How to compare objects in one array

Question

I have an array of objects and I want to find the duplicates. I'm comparing longitude/langitude and unfortunately the values aren't equal exactly.

How can I find the duplicates?

My example code:

var locations = [("Location_A", 49.5858, 9.123456), ("Location_B", 49.5858, 9.123456), ("Location_A", 49.5855, 9.123450), ...]

for location in locations {
    //Find duplicate based on longitude and latitude where values < 0.0004
}

In that case locations[0] and locations[2] should be detected as duplicate.

Thanks in advance!

matt · Accepted Answer · 2018-06-08 18:48:53Z

3

Instead of tuples, use a custom struct. Now you can make that struct Equatable, defining == for that struct in such a way as to allow for your epsilon value:

struct Loc : Equatable {
    let name : String
    let latitude : Double
    let longitude : Double
    static let epsilon = 0.0004
    static func ==(lhs:Loc, rhs:Loc) -> Bool {
        if lhs.name != rhs.name { return false }
        if abs(lhs.latitude - rhs.latitude) > epsilon { return false }
        if abs(lhs.longitude - rhs.longitude) > epsilon { return false }
        return true
    }
}

Let's test it:

let loc1 = Loc(name: "Location_A", latitude: 49.5858, longitude: 9.123456)
let loc2 = Loc(name: "Location_A", latitude: 49.5855, longitude: 9.123450)
print(loc1 == loc2) // true

At that point, the well-established techniques for eliminating duplicates will spring to life.

edited Jun 8, 2018 at 18:48

answered Jun 8, 2018 at 18:34

matt

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2 Comments

Jayesh Thanki Over a year ago

Hello @matt, can we override generic operator for all other struct/class? For example override operation fetch all properties of struct/class it self and compare all value.

AiiDee Over a year ago

Thank you very much! This is a very good solution and it works for me!

Sulthan · Accepted Answer · 2018-06-08 18:48:33Z

Tuples are not easy to work with, I recommend wrapping your data into a custom object first:

import CoreLocation

struct MyLocation: Hashable {
    let name: String
    let coordinate: CLLocation

    init(tuple: (String, Double, Double)) {
        name = tuple.0
        coordinate = CLLocation(latitude: tuple.1, longitude: tuple.2)
    }

    public static func == (lhs: MyLocation, rhs: MyLocation) -> Bool {
        return
            lhs.name == rhs.name
            && lhs.coordinate.distance(from: rhs.coordinate) < 1
    }

    public var hashValue: Int {
        return name.hashValue
    }
}

As you can see, I have already declared Equatable and also Hashable for easy indexing.

Then we can use a simple Array extension:

extension Array where Element: Hashable {
    func distinct() -> [Element] {
        var uniqueValues: Set<Element> = []
        return self.filter {
            let (inserted, _) = uniqueValues.insert($0)
            return inserted
        }
    }
}

And use it on our data:

var locations = [("Location_A", 49.5858, 9.123456), ("Location_B", 49.5858, 9.123456), ("Location_A", 49.5855, 9.123450)]
let myLocations = locations
    .map { MyLocation(tuple: $0) }
    .distinct()
print(myLocations)

Note that I have defined equality for two objects when they are closer than 1 meter. That will be slower than simply comparing longitude against longitude and latitude against latitude, but it will be also more precise.

Thanks for your help! I think this is gonna helpful for everyone!

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