Why does IndexSet say that a new element is inserted even though the element is already in the set?

Using Sets, I can conveniently insert an element into the set, and also check whether the element was in the set using one call to insert:

let array = [1,2,3,4,4,2,5,3,6,7,1]  var set = Set<Int>()  for item in array {      // set.insert not only inserts the item, but also tells me whether the item was in set before the insert      if set.insert(item).inserted {          print("Encountered new item: \(item)")      } else {          print("\(item) has already been encountered!")      }  }  

Output:

Encountered new item: 1  Encountered new item: 2  Encountered new item: 3  Encountered new item: 4  4 has already been encountered!  2 has already been encountered!  Encountered new item: 5  3 has already been encountered!  Encountered new item: 6  Encountered new item: 7  1 has already been encountered!  

However, if I rewrite the the same logic using IndexSet:

let array = [1,2,3,4,4,2,5,3,6,7,1]  var set = IndexSet()  for item in array {      // set.insert not only inserts the item, but also tells me whether the item was in set before the insert      if set.insert(item).inserted {          print("Encountered new item: \(item)")      } else {          print("\(item) has already been encountered!")      }  }  

The output becomes:

Encountered new item: 1  Encountered new item: 2  Encountered new item: 3  Encountered new item: 4  Encountered new item: 4  Encountered new item: 2  Encountered new item: 5  Encountered new item: 3  Encountered new item: 6  Encountered new item: 7  Encountered new item: 1  

It appears that the tuple returned by IndexSet.insert always matches (true, _), but even so, the set's count is not increased. I must check set.contains(item) before insert to produce the desired output.

Question: Is this intended behaviour for IndexSet?

I know that IndexSet is a Cocoa API, and not native to Swift, so I thought perhaps there is some special semantics that IndexSet has about what it means for two numbers to be "the same" that I'm not aware of. I also looked for bug reports on bugs.swift.org about IndexSet, but I didn't find anything about insert always returning true.

https://stackoverflow.com/questions/66718320/why-does-indexset-say-that-a-new-element-is-inserted-even-though-the-element-is March 20, 2021 at 12:43PM