Swift: Creating an array of UIImage

Question

Using Swift, I'm trying to create an array of UIImage objects for a simple animation. Contextual help for animationImages reads, "The array must contain UI Image objects."

I've tried to create said array as follows, but can't seem to get the syntax correct:

var logoImages: UIImage[]
logoImages[0] = UIImage(name: "logo.png")

This throws: ! Variable logoImages used before being initialized

Then I tried

var logoImages = []
logoImages[0] = UIImage(named: "logo.png")

Which throws: ! Cannot assign to the result of this expression

I've checked the docs here, but the context isn't the same: https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/CollectionTypes.html

Solution 1

You have two problems (and without a regex!)

1. You aren't creating an array. You need to do:

var logoImages: [UIImage] = []

or

var logoImages: Array<UIImage> = []

or

var logoImages = [UIImage]()

or

var logoImages = Array<UIImage>()

2. If you want to add new objects to an array, you should use Array.append() or some of the equivalent syntactic sugar:

logoImages.append(UIImage(named: "logo.png")!)

or

logoImages += [UIImage(named: "logo.png")!]

or

logoImages += [UIImage(named: "logo.png")!, UIImage(named: "logo2.png")!]

You need to append to the array because (excerpt from docs):

You can’t use subscript syntax to append a new item to the end of an array. If you try to use subscript syntax to retrieve or set a value for an index that is outside of an array’s existing bounds, you will trigger a runtime error. However, you can check that an index is valid before using it, by comparing it to the array’s count property. Except when count is 0 (meaning the array is empty), the largest valid index in an array will always be count - 1, because arrays are indexed from zero.

Of course you could always simplify it when possible:

var logoImage: [UIImage] = [
    UIImage(named: "logo1.png")!,
    UIImage(named: "logo2.png")!
]

edit: Note that UIImage now has a "failable" initializer, meaning that it returns an optional. I've updated all the bits of code to reflect this change as well as changes to the array syntax.

Solution 2

You are declaring the type for logoImages but not creating an instance of that type.

Use var logoImages = UIImage[]() which will create a new array for you.

...and then after creating a new empty Array instance, as described in the answer by @Jiaaro you can't use subscripting to add to an empty array

Solution 3

var image : UIImage = UIImage(named:"logo.png")    
var logoImages = [image]

Comments

[kmiklas]

First, this throws an error, "Constructive statements on a line must be separated by ';'. Xcode wants `UIImage[];()

[kmiklas]

Second, I saw this, but those two parenthesis () scared me off as a function invocation.

[kmiklas]

So, as commented on ackStOverflow's response, UIImage[]() throws a Constructive statements on a line must be separated by a ';' error. Also, those () parenthesis look a lot like a function call. Also this looks a little weird, eh? UIImage[];(). Finally, why do I have to use append; why can't I just assign a value to an index?

[Jiaaro]

@kmiklas Array<UIImage> is a Type, you need to instantiate it (using parens): Array<UIImage>(), alternatively if you specify the type, you can just assign an array literal (which is the first two examples). UIImage[] is just syntactic sugar for Array<UIImage>

[David Berry]

@kmiklas Sounds like you've got something else going on then. The line given by ackSt is correct and shouldn't generate errors for you. Putting a semicolon in where you are is completely changing the meaning.

[Jiaaro]

@kmiklas added excerpt from docs explaining why you need to append

[fqdn]

@kmiklas make sure you are using var logoImages = UIImage[]() which creates a new Array for storing UIImage objects and then assigns it to the logoImages variable (inferring the variable's type from the assignment)... it sounds like you might be incorrectly using a semicolon in place of the equal sign? i.e. var logoImages: UIImage[]() which would generate the type of error you describe in your comment

[kmiklas]

Honestly, I have nothing else going on. It's the first line in the function, and the first syntax option by Jiaaro below works: var logoImages:UIImage[] = []

[fqdn]

...also, those two parentheses are a function invocation :) they are calling the init() method on Array, in the same way you would do it for any other Structure or Class

[John Riselvato]

For some reason the simplified way you have doesn't work any more.

[Jiaaro]

@JohnRiselvato UIImage is now a failable initializer (meaning it returns an optional) - will update answer

[Marcello B.]

@Jiaaro when you try to run this code it breaks with the error code coder : ObjectiveC.NScoder

[Supertecnoboff]

Is there no addObject method anymore?