How to use the zip() function to join two arrays

Swift version: 5.6

Paul Hudson @twostraws May 28th 2019

The zip() function is designed to merge two sequences into a single sequence of tuples. For example, here is an array of wizards:

let wizards1 = ["Harry", "Ron", "Hermione"]

And here’s a matching array of the animals owned by those wizards:

let animals1 = ["Hedwig", "Scabbers", "Crookshanks"]

Using zip() we can combine them together:

let combined1 = zip(wizards1, animals1)

That will produce a single sequence combining the earlier two. To avoid doing extra work, Swift actually creates a special type called Zip2Sequence that stores both sequences internally – this is more efficient than doing the actual joining, but it does make the output harder to read if you’re using a playground. So, if you are using a playground you should wrap the output from zip() into a new array to make its output easier to read:

let combined2 = Array(zip(wizards1, animals1))

If you print combined you’ll see it contains this array:

[("Harry", "Hedwig"), ("Ron", "Scabbers"), ("Hermione", "Crookshanks")]

One of the helpful features of zip() is that if your two arrays differ in size it will automatically choose the shorter of the two. This avoids trying to read two arrays at the same time and accidentally going out of bounds when one is shorter.

For example, this code will print out the animals belonging to the first three wizards, but nothing for Draco because he doesn’t have a matching animal:

let wizards2 = ["Harry", "Ron", "Hermione", "Draco"]
let animals2 = ["Hedwig", "Scabbers", "Crookshanks"]

for (wizard, animal) in zip(wizards2, animals2) {
    print("\(wizard) has \(animal)")
}

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