How to find the longest initial sequence in an array

Swift version: 5.0

Paul Hudson    @twostraws    March 28th 2019

One of Swift’s lesser-known functions is prefix(while:): call this on an array along with a test to apply, and it will return as many items from the start of the array as it can, stopping only when it reaches the first element that fails your test.

For example, if we had an array of test scores:

let scores = [89, 86, 96, 78, 92, 100]

We could use prefix(while:) to return all scores over 85 that occurred before the first score below 85 – i.e., find me all the passing scores that took place before the first person failed.

Here’s that in Swift:

let initialPasses1 = scores.prefix { $0 > 85 }

You can apply any test you want, and sometimes you’ll get back an empty array if the very first element fails your test. For example, this will return an empty array:

let initialPasses2 = scores.prefix { $0 < 85 }

Available from iOS 8.0

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