Strange Behaviour of IndexSet

Using Swift
#1

With this code:

let maxIndex = 4
var indices = IndexSet(integersIn: 0 ... maxIndex)
var curIndex = indices.startIndex
while( curIndex < indices.endIndex)
{
	let curPrix = indices[curIndex]
	print("index \(curPrix); will remove 1, 3")
	indices.remove(1) 
	indices.remove(3) 
	indices.formIndex(after: &curIndex)
}

I would expect to never see indices 1, 3. But I get:

index 0; will remove 1, 3
index 1; will remove 1, 3
index 2; will remove 1, 3
index 3; will remove 1, 3
index 4; will remove 1, 3

Is this the expected behaviour?
If yes, how is this to be understood?

Gerriet.

#2

Calling indices.remove invalidates indices to elements of the collection such as indices.startIndex stored in curIndex. You can't rely on those indices to give you meaningful results after a mutation of the collection.

Some collection types have documented situations where indices are preserved after a mutation, but I don't see anything to that effect in IndexSet.

As for a solution, you could keep the elements to remove in a separate collection and only remove them at the end:

let maxIndex = 4
var indices = IndexSet(integersIn: 0 ... maxIndex)
var curIndex = indices.startIndex
var toRemove = IndexSet()
while( curIndex < indices.endIndex)
{
	let curPrix = indices[curIndex]
	// don't act on elements scheduled to be removed
	if !toRemove.contains(curPrix) {
		print("index \(curPrix); will remove 1, 3")
		toRemove.insert(1)
		toRemove.insert(3)
	}
	indices.formIndex(after: &curIndex)
}
// finally removing what needs to be removed
indices.subtract(toRemove)

Another solution would be to use a for loop where you check if elements were previously removed:

let maxIndex = 4
var indices = IndexSet(integersIn: 0 ... maxIndex)
for curPrix in indices {
	if indices.contains(curPrix) {
		print("index \(curPrix); will remove 1, 3")
		indices.remove(1)
		indices.remove(3)
	} else {
		// ignore: element already removed from indices
	}
}
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