I have code like below
let myNums = getXYZ(nums: [1,2,3,4,5])
func getXYZ(nums: [Int]) -> [Int] {
let newNum = nums.map { (num) -> Int in
if num == 2 {
//do something and continue execution with next element in list like break/fallthrough
return 0
}
return num
}
return newNum
}
print(myNums)`
This prints [1,0,3,4,5]
but i want the output to be [1,3,4,5]. How can I exclude 2? I want to alter the if statement used so as to not include in array when it sees number 2
I have to use .map here but to exclude 2..is there any possibility
Please let me know
I'd simply do a filter as described as your problem, you want to filter the numbers by removing another number.
var myNums = [1, 2, 3, 4, 5]
let excludeNums = [2]
let newNum = myNums.filter({ !excludeNums.contains($0) })
print(newNum) //1, 3, 4, 5
If you need to do a map, you could do a map first then filter.
let newNum = myNums.map({ $0*2 }).filter({ !excludeNums.contains($0) })
print(newNum) //4, 6, 8, 10
This maps to multiplying both by 2 and then filtering by removing the new 2 from the list. If you wanted to remove the initial 2 you would have to filter first then map. Since both return a [Int] you can call the operations in any order, as you deem necessary.
As suggested by #koropok, I had to make below changes
nums.compactMap { (num) -> Int? in
....
if num == 2 {
return nil
}
I suggest you to use filter instead of map:
let myNums = [1,2,3,4,5]
let result1 = myNums.filter{ return $0 != 2 }
print(result1) // This will print [1,3,4,5]
If you must definitely use map, then use compactMap:
let result2 = myNums.compactMap { return $0 == 2 ? nil : $0 }
print(result2) // This will print [1,3,4,5]
Hope this helps
filter is more appropriate than map for your use case.
If you want to exclude only 1 number:
func getXYZ(nums: [Int]) -> [Int] {
return nums.filter { $0 != 2 }
}
If you want to exclude a list of numbers, store those exclusions in a Set since Set.contains runs in O(1) time, whereas Array.contains runs in O(n) time.
func getXYZ(nums: [Int]) -> [Int] {
let excluded: Set<Int> = [2,4]
return nums.filter { !excluded.contains($0) }
}
My solution is based on enumerated() method:
let elements = nums.enumerated().compactMap { (index, value) in
( index == 0 ) ? nil : value
}
enumerated() add element's index as first closure argument
func getBArrayList(index: Int, array:[NSDictionary] ) -> [
NSDictionary]{
var barray:[NSDictionary] = []
for i in 0 ..< array.count
{
if array[i] == array[index]
{
break
}
barray[i] = array[i]
}
print(barray)
return barray
}
minuscurent = getBArrayList(index: arrayindex, array: minuscurent)
when array[i ] equals array[index] i want to break the execution of the for loop and carry on with next value
barray is initialized as an empty array, having no elements. On line:
barray[i] = array[i]
You are trying to access its ith element, but there are no elements in the array. That's the reason of the crash.
Use following to add those elements to barray:
barray.append(array[i])
Moreover, I believe it is enough to test the equality using:
if i == index
instead of:
if array[i] == array[index]
UPDATE
Now I am not 100% sure what you try to achieve, but it seems that you are just trying to remove all the elements after index arrayindex from the minuscurent.
minuscurent = getBArrayList(index: arrayindex, array: minuscurent)
If that is really the case, you don't have to implement your own implementation, and you can just use:
minuscurrent = minuscurrent.prefix(arrayindex)
If the goal is to remove just one element, again, use the standard implementation:
minuscurrent.remove(at: arrayindex)
You can not assign object of NSDictionary directly into the array, if it is not initialised or in other words, if you have not provided the memory to the array.
You should practise to use barray.append(array[i]) instead of directly assigning the object to array.
