Variables sharing the same array instance - Javascript like array [duplicate]

Question

In Javascript, when you do that :

var a = [1, 2];

var b = a;

b.push(3);

console.log(a); //Print [1, 2, 3]

a and b share the same array instance. I am looking for a way to achieve this in Swift

Here is my Swift code :

var array1 = [[1], [2], [3]];

var array2 = array1[0];

array2.append(0);

print(array1);
//print [[1], [2], [3]]
//I want [[1, 0], [2], [3]]

array1[0] & array2 are two different instances ... I'd like two variables (with different names) pointing to the same instance of an array.

Arrays are value types in Swift, you cannot share the same instance with another. — Santosh
– Santosh, Commented Nov 28, 2016 at 21:58
Oh that's sad ... Maybe a kind of pointer variables ? Does it exist ? — Santa Claus
– Santa Claus, Commented Nov 28, 2016 at 22:01
Related: Pass array by reference between viewcontrollers in swift, Is it possible to assign an array to a class property by reference rather than a copy? — Hamish
– Hamish, Commented Nov 28, 2016 at 22:03

Community · Accepted Answer · 2017-05-23 12:07:17Z

2

You have to wrap a value type into a reference type ie class.

class wrapper {
    var array = [1,2]
}

var a = wrapper()

var b = a

b.array.append(3)

print(a.array) // [1,2,3]

Reading here You can also use NSMutableArray

var c : NSMutableArray = [1,2]


var d = c

d.insert(3, at: 2)

print(c) //"(\n    1,\n    2,\n    3\n)\n"

edited May 23, 2017 at 12:07

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answered Nov 28, 2016 at 22:04

mfaani

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7 Comments

mfaani Over a year ago

I'm not sure as to which is faster/better, but I think if you only want an array, then using NSMutableArray is preferred, but if you want a model, ie something that also stores an array and needs to be persisted then perhaps wrapping it inside a class is a better choice.

Hamish Over a year ago

A wrapper type is far better, as you don't throw away the type information of what the elements are, unlike NSMutableArray (and you don't needlessly bridge the elements over to Obj-C). You can make the wrapper generic in order to store different types of arrays, such as shown in Qbyte's answer here.

mfaani Over a year ago

@Hamish Faster because no bridging? OK. What do you mean when you say "don't throw away the type information" you mean generics?

Hamish Over a year ago

I mean that in your example wrapper contains an array property of type [Int], therefore you know that it contains only Ints. NSMutableArray on the other hand just contains heterogeneous NSObjects (bridged back to Swift as Any) – the elements could be a mix of NSNumbers, UIViews and UIColors for all we know. Therefore you'll have to do some type-casting before you can do anything useful with them.

mfaani Over a year ago

Thanks. I understand the concept now. "therefore you know that it contains only Ints". So in my answer, I am throwing the type information ie I'm saying it's of type Int. I feel like my jargon is different than yours. Why did you say " as you don't throw away"

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