Compare 2 arrays and list the differences - Swift - arrays

Question

I was wondering how one would go about comparing 2 boolean arrays and listing the non matching booleans.
I have written up a simple example of 2 arrays.
let array1 = [true, false, true, false]
let array2 = [true, true, true, true]
How would I compare array1 & array2 and display the non matching. I am trying to do this to check user results for a quiz game.
Thanks!

Answer 1

Here's one implementation, but whether it is the one you are after is completely impossible to say, because you have not specified what you think the answer should be:
let answer = zip(array1, array2).map {$0.0 == $0.1}
That gives you a list of Bool values, true if the answer matches the right answer, false if it does not.
But let's say what you wanted was a list of the indexes of those answers that are correct. Then you could say:
let answer = zip(array1, array2).enumerated().filter() {
$1.0 == $1.1
}.map{$0.0}
If you want a list of the indexes of those answers that are not correct, just change == to !=.

Answer 2

Xcode 11 has it supported (only available in iOS 13 or newer)
https://developer.apple.com/documentation/swift/array/3200716-difference
let oldNames = ["a", "b", "c", "d"]
let newNames = ["a", "b", "d", "e"]
let difference = newNames.difference(from: oldNames)
for change in difference {
switch change {
case let .remove(offset, oldElement, _):
print("remove:", offset, oldElement)
case let .insert(offset, newElement, _):
print("insert:", offset, newElement)
}
}
Output
remove: 2 c
insert: 3 e

Answer 3

the easiest thing to do is use a Set. Sets have a symmetricDifference() method that does exactly this, so you just need to convert both arrays to a set, then convert the result back into an array.
Here’s an extension to make it easier:
extension Array where Element: Hashable {
func difference(from other: [Element]) -> [Element] {
let thisSet = Set(self)
let otherSet = Set(other)
return Array(thisSet.symmetricDifference(otherSet))
} }
And here’s some example code you can use to try it out:
let names1 = ["student", "class", "teacher"]
let names2 = ["class", "teacher", "classroom"]
let difference = names1.difference(from: names2)
That will set difference to be ["student", "classroom"], because those are the two names that only appear once in either array.

Answer 4

I found this post looking for a solution to situation very similar to the one posted. However, I found ordered collection diffing using inferringMoves() introduced in Swift 5.1 to be suitable for my use case. In case someone else may find it useful I've added what I used:
let array1 = [true, false, true, true, false, true]
let array2 = [true, true, true, true, true, false]
let diff = array2.difference(from: array1).inferringMoves()
for change in diff {
switch change {
case .insert(let offset, let element, _):
print("insert offset \(offset) for element \(element)")
case .remove(let offset, let element, _):
print("remove offset \(offset) for element \(element)")
}
}
This prints out the values:
remove offset 4 for element false
remove offset 1 for element false
insert offset 4 for element true
insert offset 5 for element false

