Comparing two arrays of objects in Swift - arrays

Question

I have two arrays of objects with different sizes.
First one with old data, second one with updated data from server (included old data with new), data can be mixed. I want to get difference between these arrays.
My class
class Station {
var dateOfIssue: Date
var region: String
var locality: String
var bsName: String
var freqIn: String
var freqOut: String
var networkType: String
var provider: String
var bsUsableName: String
...
}
Arrays I want to compare (example)
var a = [Station]()
var b = [Station]()
for _ in 0...5 {
a.append(Station(someRandomStationValue...)
}
b = a
for _ in 0...7{
b.append(Station(someRandomStationValue...) //array "b" will contain all that array "a" contains and some new values
}
How to compare these arrays comparing all fields between and get a new array with differences (like in java: b.removeAll(a))?

Answer 1

You can make use of Set which provides in-built .subtract() and .subtracting() methods which removes the common entries inside both the Sets
struct Station: Hashable,CustomStringConvertible {
var id: Int
var region: String
var locality: String
var bsName: String
// Just to provide a pretty print statement
var description: String {
return "ID: \(id) | region: \(region) | locality: \(locality) | bsName: \(bsName)"
}
}
var stations1 = Set<Station>()
var stations2 = Set<Station>()
for currentNumber in 0...3 {
stations1.insert(Station(id: currentNumber, region: "abc", locality: "abc", bsName: "abc"))
}
for currentNumber in 0...5 {
stations2.insert(Station(id: currentNumber, region: "abc", locality: "abc", bsName: "abc"))
}
// Caluculating the difference here
print(stations2.subtracting(stations1))

Answer 2

As pointed out by #koropok, a good solution is using Set. The first step is to conform your type to Hashable. For classes, you'd have to implement == and hash(into:) functions, but if you use struct you don't have to do anything else other than declaring the conformance. So:
struct Station: Hashable {
var dateOfIssue: Date
var region: String
...
}
Now you should be able to add Station into a Set. Thus:
var a = Set<Station>()
for _ in 0...5 {
a.insert(Station(...))
}
var b = Set<Station>()
a.forEach { b.insert($0) }
for _ in 0...7 {
b.insert(Station(...))
}
let c = b.subtracting(a)
Set also provides a handy initializer that you can use to turn your Station array into a set:
let s = Set(arrayLiteral: [your, station, objects])

Answer 3

As mentioned in comments by koropok you may use subtract method:
// Added to make original code functional
// Station must conform to Hashable protocol in order to be stored in the Set
struct Station: Hashable {
let id: Int
}
var a = [Station]()
for i in 0...5 {
a.append(Station(id:i))
}
var b = [Station]()
for i in 0...7{
//array "b" will contain all that array "a" contains and some new values
b.append(Station(id:i))
}
var c = Set(b)
// c will contain 6 and 7
c.subtract(a)