Ternary Conditional Operator: Why do I need to use parentheses?

Using Swift
1

I wrote the following code in Swift Playgrounds:

var teen: String
myAge > 12 && myAge < 19 ? teen = "You're a Teen" : teen = "You're not a teen"

Upon running my code, I got an error message stating "result of conditional operator '?:' is never mutable.

I tried searching online for a solution and found someone with a similar issue who said he resolved this by placing each result in parentheses, so I did the same and it worked.

var teen: String
myAge > 12 && myAge < 19 ? (teen = "You're a Teen") : (teen = "You're not a teen")

Now when I clicked "run code" it worked just fine. However, I'd like to know WHY this worked. Why do I have to place each result of the condition in parentheses? Please help. Thank you.

2

It's because of the way the operator precendese rules work out with what you've written. I'll simplify this by replacing myAge > 12 && myAge < 19 with just b. Your code is parsed like so:

(b ? teen = "You're a Teen" : teen) = "You're not a teen"

Which clearly doesn't make sense.

By adding parenthesis, you've forced a particular parsing that actually makes the code valid.

However, there's an even simply, clear way to do this:

  1. There's no need for teen to be mutable
  2. There's no need for teen to be declared and assigned separately
  3. When you're using a conditional operator, it's often nice to group up a complex predicate in parenthesis, but that's a matter of personal style.

Here's how you might improve this:

let teen = (myAge > 12 && myAge < 19) ? "You're a Teen" : "You're not a teen"

I would go further and improve the naming:

let isTeen = (13...19).contains(myAge)
let teenStatusMessage = isTeen ? "You're a Teen" : "You're not a teen"
4 Likes
3

Oh and by the way, you have a bug in your original snippet :smiley:

myAge < 19 should read myAge <= 19, because nineteen year-olds are teens, too.

1 Like
4

Thank you, I understand this now (though I don't know why it would be parsed so strangely). :frowning: And you're right, 19-year olds ARE teens too! :slight_smile:

5

I can most easily explain it by analogy. Imagine you're writing a math expression like:

1 + 2 * 3

and was wondering why the result was 7 instead of 9. Of course, that's because it evaluates as 1 + (2 * 3), whereas you intended (1 + 2) * 3. Then you might ask: "Why does it parse so strangely?". Precedence are man-made, and one could imagine an alternative convention on mathematical notation where that + takes precedence over *. It would work the way you wanted it to. But one can also imagine another post saying: "Why does it evaluate to 9, I wanted 7!"

Operator precedence rules are kind of like which side of the road a country drives on. It doesn't really matter one way or another, but there needs to be a consistent standard or else chaos ensues.

2 Likes
6

Okay, got it, thank you for your help! :slight_smile: