In Swift, we can assign a value to a variable if it doesn't exist using the nil-coalescing operator (??). Here's an example:
var value: Int? // the value variable does not have a value assigned to it yet
let defaultValue = 0 // a default value we can assign if value is nil
value = value ?? defaultValue // if value is nil, assign defaultValue to it
print(value) // output: 0 since value was previously nil and defaultValue was assigned to it
In this example, we declare a variable value without assigning a value to it. Then we define a defaultValue that we want to assign to value if it is nil. The line value = value ?? defaultValue uses the nil-coalescing operator to assign defaultValue to value if value is nil. Finally, we print out value to ensure it has been assigned the correct value.
Keep in mind that ?? can only be used with optionals. If you try to apply it to a non-optional variable, you'll get a compiler error.