And, instead initialising the empty barray, you can use this so to always start with 0 index, instead 0 will be occupied by empty data.
var barray = [NSDictionary]()
Try this function, i think there is no require for loop you can use ArraySlice https://developer.apple.com/documentation/swift/arrayslice
func getBArrayList(index: Int, array:[NSDictionary] ) -> [NSDictionary] {
var barray:[NSDictionary] = []
if index >= 0 && index < array.count {
barray.append(contentsOf: array[0..<index])
}
return barray
}
Here is my code so far
var counter = 0
for i in 0...9 {
var val = NamePicker()
// array to find duplicates
var buttonValues = ["", "", "", "", "", "", "", "", "", ""] // array for button names
buttonValues.insert(val, at: counter)
print(buttonValues[counter])
counter += 1
}
This code is putting 10 string values into my array. What I would like to do is find a way to check each value in my array. for eample if my end result array is ["a","a","a","b","b","c","c","e","f","c"] I want to see if there is a triple of the same name(single and duplicates are fine). However if there is a triple I would like to change the 3rd value to another val from my NamePicker() function.
so with my array of
["a","a","a","b","b","c","c","e","f","c"]
there are 3 "a" and 3 "c", having two of the same is ok, i would like to change the 3rd to a new values and if the new value makes another triple it will change until there are no more triples.
so that array could possible have an end result of
["a","a","f","b","b","c","c","e","f","z"]
this is where the triples where changed.
Any help on how to do this efficiently?
Both options below asume that your NamePciker() function can generate at least 5 distinct values so there exists an array that satisfies your requirement.
Your requirement is better handled by not generating so many duplicates to begin with. If all you want is an array of names when each name cannot be repeated more than twice, try this:
var buttonValues = [String]()
var dict = [String: Int]()
while buttonValues.count < 10 {
let name = NamePicker()
let count = dict[name] ?? 0
guard count < 2 else { continue }
buttonValues.append(name)
dict[name] = count + 1
}
If you already have the array and want to correct it, do this:
var buttonValues = ["a","a","a","b","b","c","c","e","f","c"]
// Scan the array to tally how many times each name appears
var totalDict = [String: Int]()
buttonValues.forEach { totalDict[$0] = (totalDict[$0] ?? 0) + 1 }
// Now scan it again to update names that appear too many times
var runningDict = [String: Int]()
for (index, value) in buttonValues.enumerated() {
let count = runningDict[value] ?? 0
if count >= 2 {
while true {
let newValue = NamePicker()
let newTotal = (totalDict[newValue] ?? 0) + 1
if newTotal < 3 {
buttonValues[index] = newValue
totalDict[newValue] = newTotal
break
}
}
} else {
runningDict[value] = count + 1
}
}
Dictionary is the best way I think. Have the key be the character and the value be the count of that character. Your runtime will be O(n) since you only have to run through each input once. Here is an example:
let chars = ["a","a","a","b","b","c","c","e","f","c"]
var dict = [String: Int]()
for char in chars {
//If already in Dictionary, increase by one
if var count = dict[char] {
count += 1
dict[char] = count
} else {//else is not in the dictionary already, init with 1
dict[char] = 1
}
}
Output:
["b": 2, "e": 1, "a": 3, "f": 1, "c": 3]
Now I'm not sure how you want to replace the value that's the same character for a third time, but this is probably the best way to group the strings to determine which are over the limit.
Instead of inserting the wrong value and then checking if the values are correct, I would suggest to automatically create the correct array.
//array for button names
var buttonValues = Array<String>()
//tracks what value has been inserted how many times
var trackerDict = [String: Int]()
for i in 0...9 {
//we initialize a new variable that tells us if we found a valid value (if the value has not been inserted 2 times already)
var foundValidValue = false
while !foundValidValue{
var val = NamePicker()
//now we check if the value exists and if it is inserted less than 2 times
if let count = trackerDict[val] {
if count < 2 {
foundValidValue = true
}
}
//if we found the value, we can add it
if foundValidValue {
trackerDict[val] = (trackerDict[val] ?? 0) + 1
buttonValues.append(val)
}
//if we did not find it, we just run through the loop again
}
}
I added a dictionary because it is faster to keep track of the count in a dictionary than counting the number of occurrences in the array every time.
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.