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How to filter a loop using a where clause

Swift version: 5.1

Paul Hudson    @twostraws   

A regular for-in loop goes over all the items in an array, allowing you to manipulate them as you need. However, sometimes you don’t need all items and instead only want a subset, and in those circumstances the where keyword is useful.

For example, consider this array:

let names = ["Michael Jackson", "Taylor Swift", "Michael Caine", "Adele Adkins", "Michael Jordan"]

A regular for-in loop could print out all those names, but by adding a where clause we could restrict the loop so it operates only on people named Michael:

for name in names where name.hasPrefix("Michael") {
    print(name)
}

If you need multiple conditions in your where clause, join them using &&.

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