Find if sequence of elements exists in array - arrays

Question

Is it possible to find if a sequence of elements in an array exists?
Lets take some digits from the Pi,
let piDigits=[3,1,4,1,5,9,2,6,5,3,5,8,9,7,9,3,2,3,8,4,6,2,6,4,3,3,8,3,2,7,9,5,0,2,8,8,4,1,9,7,1,6,9,3,9,9,3,7,5,1,0,5,8,2,0,9,7,4,9,4,4]
Now, i want to find if, 5 and 9 exist as sequence elements in the array- in this case they do, once, in positions 4 & 5.
Ideally, i wouldn't like to iterate over the array with a loop, i would like something similar to array.contains(element) .
#Bawpotter, the code snippet:
for element in piDigits{ //check every element
if element == 5 { //if element is equal with the element i want
var currentPosition = piDigits.index(of: element) //get the position of that element
if piDigits[currentPosition!+1] == 9 { //if the element at the next position is equal to the other element i want
print("true") // it prints true 7 times, instead of 1!
}
}
}

Answer 1

You can filter your indices where its subsequence elementsEqual is true:
extension Collection where Element: Equatable {
func firstIndex<C: Collection>(of collection: C) -> Index? where C.Element == Element {
guard !collection.isEmpty else { return nil }
let size = collection.count
return indices.dropLast(size-1).first {
self[$0..<index($0, offsetBy: size)].elementsEqual(collection)
}
}
func indices<C: Collection>(of collection: C) -> [Index] where C.Element == Element {
guard !collection.isEmpty else { return [] }
let size = collection.count
return indices.dropLast(size-1).filter {
self[$0..<index($0, offsetBy: size)].elementsEqual(collection)
}
}
func range<C: Collection>(of collection: C) -> Range<Index>? where C.Element == Element {
guard !collection.isEmpty else { return nil }
let size = collection.count
var range: Range<Index>!
guard let _ = indices.dropLast(size-1).first(where: {
range = $0..<index($0, offsetBy: size)
return self[range].elementsEqual(collection)
}) else {
return nil
}
return range
}
func ranges<C: Collection>(of collection: C) -> [Range<Index>] where C.Element == Element {
guard !collection.isEmpty else { return [] }
let size = collection.count
return indices.dropLast(size-1).compactMap {
let range = $0..<index($0, offsetBy: size)
return self[range].elementsEqual(collection) ? range : nil
}
}
}
[1, 2, 3, 1, 2].indices(of: [1,2]) // [0,3]
[1, 2, 3, 1, 2].ranges(of: [1,2]) // [[0..<2], [3..<5]]
If you only need to check if a collection contains a subsequence:
extension Collection where Element: Equatable {
func contains<C: Collection>(_ collection: C) -> Bool where C.Element == Element {
guard !collection.isEmpty else { return false }
let size = collection.count
for i in indices.dropLast(size-1) where self[i..<index(i, offsetBy: size)].elementsEqual(collection) {
return true
}
return false
}
}
[1, 2, 3].contains([1, 2]) // true

Answer 2

A very simple implementation using linear search:
let piDigits: [Int] = [3,1,4,1,5,9,2,6,5,3,5,8,9,7,9,3,2,3,8,4,6,2,6,4,3,3,8,3,2,7,9,5,0,2,8,8,4,1,9,7,1,6,9,3,9,9,3,7,5,1,0,5,8,2,0,9,7,4,9,4,4]
let searchedSequence: [Int] = [5, 9]
var index = 0
var resultIndices: [Int] = []
while index < (piDigits.count - searchedSequence.count) {
let subarray = piDigits[index ..< (index + searchedSequence.count)]
if subarray.elementsEqual(searchedSequence) {
resultIndices.append(index)
}
index += 1
}
print("Result: \(resultIndices)")
There are other variants as well, you could, for example, keep dropping the first character from piDigits during iteration and check whether piDigits start with the searchedSequence.
If performance is critical, I recommend using a string searching algorithm, e.g. Aho-Corasick (see https://en.wikipedia.org/wiki/String_searching_algorithm) which builds a state machine first for fast comparison (similar to regular expressions).
Let's see how regular expressions can be used:
let searchedSequences: [[Int]] = [[5, 9], [7], [9, 2]]
let stringDigits = piDigits.map { String($0) }.joined()
let stringSearchedSequences = searchedSequences.map { sequence in sequence.map { String($0) }.joined() }
let regularExpressionPattern = stringSearchedSequences.joined(separator: "|")
let regularExpression = try! NSRegularExpression(pattern: regularExpressionPattern, options: [])
let matches = regularExpression.matches(in: stringDigits, options: [], range: NSRange(location: 0, length: stringDigits.characters.count))
let matchedIndices = matches.map { $0.range.location }
print("Matches: \(matchedIndices)")
The downside of the approach is that it won't search overlapping ranges (e.g. "592" matches two ranges but only one is reported).

Answer 3

Inside the contains method iterates over the array and here you have to do the same thing. Here an example:
extension Array where Element: Equatable {
func contains(array elements: [Element]) -> Int {
guard elements.count > 0 else { return 0 }
guard count > 0 else { return -1 }
var ti = 0
for (index, element) in self.enumerated() {
ti = elements[ti] == element ? ti + 1 : 0
if ti == elements.count {
return index - elements.count + 1
}
}
return -1
}
}
And here how to use it:
let index = [1, 4, 5, 6, 6, 9, 6, 8, 10, 3, 4].contains(array: [6, 8, 10])
// index = 6
let index = [1, 4, 5, 6, 6, 9, 6, 8, 10, 3, 4].contains(array: [6, 8, 1])
// index = -1

Answer 4

let firstSeqNum = 5
let secondSeqNum = 9
for (index, number) in array.enumerated() {
if number == firstSeqNum && array[index+1] == secondSeqNum {
print("The sequence \(firstSeqNum), \(secondSeqNum) was found, starting at an index of \(index).")
}
}
Since there's no built-in method for this, this would be your best option.