Updated for Xcode 13.2
Both sets and arrays are important in Swift, and understanding what their differences are will help you understand which one to choose for any given circumstance.
Both sets and arrays are collections of data, meaning that they hold multiple values inside a single variable. However, how they hold their values is what matters: sets are unordered and cannot contain duplicates, whereas arrays retain their order and can contain duplicates.
Those two differences might seem small, but they have an interesting side effect: because sets don’t need to store your objects in the order you add them, they can instead store them in a seemingly random order that optimizes them for fast retrieval. So, when you say “does this set contain item X,” you’ll get an answer in a split second no matter how big the set is.
In comparison, arrays must store their items in the order you give them, so to check whether item X exists in an array containing 10,000 items Swift needs to start at the first item and check every single item until it’s found – or perhaps it isn’t found at all.
This difference means that sets are more useful for times when you want to say “does this item exist?” For example, if you want to check whether a word appears in a dictionary, you should use a set: you don’t have duplicates, and you want to do a fast look up.
For more information on this topic, check out Antoine van der Lee’s post: https://www.avanderlee.com/swift/array-vs-set-differences-explained/
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