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

Answer 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.

Answer 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