> On Jul 2, 2016, at 11:40 AM, Jens Alfke via swift-users
> <swift-users@swift.org> wrote:
>
> It’s hard to handle empty collections, otherwise. If endIndex were the last
> valid subscript, then in an empty collection what would its value be?
> Presumably it would have to point to one before the start, which is rather
> weird because that’s never a valid index (in an int-indexed collection it
> would be negative.) Even worse, endIndex is never reachable from startIndex,
> so an iteration can’t simply continue until it reaches the endIndex. Instead
> the Indexable type would have to implement a (cheap, constant-time) >=
> operator to use as the test for an iteration.
To add to Jens's point, all of this adds up to simplify iterating over a
collection with a `while` loop (remember, all loops are `while` loops
eventually):
// Without invalid endIndex:
var index = c.startIndex
var done = c.isEmpty
while !done {
...
if index == c.endIndex {
done = true
}
else {
index = c.index(after: index)
}
}
// With invalid endIndex:
var index = c.startIndex
while index != c.endIndex {
...
index = c.index(after: index)
}
There are even more reasons than this one. For instance, having `endIndex` be
past the end means that `insert(_:at:)` can insert after the last element of
the collection, in addition to before any other element. It gives other
range-based APIs similar powers. It means that `distance(from: startIndex, to:
endIndex)` is the count of the elements in the collection. It means you
natively use `Range` instead of `ClosedRange`, which (perhaps
counterintuitively) is easier to reason about. And so on.
The manuscripts of famed computer scientist Edsger Dijkstra include a little
note about ranges and zero-based indexes; its logic is intimately related to
this: https://www.cs.utexas.edu/users/EWD/transcriptions/EWD08xx/EWD831.html
--
Brent Royal-Gordon
Architechies
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