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How to convert a multidimensional array to a single-dimensional array

Swift version: 5.4

Paul Hudson    @twostraws   

If you have an array of arrays – for example, an array of an array of integers – you can convert it to a single, flat array by using the joined() method. Because Swift sends back an optimized type (FlattenSequence<[[YourType]]>, in this case), you might also want to add an array conversion for easier use.

For example, here’s an array of arrays of strings, such as you might find if you had one array for each class of students, grouped into a larger array to represent the whole school:

let classes = [
    ["Ash", "Brock", "Misty"],
    ["Gloria", "Piper", "Suzanne"],
    ["Picard", "Riker", "Troi"]
]

We can use joined() to get a single array of names like this:

let allStudents = classes.joined()

As noted, that will make allStudents an instance of FlattenSequence<[[String]]> rather than an array, so if you need to send it back from a method or want to subscript it you should convert it to an array first, like this:

let allStudents = Array(classes.joined())

That will set allStudents to an array containing Ash, Brock, Misty, Gloria, Piper, Suzanne, Picard, Riker, and Troi.

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