Swift Unable to Append String to an Array looked up with a dictionary

Question

Any idea why this code is not working?

var apples = [String]()
var oranges = [String]()
var bananas = [String]()

var optionArrays : [String : [String]] = [
    "apple" : apples,
    "orange" : oranges,
    "banana" : bananas
]

optionArrays["apple"]!.append("Macintosh")
optionArrays["apple"]!.count // 1

apples.count // 0 -> Why isn't there already one apple?
apples.append("Golden Delicious")
apples.count // 1

For some reason the optionArrays["apple"] seems to be working, but no the actual apples array. Shouldn't this work?

You never assigned something to the apples array. You did it for the optionsArrays. — Guy Daher
– Guy Daher, Commented Feb 13, 2017 at 7:43
@GuyDaher is there anyway to do what I am trying to accomplish? — GED125
– GED125, Commented Feb 13, 2017 at 7:48
Basically I want to populate a variable array, if that makes any sense. — GED125
– GED125, Commented Feb 13, 2017 at 7:49

Sweeper · Accepted Answer · 2017-02-13 07:56:12Z

3

Arrays are value types in swift. When you put them into dictionaries:

var optionArrays : [String : [String]] = [
    "apple" : apples,
    "orange" : oranges,
    "banana" : bananas
]

Copies of the actual apples, oranges and bananas arrays are created and put into the dictionary. When you modify the dictionary:

optionArrays["apple"]!.append("Macintosh")

you only modify the copy of apples that is in the dictionary.

A workaround is to assign all the copies of the arrays to the actual arrays whenever the dictionary is modfied:

var optionArrays : [String : [String]] = [
    "apple" : apples,
    "orange" : oranges,
    "banana" : bananas
    ] {
didSet {
    apples = optionArrays["apple"]!
    oranges = optionArrays["orange"]!
    bananas = optionArrays["banana"]!
}
}

Another workaround is to create a reference type wrapper for Array:

class RefArray<T> { // you can consider conforming to ExpressibleByArrayLiteral
    var innerArray = [T]()
}

var apples = RefArray<String>()
var oranges = RefArray<String>()
var bananas = RefArray<String>()

var optionArrays : [String : RefArray<String>] = [
    "apple" : apples,
    "orange" : oranges,
    "banana" : bananas
    ]

optionArrays["apple"]!.innerArray.append("Macintosh")
optionArrays["apple"]!.innerArray.count // 1

apples.innerArray.count

answered Feb 13, 2017 at 7:56

Sweeper

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1 Comment

GED125 Over a year ago

thank you, your first work around worked great! Using the didSet event is exactly what I needed! Thank you so much, I've been scratching my head all day on this!

ERbittuu · Accepted Answer · 2017-02-13 08:02:59Z

It like that because Swift array is value type not reference type

var apples = [String]()
var oranges = [String]()
var bananas = [String]()

var optionArrays : [String : [String]] = [
    "apple" : apples,
    "orange" : oranges,
    "banana" : bananas
]

print("Initial values")
print(apples)

[]

print(optionArrays)

["apple": [], "orange": [], "banana": []]

optionArrays["apple"]!.append("Macintosh")
optionArrays["apple"]!.count // 1

print("Values after update optionArrays")
print(apples)

[]

print(optionArrays)

["apple": ["Macintosh"], "orange": [], "banana": []]

You will see here that apples array is not updated just an optionArrays is updated. Because swift's array is not reference typr it is value typr so apples will copied to an optionArrays array, so optionArrays will update separately. and also apples will update separately.

So apples.count will be 0

Now try update apple array values apples.append("Golden Delicious") apples.count // 1

But now optionArrays value remain same as before

print(optionArrays)

["apple": ["Macintosh"], "orange": [], "banana": []] optionArrays["apple"]

print(apples)

["Golden Delicious"]

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Swift Unable to Append String to an Array looked up with a dictionary

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It like that because Swift array is value type not reference type

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It like that because Swift array is value type not reference type

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