Why use a wildcard (_) in Swift?

Question

Going through the Apple documentation for Swift, I'm designed a for loop that creates 5 buttons to act as "stars" in a 5-star rating feature.

I noticed that the for loop is constructed as follows:

for _ in 0...<5

And in the explanation for this, Apple mentions that you can use a wildcard _ operator when you don't need to know which iteration of the loop is currently executing.

But what's the upside to not knowing the iteration? Is it a memory saving issue optimization? Is there ever a scenario where you don't want to know the iteration?

Maybe if you just want to run something more than once? (happens sometimes) — gcampbell, Jun 07 '16 at 17:39

score 4 · Answer 1 · answered Jun 07 '16 at 17:45

In general, unused variables add semantic overhead to code. When someone reads your code, they need to make an effort to understand the function of each line and identifier, so that they can accurately anticipate the impact of changing the code. A wildcard operator allows you to communicate that a value that is required by the syntax of the language is not relevant to the code that follows (and is not, in fact, even referenced).

In your specific example, a loop might well need executing a certain number of times, but if you want to do the exact same thing on each iteration, the iteration count is irrelevant. It's not especially common, but it does happen. The semantic overhead in this case is low, but it's meaningful (you're making it clear from the start that you intend to do the same thing every time), and it's a good habit to get in, broadly.

score 3 · Answer 2 · answered Jun 07 '16 at 17:43

The for loop index needs to be stored in memory, so this won't serve as any kind of memory optimization.

I think most importantly it easily conveys to readers of the code that the loop index is not important.

Additionally, it prevents you from having to come up with some arbitrary dummy variable name. It also declutters your name space when you're debugging inside the loop, since you won't be shown this dummy variable.

score 3 · Answer 3 · answered Jun 07 '16 at 17:55

3

Is there ever a scenario where you don't want to know the iteration?

Certainly. Consider:

func echo(_ s:String, times:Int) -> String {
    var result = ""
    for _ in 1...times { result += s }
    return result
}

How would you write it?

answered Jun 07 '16 at 17:55

matt

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`Repeat(count: times, repeatedValue: s).joinWithSeparator("")` (SCNR :) – Martin R Jun 07 '16 at 18:28
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@MartinR Sssssssh! :) – matt Jun 07 '16 at 18:44
How about `(0.. – vacawama Jun 07 '16 at 20:37
@vacawama Reminds me of how I used to use `reduce` before `flatten` existed! – matt Jun 07 '16 at 20:54
@matt, why the `_` for the first parameter? Isn't that the default behavior? – vacawama Jun 07 '16 at 20:58
@vacawama Sorry, I'm using Swift 3 already. – matt Jun 07 '16 at 21:21
I should have guessed. Swift, the moving target. :-) – vacawama Jun 07 '16 at 21:46
@vacawama If you haven't been following along, you are about to be _amazed_ at just how moving a target it will be! – matt Jun 07 '16 at 21:58

score 1 · Answer 4 · edited May 23 '17 at 11:53

1

This is not necessarily about knowing the iteration. When you don't need to know the element inside the loop you are iterating. The underscore is that placeholder - just as the underscore is the symbol in the declaration suppressing externalization of the name. Check this SO Thread for more detail about this.

edited May 23 '17 at 11:53

Community

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answered Jun 07 '16 at 17:46

Jeet

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Thanks for the link to the other thread. – Dupontrocks11 Jun 07 '16 at 17:50

Why use a wildcard (_) in Swift?

4 Answers4