Type Int does not conform to protocol sequence

This maybe a little late but you could have done:

for number in numbers { }

instead of:

for number in numbers.count - 1 { }

For a for loop to work a sequence (range) is needed. A sequence consists of a stating a value, an ending value and everything in between. This means that a for loop can be told to loop through a range with ether

for number in 0...numbers.count-1 { }   `or`   for number in numbers { } 

Both example give the nesasery sequences. Where as:

 for number in numbers.count - 1 { }

Only gives one value that could either be the starting or the ending value, making it impossible to work out how many time the for loop will have to run.

For more information see Apple's swift control flow documnetation

It may be swift. You can use this iteration.

for number in 0..<(numbers.count-1)

The error is because Int is not a Sequence. You can create a range as already suggested, which does conform to a sequence and will allow iteration using for in.

One way to make Int conform to a sequence is:

extension Int: Sequence {
    public func makeIterator() -> CountableRange<Int>.Iterator {
        return (0..<self).makeIterator()
    }
}

Which would then allow using it as a sequence with for in.

for i in 5 {
    print(i)
}

but I wouldn't recommend doing this. It's only to demonstrate the power of protocols but would probably be confusing in an actual codebase.

From you example, it looks like you are trying to compare consecutive elements of the collection. A custom iterator can do just that while keeping the code fairly readable:

public struct ConsecutiveSequence<T: IteratorProtocol>: IteratorProtocol, Sequence {
    private var base: T
    private var index: Int
    private var previous: T.Element?

    init(_ base: T) {
        self.base = base
        self.index = 0
    }

    public typealias Element = (T.Element, T.Element)

    public mutating func next() -> Element? {
        guard let first = previous ?? base.next(), let second = base.next() else {
            return nil
        }

        previous = second

        return (first, second)
    }
}

extension Sequence {
    public func makeConsecutiveIterator() -> ConsecutiveSequence<Self.Iterator> {
        return ConsecutiveSequence(self.makeIterator())
    }
}

which can be used as:

for (x, y) in [1,2,3,4].makeConsecutiveIterator() {
    if (x < y) {
        print(x)
    }
}

In the above example, the iterator will go over the following pairs:

(1, 2)
(2, 3)
(3, 4)