How to find index of list item in Swift?

As swift is in some regards more functional than object-oriented (and Arrays are structs, not objects), use the function "find" to operate on the array, which returns an optional value, so be prepared to handle a nil value:

let arr:Array = ["a","b","c"]find(arr, "c")!              // 2find(arr, "d")               // nil

Use firstIndex and lastIndex - depending on whether you are looking for the first or last index of the item:

let arr = ["a","b","c","a"]let indexOfA = arr.firstIndex(of: "a") // 0let indexOfB = arr.lastIndex(of: "a") // 3

tl;dr:

For classes, you might be looking for:

let index = someArray.firstIndex{$0 === someObject}

Full answer:

I think it's worth mentioning that with reference types (class) you might want to perform an identity comparison, in which case you just need to use the === identity operator in the predicate closure:

Swift 5, Swift 4.2:

let person1 = Person(name: "John")let person2 = Person(name: "Sue")let person3 = Person(name: "Maria")let person4 = Person(name: "Loner")let people = [person1, person2, person3]let indexOfPerson1 = people.firstIndex{$0 === person1} // 0let indexOfPerson2 = people.firstIndex{$0 === person2} // 1let indexOfPerson3 = people.firstIndex{$0 === person3} // 2let indexOfPerson4 = people.firstIndex{$0 === person4} // nil

Note that the above syntax uses trailing closures syntax, and is equivalent to:

let indexOfPerson1 = people.firstIndex(where: {$0 === person1})

Swift 4 / Swift 3 - the function used to be called index

Swift 2 - the function used to be called indexOf

* Note the relevant and useful comment by paulbailey about class types that implement Equatable, where you need to consider whether you should be comparing using === (identity operator) or == (equality operator). If you decide to match using ==, then you can simply use the method suggested by others (people.firstIndex(of: person1)).

You can filter an array with a closure:

var myList = [1, 2, 3, 4]var filtered = myList.filter { $0 == 3 }  // <= returns [3]

And you can count an array:

filtered.count // <= returns 1

So you can determine if an array includes your element by combining these:

myList.filter { $0 == 3 }.count > 0  // <= returns true if the array includes 3

If you want to find the position, I don't see fancy way, but you can certainly do it like this:

var found: Int?  // <= will hold the index if it was found, or else will be nilfor i in (0..x.count) {    if x[i] == 3 {        found = i    }}

EDIT

While we're at it, for a fun exercise let's extend Array to have a find method:

extension Array {    func find(includedElement: T -> Bool) -> Int? {        for (idx, element) in enumerate(self) {            if includedElement(element) {                return idx            }        }        return nil    }}

Now we can do this:

myList.find { $0 == 3 }// returns the index position of 3 or nil if not found