Answer 5

Ported from the Swift source code, works with Swift5, iOS 12
usage:
let difference = newArray.differenceFrom(from: originalArray) { (s1: String, s2: String) in
return s1 == s2
}
// or
let difference = Array<String>.difference(from: originalArray, to: newArray) { (s1: String, s2: String) in
return s1 == s2
}
for change in diff {
switch change {
case let .remove(offset, oldElement, _):
print("remove:", offset, oldElement)
case let .insert(offset, newElement, _):
print("insert:", offset, newElement)
}
}
Source
Array Extension
extension Array {
public func differenceFrom<C: BidirectionalCollection>(
from other: C,
by areEquivalent: (C.Element, Element) -> Bool
) -> CollectionDifference<Element>
where C.Element == Self.Element {
return Array<Element>.difference(from: other, to: self, using: areEquivalent)
}
public static func difference<C, D>(
from old: C, to new: D,
using cmp: (C.Element, D.Element) -> Bool
) -> CollectionDifference<C.Element>
where
C: BidirectionalCollection,
D: BidirectionalCollection,
C.Element == D.Element {
// Core implementation of the algorithm described at http://www.xmailserver.org/diff2.pdf
// Variable names match those used in the paper as closely as possible
func _descent(from a: UnsafeBufferPointer<C.Element>, to b: UnsafeBufferPointer<D.Element>) -> [_V] {
let n = a.count
let m = b.count
let max = n + m
var result = [_V]()
var v = _V(maxIndex: 1)
v[1] = 0
var x = 0
var y = 0
iterator: for d in 0...max {
let prev_v = v
result.append(v)
v = _V(maxIndex: d)
// The code in this loop is _very_ hot—the loop bounds increases in terms
// of the iterator of the outer loop!
for k in stride(from: -d, through: d, by: 2) {
if k == -d {
x = prev_v[k &+ 1]
} else {
let km = prev_v[k &- 1]
if k != d {
let kp = prev_v[k &+ 1]
if km < kp {
x = kp
} else {
x = km &+ 1
}
} else {
x = km &+ 1
}
}
y = x &- k
while x < n && y < m {
if !cmp(a[x], b[y]) {
break;
}
x &+= 1
y &+= 1
}
v[k] = x
if x >= n && y >= m {
break iterator
}
}
if x >= n && y >= m {
break
}
}
//_internalInvariant(x >= n && y >= m)
return result
}
// Backtrack through the trace generated by the Myers descent to produce the changes that make up the diff
func _formChanges(
from a: UnsafeBufferPointer<C.Element>,
to b: UnsafeBufferPointer<C.Element>,
using trace: [_V]
) -> [CollectionDifference<C.Element>.Change] {
var changes = [CollectionDifference<C.Element>.Change]()
changes.reserveCapacity(trace.count)
var x = a.count
var y = b.count
for d in stride(from: trace.count &- 1, to: 0, by: -1) {
let v = trace[d]
let k = x &- y
let prev_k = (k == -d || (k != d && v[k &- 1] < v[k &+ 1])) ? k &+ 1 : k &- 1
let prev_x = v[prev_k]
let prev_y = prev_x &- prev_k
while x > prev_x && y > prev_y {
// No change at this position.
x &-= 1
y &-= 1
}
//_internalInvariant((x == prev_x && y > prev_y) || (y == prev_y && x > prev_x))
if y != prev_y {
changes.append(.insert(offset: prev_y, element: b[prev_y], associatedWith: nil))
} else {
changes.append(.remove(offset: prev_x, element: a[prev_x], associatedWith: nil))
}
x = prev_x
y = prev_y
}
return changes
}
/* Splatting the collections into contiguous storage has two advantages:
*
* 1) Subscript access is much faster
* 2) Subscript index becomes Int, matching the iterator types in the algorithm
*
* Combined, these effects dramatically improves performance when
* collections differ significantly, without unduly degrading runtime when
* the parameters are very similar.
*
* In terms of memory use, the linear cost of creating a ContiguousArray (when
* necessary) is significantly less than the worst-case n² memory use of the
* descent algorithm.
*/
func _withContiguousStorage<C: Collection, R>(
for values: C,
_ body: (UnsafeBufferPointer<C.Element>) throws -> R
) rethrows -> R {
if let result = try values.withContiguousStorageIfAvailable(body) {
return result
}
let array = ContiguousArray(values)
return try array.withUnsafeBufferPointer(body)
}
return _withContiguousStorage(for: old) { a in
return _withContiguousStorage(for: new) { b in
return CollectionDifference(_formChanges(from: a, to: b, using: _descent(from: a, to: b)))!
}
}
}
}
CollectionDifference
public struct CollectionDifference<ChangeElement> {
/// A single change to a collection.
#frozen
public enum Change {
/// An insertion.
///
/// The `offset` value is the offset of the inserted element in the final
/// state of the collection after the difference is fully applied.
/// A non-`nil` `associatedWith` value is the offset of the complementary
/// change.
