Questions on FunFacts exercise

Mark Goldenson

This is actually a tad bit more complicated than it seems. I'm also tagging the mods Kevin Lozandier and Ricardo Hill-Henry so that they're aware if this issue comes up again.

Let's start with the first problem. You can't append an item to your array. As Kevin mentioned, the first issue is a syntactical problem where you need to be writing

var factIndexes: [Int] = []

to create the array. Note that the semi colons aren't necessary since they are optional in Swift. But even with the right syntax you still won't be able to add items and this is because the way Swift is designed. (Fair warning: It's about to get computer sciency here). In the Fun Facts app, we're creating FactBook as a structure, or struct. In Swift, structures (and enums) are value types. This is in contrast to a Swift class, which is a reference type.

These two types differ in how data is stored and accessed. In simple terms, a value type copies the data directly while a reference type stores the memory address and uses that to refer to the data. This is also known as deep copy (value type) vs shallow copy (reference type). In the former, the data is safer because it is copied over before use so that if it is changed, the data you are working with remains unchanged.

Because of this, despite creating a mutable array using the keyword var, when the struct is instantiated, it creates an immutable copy instead by deep copying the data. This is why it's telling you that you can't append items. There's two solutions to this issue (neither of which I've covered yet in courses).

• To keep using a struct as your data model but be able to append items or change the data after creation, you need to append the mutating keyword to your methods. The following code will work.

var factIndex: [Int] = []

mutating func randomFact() -> String {
    factIndex.append(1)
    var unsignedArrayCount = UInt32(factsArray.count)
    var unsignedRandomNumber = arc4random_uniform(unsignedArrayCount)
    var randomNumber = Int(unsignedRandomNumber)

    return factsArray[randomNumber]
}

Note to do this, when you instantiate your struct in your view controller, you can't assign it to a constant, it needs to be a variable.

• Use a class instead. Classes are reference types and you won't run into this problem.

Phew! That should help you with your first problem of appending stuff to your index. Problem 2! How to improve your code to make sure facts aren't repeated.

Here's a simpler approach that should work for what you are trying to achieve.

• Create a stored property of type Int to store the previous randomNumber generated
• Every time you generate a random number, check to see if it is the same as the previously generated one.
• If it is, generate another number, otherwise use that one and store it for the next time

Now you ensure that you're not repeating the same random number twice, which would mean the same fact is not displayed twice. Your approach is fine, but using an array makes more sense if you wanted to ensure that a couple facts don't repeat in a row.

I think I covered everything!

Happy learning :D