I recently asked the following quiz question on this list, as well as on social media.

What does the following code print?

var hmm = [...]int{1, 2, 3, 12: 9, 8: 3}

fmt.Println(len(hmm))

On social media, I provided the following options:

• 5
• 7
• 13
• Doesn’t compile

And now the results are in! I got a total of 428 responses across LinkedIn, Twitter, and Mastodon:

The good news is: The majority are right! 13 is the correct answer. But why?

That’s what I’ll explain today.

(As an aside, the second most popular answer, Doesn’t compile is how I probably would have answered a week ago, too.)

First off, let’s examine [...]. This should be familiar to anyone who’s a regular reader of my daily list, as we just discussed this on Monday. But here’s a refresher:

The notation ... specifies an array length equal to the maximum element index plus one.
— The Go Spec

So from this we can deduce that we’re defining a variable of array type (not a slice, as was often assumed in some of the comments I received). This means it will have a fixed length, and from the quote above, that lenght is defined as equal to the maximum element index, plus one.

Which takes us to the next potentially confusing bit:

This code snippet mixes keyed and unkeyed elements in the declaration. The first three elements, 1, 2, 3 are pretty straight forward, and probably what you’re accustomed to. The next two elements 12: 9, 8: 3 are keyed. Specifically, it’s saying that value 9 should go to index 12, and value 3 should go to index 8.

From this, we can conclude that the maximum element index is 12. Therefore, the length is that, plus one, or 13. Tada! 🎉

If we print the contents of the hmm variable, we’ll also see a bit more about what actually happened. We created a sparsely-populated array:

[1 2 3 0 0 0 0 0 3 0 0 0 9]

Indexes 3, 4, 5, 6, 7, 9, 10, and 11 are unset, so default to the zero value (or 0 in this case).