case insert(offset: Int, element: ChangeElement, associatedWith: Int?)
/// A removal.
///
/// The `offset` value is the offset of the element to be removed in the
/// original state of the collection. A non-`nil` `associatedWith` value is
/// the offset of the complementary change.
case remove(offset: Int, element: ChangeElement, associatedWith: Int?)
// Internal common field accessors
internal var _offset: Int {
get {
switch self {
case .insert(offset: let o, element: _, associatedWith: _):
return o
case .remove(offset: let o, element: _, associatedWith: _):
return o
}
}
}
internal var _element: ChangeElement {
get {
switch self {
case .insert(offset: _, element: let e, associatedWith: _):
return e
case .remove(offset: _, element: let e, associatedWith: _):
return e
}
}
}
internal var _associatedOffset: Int? {
get {
switch self {
case .insert(offset: _, element: _, associatedWith: let o):
return o
case .remove(offset: _, element: _, associatedWith: let o):
return o
}
}
}
}
/// The insertions contained by this difference, from lowest offset to
/// highest.
public let insertions: [Change]
/// The removals contained by this difference, from lowest offset to highest.
public let removals: [Change]
/// The public initializer calls this function to ensure that its parameter
/// meets the conditions set in its documentation.
///
/// - Parameter changes: a collection of `CollectionDifference.Change`
/// instances intended to represent a valid state transition for
/// `CollectionDifference`.
///
/// - Returns: whether the parameter meets the following criteria:
///
/// 1. All insertion offsets are unique
/// 2. All removal offsets are unique
/// 3. All associations between insertions and removals are symmetric
///
/// Complexity: O(`changes.count`)
private static func _validateChanges<Changes: Collection>(
_ changes: Changes
) -> Bool where Changes.Element == Change {
if changes.isEmpty {
return true
}
var insertAssocToOffset = Dictionary<Int, Int>()
var removeOffsetToAssoc = Dictionary<Int, Int>()
var insertOffset = Set<Int>()
var removeOffset = Set<Int>()
for change in changes {
let offset = change._offset
if offset < 0 {
return false
}
switch change {
case .remove(_, _, _):
if removeOffset.contains(offset) {
return false
}
removeOffset.insert(offset)
case .insert(_, _, _):
if insertOffset.contains(offset) {
return false
}
insertOffset.insert(offset)
}
if let assoc = change._associatedOffset {
if assoc < 0 {
return false
}
switch change {
case .remove(_, _, _):
if removeOffsetToAssoc[offset] != nil {
return false
}
removeOffsetToAssoc[offset] = assoc
case .insert(_, _, _):
if insertAssocToOffset[assoc] != nil {
return false
}
insertAssocToOffset[assoc] = offset
}
}
}
return removeOffsetToAssoc == insertAssocToOffset
}
/// Creates a new collection difference from a collection of changes.
///
/// To find the difference between two collections, use the
/// `difference(from:)` method declared on the `BidirectionalCollection`
/// protocol.
///
/// The collection of changes passed as `changes` must meet these
/// requirements:
///
/// - All insertion offsets are unique
/// - All removal offsets are unique
/// - All associations between insertions and removals are symmetric
///
/// - Parameter changes: A collection of changes that represent a transition
/// between two states.
///
/// - Complexity: O(*n* * log(*n*)), where *n* is the length of the
/// parameter.
public init?<Changes: Collection>(
_ changes: Changes
) where Changes.Element == Change {
guard CollectionDifference<ChangeElement>._validateChanges(changes) else {
return nil
}
self.init(_validatedChanges: changes)
}
/// Internal initializer for use by algorithms that cannot produce invalid
/// collections of changes. These include the Myers' diff algorithm,
/// self.inverse(), and the move inferencer.
///
/// If parameter validity cannot be guaranteed by the caller then
/// `CollectionDifference.init?(_:)` should be used instead.
///
/// - Parameter c: A valid collection of changes that represent a transition
/// between two states.
///
/// - Complexity: O(*n* * log(*n*)), where *n* is the length of the
/// parameter.
internal init<Changes: Collection>(
_validatedChanges changes: Changes
) where Changes.Element == Change {
let sortedChanges = changes.sorted { (a, b) -> Bool in
switch (a, b) {
case (.remove(_, _, _), .insert(_, _, _)):
return true
case (.insert(_, _, _), .remove(_, _, _)):
return false
default:
return a._offset < b._offset
}
}
// Find first insertion via binary search
let firstInsertIndex: Int
