Swift Fatal error: Index out of range (out of bounds index)

In your code trying to access 4th index due to you have used ... in loop control syntax. And 4th index not in array.

Here is some details about for swift loop.

for index in 0...4 {
 ...
}

The above snippet says, iterate over the range starting at 0 and inclusive of 4 i.e from 0–4

If you do not want 4 included, you use this called the half-open range operator (..<).

for index in 0..<4 {
 ...
}

This would loop from 0 to 3 and stop execution.

In swift, there're more efficient ways to loop...but to better understand what you implemented...

I've updated your code...it will run properly.

let dir: [String] = ["north", "east", "south", "west"]
var num = dir.count
func move(){
    for i in 0..<num{
        var holder = dir[i]
        switch holder{
        case "north":
            print("you've moved north")
        case "east":
            print("you've moved east")
        case "south":
            print("you've moved south")
        case "west":
            print("you've moved west")
        default:
            print("where you going?")
        }
        if i == 3{
            print("round the world")
        }

    }
}

move()

Output :-

you've moved north
you've moved east
you've moved south
you've moved west
round the world

Happy Coding in Swift :-)

import UIKit

class ViewController: UIViewController {

    let dir: [String] = ["north", "east", "south", "west"]

    override func viewDidLoad() {
        super.viewDidLoad()
         move()
        // Do any additional setup after loading the view.
    }

    func move(){
        for (index, element) in dir.enumerated() {
       //   print("Item \(index): \(element)")
            switch element{
                        case "north":
                            print("you've moved north")
                        case "east":
                            print("you've moved east")
                        case "south":
                            print("you've moved south")
                        case "west":
                            print("you've moved west")
                        default:
                            print("where you going?")
                        }
                        if index == 3{
                            print("round the world")
                        }
        }
    }
}

So, here you are first taking count of the num which is here 4.