Returning a mutable array from a function

Question

How to return a mutable array from a function?

Here is a short snippet of code to make it more clear:

var tasks = ["Mow the lawn", "Call Mom"]
var completedTasks = ["Bake a cake"]

func arrayAtIndex(index: Int) -> String[] {
    if index == 0 {
        return tasks
    } else {
        return completedTasks
    }
}

arrayAtIndex(0).removeAtIndex(0)
// Immutable value of type 'String[]' only has mutating members named 'removeAtIndex'

The following snippet works but I have to return an Array, not an NSMutableArray

var tasks: NSMutableArray = ["Mow the lawn", "Call Mom"]
var completedTasks: NSMutableArray = ["Bake a cake"]

func arrayAtIndex(index: Int) -> NSMutableArray {
    if index == 0 {
        return tasks
    } else {
        return completedTasks
    }
}

arrayAtIndex(0).removeObjectAtIndex(0)

tasks // ["Call Mom"]

Thanks!

drewag · Accepted Answer · 2014-06-08 18:58:58Z

3

This whole paradigm is discouraged in Swift. Arrays in swift are "Value Types" meaning that they get copied every time they are passed around. This means that once you pass the array into a function, you cannot have that function modify the contents of the original array. This is much safer.

What you could do is:

var newArray = arrayAtIndex(0)
newArray.removeObjectAtIndex(0)

But note that tasks will not be modified. NewArray will be a copy of tasks with the first object removed

The reason this works with NSMutableArray, is that NSArray and NSMutableArray are copied by reference, so they always refer to the original array unless explicitly copied.

answered Jun 8, 2014 at 18:58

drewag

95.1k28 gold badges142 silver badges131 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Damien Over a year ago

Thanks for your answer, it totally makes sense. However, it worths noting that in the whole program, tasks and completedTasks are instance variables (declared with var) and I am returning self.tasks and self.completedTasks. I can't make a copy of the returned array, remove an element and then reassign the instance variable to it because the instance variables themselves are transformed as non-mutable. How can instance variables possibly be transformed as non-mutable after being returned by a function?

Renetik Over a year ago

swift is so safe that it's pain in the ass

Collectives™ on Stack Overflow

Returning a mutable array from a function

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related