Swift is what’s known as a type-safe language, which means that every variable must be of one specific type. The str variable that Xcode created for us holds a string of letters that spell “Hello, playground”, so Swift assigns it the type String.
On the other hand, if we want to store someone’s age, we might make a variable like this:
var age = 38That holds a whole number, so Swift assigns the type Int – short for “integer”.
If you have large numbers, Swift lets you use underscores as thousands separators – they don’t change the number, but they do make it easier to read. For example:
var population = 8_000_000Strings and integers are different types, and they can’t be mixed. So, while it’s safe to change str to “Goodbye”, I can’t make it 38 because that’s an Int not a String.