Randomly select 5 elements from an array list with

Question

I am currently trying to make an app for iOS but I can't get some simple code down. Basically I need to randomly select 5 elements from an array list without repeating an element. I have a rough draft, but it only displays one element.

Here is my code:

let array1 = ["salmon", "turkey", "salad", "curry", "sushi", "pizza"]

let randomIndex1 = Int(arc4random_uniform(UInt32(array1.count)))

print(array1[randomIndex1])

Answer 1

Just my ¢2: Moe Abdul-Hameed's solution has one theoretical drawback: if you roll the same randomIndex in every iteration, the while loop will never exit. It's very unlike tho.

Another approach is to create mutable copy of original array and then exclude picked items:

var source = array1
var dest = [String]()
for _ in 1...5 {
    let randomIndex = Int(arc4random_uniform(UInt32(source.count)))
    dest.append(source[randomIndex])
    source.remove(at: randomIndex)
}

print(dest)

Answer 2

var array1 = ["salmon", "turkey", "salad", "curry", "sushi", "pizza"]

while array1.count > 0 {

    // random key from array
    let arrayKey = Int(arc4random_uniform(UInt32(array1.count)))

    // your random number
    let randNum = array1[arrayKey] 

    // make sure the number ins't repeated
    array1.removeAtIndex(arrayKey)
}

By removing your picked value from array, prevent's from duplicates to be picked

Answer 3

You can do it like this:

let array1 = ["salmon", "turkey", "salad", "curry", "sushi", "pizza", "curry", "sushi", "pizza"]
var resultSet = Set<String>()

while resultSet.count < 5 {
    let randomIndex = Int(arc4random_uniform(UInt32(array1.count)))
    resultSet.insert(array1[randomIndex])
}

let resultArray = Array(resultSet)

print(resultArray)

A set can contain unique elements only, so it can't have the same element more than once.

I created an empty set, then as long as the array contains less than 5 elements (the number you chose), I iterated and added a random element to the set.

In the last step, we need to convert the set to an array to get the array that you want.

Answer 4

If you don't care about changing the original array, the following code will put the picked elements at the end of the array and then it will return the last part of the array as a slice. This is useful if you don't care about changing the original array, the advantage is that it doesn't use extra memory, and you can call it several times on the same array.

extension Array {
    mutating func takeRandomly(numberOfElements n: Int) -> ArraySlice<Element> {
        assert(n <= self.count)
        for i in stride(from: self.count - 1, to: self.count - n - 1, by: -1) {
            let randomIndex = Int(arc4random_uniform(UInt32(i + 1)))
            self.swapAt(i, randomIndex)
        }
        return self.suffix(n)
    }
}

Example:

var array = [1,2,3,4]
let a1 = array.takeRandomly(numberOfElements: 2)
let a2 = array.takeRandomly(numberOfElements: 2)