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Why would you want to create an empty collection?

Paul Hudson @twostraws May 28th 2020

Updated for Xcode 14.2

When you’re just learning Swift, it’s common to see examples such as this one:

let names = ["Eleanor", "Chidi", "Tahani", "Jianyu", "Michael", "Janet"]

That’s a constant array of six strings, and because it’s constant it means we can’t add more things to the array – we know all our items when the array is created, so the rest of our program is just using that fixed data.

But sometimes you don’t know all your data up front, and in those cases it’s more common to create an empty collection then add things as you calculate your data.

For example, we have our fixed names array above, and if we wanted to figure out which names started with J then we would:

Creating an empty string array called something like jNames
Go over every name in the original names array and check whether it starts with “J”
If it does, add it to the jNames array.

When we’ve gone over all the names, we’ll end up with two strings in jNames: Jianyu and Janet. Of course, if our check was which names started with “X” then we’d end up with no names in the array – and that’s OK. It started as empty and ended as empty.

Later on in this book we’ll be looking at the Swift code needed to make all this work for real.