JuniorTheoryCommon

What does the program print?

SWIFT

let pack = [4, 7, 4, 9, 7]
var album = Set(pack)
album.insert(9)
album.insert(12)
print(pack.count, album.count)

5 4

Expecting 5 5: thinking insert always adds, like append. A set skips a value it already holds, so inserting 9 again changes nothing.
Expecting 5 2: reading "no repeats" as "appears once". Set(pack) keeps every distinct value once, so 4 and 7 stay too.

JuniorTheoryCommon

SWIFT

let pack = [4, 7, 4, 9, 7]
var album = Set(pack)
album.insert(9)
album.insert(12)
print(pack.count, album.count)

5 4

Expecting 5 5: thinking insert always adds, like append. A set skips a value it already holds, so inserting 9 again changes nothing.
Expecting 5 2: reading "no repeats" as "appears once". Set(pack) keeps every distinct value once, so 4 and 7 stay too.