Swift: dictionary from array of keys and values

You can get your collection in subsequences of two and reduce them into a dictionary:

extension Collection {
    func unfoldSubSequences(limitedTo maxLength: Int) -> UnfoldSequence<SubSequence,Index> {
        sequence(state: startIndex) { start in
            guard start < self.endIndex else { return nil }
            let end = self.index(start, offsetBy: maxLength, limitedBy: self.endIndex) ?? self.endIndex
            defer { start = end }
            return self[start..<end]
        }
    }
}

let kvs = ["key1", "value1", "key2", "value2"]

let dictionary = kvs
    .unfoldSubSequences(limitedTo: 2)
    .reduce(into: [:]) {
        guard $1.count == 2, let key = $1.first, let value = $1.last else { return }
        $0[key] = value
    }
dictionary  // ["key1": "value1", "key2": "value2"]

Another approach using stride. This assumes your collection has an even number of elements:

let kvs = ["key1", "value1", "key2", "value2"]

let dictionary = stride(from: 0, to: kvs.count, by: 2)
    .reduce(into: [:]) { $0[kvs[$1]] = kvs[$1+1] }
dictionary  // ["key1": "value1", "key2": "value2"]