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
}
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.
Given a Dictionary<String, Arrary<Int>> find the how many entries have the same two specified values in the first 5 entries in the Array<Int>.
For example:
Given:
let numberSeries = [
"20022016": [07,14,36,47,50,02,05],
"13022016": [16,07,32,36,41,07,09],
"27022016": [14,18,19,31,36,04,05],
]
And the values: 7 and 36, the result should be 2 since the first and second entry have both the values 7 and 36 in the first 5 entries of the entry's array.
I've tried to accomplish this many ways, but I haven't been able to get it to work.
This is my current attempt:
//created a dictionary with (key, values)
let numberSeries = [
"20022016": [07,14,36,47,50,02,05],
"13022016": [16,07,32,36,41,07,09],
"27022016": [14,18,19,31,36,04,05],
]
var a = 07 //number to look for
var b = 36 // number to look for
// SearchForPairAB // search for pair // Doesn't Work.
var ab = [a,b] // pair to look for
var abPairApearedCount = 0
for (kind, numbers) in numberSeries {
for number in numbers[0...4] {
if number == ab { //err: Cannot invoke '==' with argument listof type Int, #Value [Int]
abPairApearedCount++
}
}
}
This gives the error: Cannot invoke '==' with argument listof type Int, #Value [Int] on the line: if number == ab
You can't use == to compare an Int and Array<Int>, that just doesn't make any sense from a comparison perspective. There are lots of different ways you can achieve what you're trying to do though. In this case I'd probably use map/reduce to count your pairs.
The idea is to map the values in your ab array to Bool values determined by whether or not the value is in your numbers array. Then, reduce those mapped Bools to a single value: true if they're all true, or false. If that reduced value is true, then we found the pair so we increment the count.
var ab = [a,b] // pair to look for
var abPairApearedCount = 0
for (kind, numbers) in numberSeries {
let found = ab.map({ number in
// find is a built-in function that returns the index of the value
// in the array, or nil if it's not found
return find(numbers[0...4], number) != nil
}).reduce(true) { (result, value: Bool) in
return result && value
}
if found {
abPairApearedCount++
}
}
That can actually be compacted quite a bit by using some of Swift's more concise syntax:
var ab = [a,b] // pair to look for
var abPairApearedCount = 0
for (kind, numbers) in numberSeries {
let found = ab.map({ find(numbers[0...4], $0) != nil }).reduce(true) { $0 && $1 }
if found {
abPairApearedCount++
}
}
And, just for fun, can be compacted even further by using reduce instead of a for-in loop:
var ab = [a,b] // pair to look for
var abPairApearedCount = reduce(numberSeries, 0) { result, series in
result + (ab.map({ find(series.1[0...4], $0) != nil }).reduce(true) { $0 && $1 } ? 1 : 0)
}
That's getting fairly unreadable though, so I'd probably expand some of that back out.
So here's my FP solution, aimed at decomposing the problem into easily digestible and reusable bite-sized chunks:
First, we define a functor that trims an array to a given length:
func trimLength<T>(length: Int) -> ([T]) -> [T] {
return { return Array($0[0...length]) }
}
Using this we can trim all the elements using map(array, trimLength(5))
Now, we need an predicate to determine if all the elements of one array are in the target array:
func containsAll<T:Equatable>(check:[T]) -> ([T]) -> Bool {
return { target in
return reduce(check, true, { acc, elem in return acc && contains(target, elem) })
}
}
This is the ugliest bit of code here, but essentially it's just iterating over check and insuring that each element is in the target array. Once we've got this we can use filter(array, containsAll([7, 26])) to eliminate all elements of the array that don't contain all of our target values.
At this point, we can glue the whole thing together as:
filter(map(numberSeries.values, trimLength(5)), containsAll([7, 36])).count
But long lines of nested functions are hard to read, let's define a couple of helper functions and a custom operator:
func rmap<S:SequenceType, T>(transform:(S.Generator.Element)->T) -> (S) -> [T] {
return { return map($0, transform) }
}
func rfilter<S:SequenceType>(predicate:(S.Generator.Element)->Bool) -> (S) -> [S.Generator.Element] {
return { sequence in return filter(sequence, predicate) }
}
infix operator <^> { associativity left }
func <^> <S, T>(left:S, right:(S)->T) -> T {
return right(left)
}
And a convenience function to count it's inputs:
func count<T>(array:[T]) -> Int {
return array.count
}
Now we can condense the whole thing as:
numberSeries.values <^> rmap(trimLength(5)) <^> rfilter(containsAll([7, 36])) <^> count