if sortedChanges.isEmpty {
firstInsertIndex = 0
} else {
var range = 0...sortedChanges.count
while range.lowerBound != range.upperBound {
let i = (range.lowerBound + range.upperBound) / 2
switch sortedChanges[i] {
case .insert(_, _, _):
range = range.lowerBound...i
case .remove(_, _, _):
range = (i + 1)...range.upperBound
}
}
firstInsertIndex = range.lowerBound
}
removals = Array(sortedChanges[0..<firstInsertIndex])
insertions = Array(sortedChanges[firstInsertIndex..<sortedChanges.count])
}
public func inverse() -> Self {
return CollectionDifference(_validatedChanges: self.map { c in
switch c {
case .remove(let o, let e, let a):
return .insert(offset: o, element: e, associatedWith: a)
case .insert(let o, let e, let a):
return .remove(offset: o, element: e, associatedWith: a)
}
})
}
}
CollectionDifference Extension
extension CollectionDifference: Collection {
public typealias Element = Change
/// The position of a collection difference.
#frozen
public struct Index {
// Opaque index type is isomorphic to Int
#usableFromInline
internal let _offset: Int
internal init(_offset offset: Int) {
_offset = offset
}
}
public var startIndex: Index {
return Index(_offset: 0)
}
public var endIndex: Index {
return Index(_offset: removals.count + insertions.count)
}
public func index(after index: Index) -> Index {
return Index(_offset: index._offset + 1)
}
public subscript(position: Index) -> Element {
if position._offset < removals.count {
return removals[removals.count - (position._offset + 1)]
}
return insertions[position._offset - removals.count]
}
public func index(before index: Index) -> Index {
return Index(_offset: index._offset - 1)
}
public func formIndex(_ index: inout Index, offsetBy distance: Int) {
index = Index(_offset: index._offset + distance)
}
public func distance(from start: Index, to end: Index) -> Int {
return end._offset - start._offset
}
}
extension CollectionDifference.Index: Equatable {
#inlinable
public static func ==(
lhs: CollectionDifference.Index,
rhs: CollectionDifference.Index
) -> Bool {
return lhs._offset == rhs._offset
}
}
extension CollectionDifference.Index: Comparable {
#inlinable
public static func <(
lhs: CollectionDifference.Index,
rhs: CollectionDifference.Index
) -> Bool {
return lhs._offset < rhs._offset
}
}
extension CollectionDifference.Index: Hashable {
#inlinable
public func hash(into hasher: inout Hasher) {
hasher.combine(_offset)
}
}
extension CollectionDifference.Change: Equatable where ChangeElement: Equatable {}
extension CollectionDifference: Equatable where ChangeElement: Equatable {}
extension CollectionDifference.Change: Hashable where ChangeElement: Hashable {}
extension CollectionDifference: Hashable where ChangeElement: Hashable {}
extension CollectionDifference where ChangeElement: Hashable {
/// Returns a new collection difference with associations between individual
/// elements that have been removed and inserted only once.
///
/// - Returns: A collection difference with all possible moves inferred.
///
/// - Complexity: O(*n*) where *n* is the number of collection differences.
public func inferringMoves() -> CollectionDifference<ChangeElement> {
let uniqueRemovals: [ChangeElement: Int?] = {
var result = [ChangeElement: Int?](minimumCapacity: Swift.min(removals.count, insertions.count))
for removal in removals {
let element = removal._element
if result[element] != .none {
result[element] = .some(.none)
} else {
result[element] = .some(removal._offset)
}
}
return result.filter { (_, v) -> Bool in
v != .none
}
}()
let uniqueInsertions: [ChangeElement: Int?] = {
var result = [ChangeElement: Int?](minimumCapacity: Swift.min(removals.count, insertions.count))
for insertion in insertions {
let element = insertion._element
if result[element] != .none {
result[element] = .some(.none)
} else {
result[element] = .some(insertion._offset)
}
}
return result.filter { (_, v) -> Bool in
v != .none
}
}()
return CollectionDifference(_validatedChanges: map({ (change: Change) -> Change in
switch change {
case .remove(offset: let offset, element: let element, associatedWith: _):
if uniqueRemovals[element] == nil {
return change
}
if let assoc = uniqueInsertions[element] {
return .remove(offset: offset, element: element, associatedWith: assoc)
}
case .insert(offset: let offset, element: let element, associatedWith: _):
if uniqueInsertions[element] == nil {
return change
}
if let assoc = uniqueRemovals[element] {
return .insert(offset: offset, element: element, associatedWith: assoc)
}
}
return change
}))
}
}