds indexes.
Please consider that having such an error fail silently (as suggested by your code above) by returning nil is prone to producing even more complex, more intractable errors.
You could do your override in a similar fashion you used and just write the subscripts in your own way. Only drawback is that existing code will not be compatible. I think to find a hook to override the generic x[i] (also without a text preprocessor as in C) will be challenging.
The closest I can think of is
// compile error:
if theIndex < str.count && let existing = str[theIndex]
EDIT: This actually works. One-liner!!
func ifInBounds(array: [AnyObject], idx: Int) -> AnyObject? {
return idx < array.count ? array[idx] : nil
}
if let x: AnyObject = ifInBounds(swiftarray, 3) {
println(x)
}
else {
println("Out of bounds")
}
I have made a simple extension for array
extension Array where Iterator.Element : AnyObject {
func iof (_ i : Int ) -> Iterator.Element? {
if self.count > i {
return self[i] as Iterator.Element
}
else {
return nil
}
}
}
it works perfectly as designed
Example
if let firstElemntToLoad = roots.iof(0)?.children?.iof(0)?.cNode,
You can try
if index >= 0 && index < array.count {
print(array[index])
}
To be honest I faced this issue too. And from performance point of view a Swift array should be able to throw.
let x = try a[y]
This would be nice and understandable.
When you only need to get values from an array and you don't mind a small performance penalty (i.e. if your collection isn't huge), there is a Dictionary-based alternative that doesn't involve (a too generic, for my taste) collection extension:
// Assuming you have a collection named array:
let safeArray = Dictionary(uniqueKeysWithValues: zip(0..., array))
let value = safeArray[index] ?? defaultValue;
2022
infinite index access and safe idx access(returns nil in case no such idex):
public extension Collection {
subscript (safe index: Index) -> Element? {
return indices.contains(index) ? self[index] : nil
}
subscript (infinityIdx idx: Index) -> Element where Index == Int {
return self[ abs(idx) % self.count ]
}
}
but be careful, it will throw an exception in case of array/collection is empty
usage
(0...10)[safe: 11] // nil
(0...10)[infinityIdx: 11] // 0
(0...10)[infinityIdx: 12] // 1
(0...10)[infinityIdx: 21] // 0
(0...10)[infinityIdx: 22] // 1
Swift 5 Usage
extension WKNavigationType {
var name : String {
get {
let names = ["linkAct","formSubm","backForw","reload","formRelo"]
return names.indices.contains(self.rawValue) ? names[self.rawValue] : "other"
}
}
}
ended up with but really wanted to do generally like
[<collection>][<index>] ?? <default>
but as the collection is contextual I guess it's proper.
Im trying to ask for some values from a variable.
The variable is going to have the description of the weather and i want to ask for specific words in order to show different images (like a sun, rain or so)
The thing is i have code like this:
if self.descriptionWeather.description.rangeOfString("Clear") != nil
{
self.imageWeather.image = self.soleadoImage
}
if self.descriptionWeather.description.rangeOfString("rain") != nil
{
self.imageWeather.image = self.soleadoImage
}
if self.descriptionWeather.description.rangeOfString("broken clouds") != nil
{
self.imageWeather.image = self.nubladoImage
}
Because when i tried to add an "OR" condition xcode gives me some weird errors.
Is it possible to do a swich sentence with that? Or anyone knows how to do add an OR condition to the if clause?
I had a similar problem today and realized this question hasn't been updated since Swift 1! Here's how I solved it in Swift 4:
switch self.descriptionWeather.description {
case let str where str.contains("Clear"):
print("clear")
case let str where str.contains("rain"):
print("rain")
case let str where str.contains("broken clouds"):
print("broken clouds")
default:
break
}
Swift 5 Solution
func weatherImage(for identifier: String) -> UIImage? {
switch identifier {
case _ where identifier.contains("Clear"),
_ where identifier.contains("rain"):
return self.soleadoImage
case _ where identifier.contains("broken clouds"):
return self.nubladoImage
default: return nil
}
}
You can do this with a switch statement using value binding and a where clause. But convert the string to lowercase first!
var desc = "Going to be clear and bright tomorrow"
switch desc.lowercaseString as NSString {
case let x where x.rangeOfString("clear").length != 0:
println("clear")
case let x where x.rangeOfString("cloudy").length != 0:
println("cloudy")
default:
println("no match")
}
// prints "clear"
Swift language has two kinds of OR operators - the bitwise ones | (single vertical line), and the logical ones || (double vertical line). In this situation you need a logical OR:
if self.descriptionWeather.description.rangeOfString("Clear") != nil || self.descriptionWeather.description.rangeOfString("clear") != nil {
self.imageWeather.image = self.soleadoImage
}
Unlike Objective-C where you could get away with a bitwise OR in exchange for getting a slightly different run-time semantic, Swift requires a logical OR in the expression above.
If you do this a lot, you can implement a custom ~= operator that defines sub-string matching. It lends itself to this nice syntax:
switch "abcdefghi".substrings {
case "def": // calls `"def" ~= "abcdefghi".substrings`
print("Found substring: def")
case "some other potential substring":
print("Found \"some other potential substring\"")
default: print("No substring matches found")
}
Implementation:
import Foundation
public struct SubstringMatchSource {
private let wrapped: String
public init(wrapping wrapped: String) {
self.wrapped = wrapped
}
public func contains(_ substring: String) -> Bool {
return self.wrapped.contains(substring)
}
public static func ~= (substring: String, source: SubstringMatchSource) -> Bool {
return source.contains(substring)
}
}
extension String {
var substrings: SubstringMatchSource {
return SubstringMatchSource(wrapping: self)
}
}
I'd recommend using a dictionary instead, as a mapping between the substring you're searching for and the corresponding image:
func image(for weatherString: String) -> UIImage? {
let imageMapping = [
"Clear": self.soleadoImage,
"rain": self.soleadoImage,
"broken clouds": self.nubladoImage]
return imageMapping.first { weatherString.contains($0.key) }?.value
}
A dictionary gives you flexibility, adding new mappings is easy to do.
This link also describes overloading operator ~= which is actually used by the switch statement for matching cases to allow you to match regular expressions.