In-place collection element removal

Paul Hudson      @twostraws

SE-0197 introduced a new removeAll(where:) method that performs a high-performance, in-place filter for collections. You give it a closure condition to run, and it will strip out all objects that match the condition.

For example, if you have a collection of names and want to remove people called “Terry”, you’d use this:

var pythons = ["John", "Michael", "Graham", "Terry", "Eric", "Terry"]
pythons.removeAll { $0.hasPrefix("Terry") }
print(pythons)

Now, you might very well think that you could accomplish that by using filter() like this:

pythons = pythons.filter { !$0.hasPrefix("Terry") }

However, that doesn’t use memory very efficiently, it specifies what you don’t want rather than what you want, and more advanced in-place solutions come with a range of complexities that are off-putting to novices. Ben Cohen, the author of SE-0197, gave a talk at dotSwift 2018 where he discussed the implementation of this proposal in more detail – if you’re keen to learn why it’s so efficient, you should start there!

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Other changes in Swift 4.2…

• Derived collections of enum cases
• Warning and error diagnostic directives
• Dynamic member look up
• Enhanced conditional conformances
• Random number generation and shuffling
• Simpler, more secure hashing
• Checking sequence elements match a condition
• Boolean toggling

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