What are sets?

Paul Hudson    February 12th 2018    @twostraws

Sets are a type of sequence similar to arrays, except they may not store any item more than once and are unordered. This eliminates them from many places where you would use an array, but they do have one special super-power: it’s extremely fast to check whether a set contains a specific value. In fact, sets perform this operation at the same speed whether they contain 10 items or 10,000 items.

Sets can be created directly from arrays, then used like them in many ways. For example:

var numbers = Set([1, 2, 3])

You can then go ahead and check whether that contains a specific value, like this:

print(numbers.contains(5))

Because sets don’t have any concept of ordering, they don’t have an append() method. Instead, they use insert(), like this:

numbers.insert(5)

Available from iOS – learn more in my book Pro Swift

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