Comparing two arrays of objects in Swift

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))?

Firstly, you need to use var instead of let for initialising your arrays in order to append to it. Secondly, one easy way to get the difference between 2 collections is to use Set. There is a subtract function for that. You can read more here: developer.apple.com/documentation/swift/set/1779475-subtract — koropok
– koropok, Commented Apr 30, 2019 at 1:16
Oh, about let, it's not a "real" code, wrote it for example, mechanical mistake. About Set thanks, will try — Ruslan Duda
– Ruslan Duda, Commented Apr 30, 2019 at 1:18
tried use Set, it says me "Cannot convert value of type '[Station]' to specified type 'Set'" — Ruslan Duda
– Ruslan Duda, Commented Apr 30, 2019 at 1:24

Shubham Bakshi · Accepted Answer · 2019-04-30 07:14:29Z

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))

answered Apr 30, 2019 at 7:14

Shubham Bakshi

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4 Comments

ielyamani Over a year ago

Using a Set loses the original order

Shubham Bakshi Over a year ago

Yeah, but if all you want to know is what changes there are between the two, how does the order matter? Plus there is no need for writing your custom logic for eliminating the similar items from the two

Shubham Bakshi Over a year ago

Plus the model has a dateOfIssue with which you can later sort the final output

nayem Over a year ago

How does your answer differ from @paivaalan's answer?

alanpaivaa · Accepted Answer · 2019-04-30 01:49:34Z

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])

NShiny · Accepted Answer · 2019-04-30 01:54:39Z

0

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)

edited Apr 30, 2019 at 1:54

answered Apr 30, 2019 at 1:49

NShiny

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