Efficient Swift Array Comparison

Question

struct Area {
var name: String
var isSelected: Bool
}

I have two lists:

List One: It has the full list of Areas.
List Two: It has a subset of Areas from List One the user has selected (but selected state is not set)

What I want to do is to create a new list of Areas that has items from both lists but if an element from List One is in List Two I want to update the isSelected property to true.

I wrote the current method but its flawed and inefficient:

private func didGetCurrentUserSession(_ usersAreas: [Area]?, allAreas: [Area]){
        guard let usersAreas = usersAreas else {return}
        var newAreasList = [Area]()
        for area in allAreas {
            for userArea in usersAreas {
                if userArea.name == area.name {
                    newAreasList.append(Area(name: area.name, isSelected: true))
                    break
                }
            }
            newAreasList.append(Area(name: area.name, isSelected: false))
        }

        _loadingAreas.onNext(false)
        _areas.onNext(newAreasList)
    }

Any help with this would be highly appreciated. Would prefer to accomplish this by using very Swifty approach.

Sulthan · Accepted Answer · 2020-04-26 11:00:33Z

4

Let's get names of user areas:

let userAreaNames = Set((usersAreas ?? []).map { $0.name })

map the other areas

let newAreas = allAreas.map { area in
   return Area(
       name: area.name,
       isSelected: userAreaNames.contains(area.name)
   )
}

edited Apr 26, 2020 at 11:00

answered Apr 26, 2020 at 8:57

Sulthan

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5 Comments

user652038 Over a year ago

??, not ||! usersAreas.map { Set( $0.map(\.name) ) } is more meaningful though.

Sulthan Over a year ago

@Jessy thanks for the ?? fix :) Lots of Javascript coding recently. I actually prefer using $0.name over key paths. Both are possible. I think that removing the optional makes more sense here than using Optional.map.

user652038 Over a year ago

Not preferring key paths is bad because $0 is worse than \ and >= 3 spaces are worse than zero.

Sulthan Over a year ago

@Jessy I wanted to verify you statement but there is no map function that would accept a key path. That means you are probably using an extension which adds that using the closure map. And that means it cannot have better performance. I don't understand your statement about spaces.

user652038 Over a year ago

Not sure what you’re saying there. Maybe you’re not using Swift 5.2? github.com/apple/swift-evolution/blob/master/proposals/…

gnasher729 · Accepted Answer · 2020-04-26 08:46:04Z

2

Two choices: Sort both arrays, or turn one into a dictionary for fast lookup. And whatever you do, turn it into a generic method.

answered Apr 26, 2020 at 8:46

gnasher729

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2 Comments

Sulthan Over a year ago

No need for a generic if the functionality won't be ever used with a different type.

Minon Weerasinghe Over a year ago

Thank you for your fast response. I have chosen to go with Sulthans answer. It's a very clean and Swift approach.

Simba · Accepted Answer · 2020-04-26 09:01:49Z

2

Sort the arrays first to increase efficiency.

The main flaw in your code at the moment is that a false selected area is appended to your new array even if a true selected version of that same area has already been appended. Try something like this instead:

private func didGetCurrentUserSession(_ usersAreas: [Area]?, allAreas: [Area]){
    guard let usersAreas = usersAreas else {return}
    var newAreasList = [Area]()
    var selected = Bool()
    for area in allAreas {
        for userArea in usersAreas {
            if userArea.name == area.name {
                newAreasList.append(Area(name: area.name, isSelected: true))
                selected = true
                break
            }
        }
        if selected == false {
            newAreasList.append(Area(name: area.name, isSelected: false))
        } else {
            selected = false
        }
    }

    _loadingAreas.onNext(false)
    _areas.onNext(newAreasList)
}

answered Apr 26, 2020 at 9:01

Simba

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2 Comments

Minon Weerasinghe Over a year ago

Hi, Simba thank you very much for your answer. You identified the flaw to the point. I have chosen to go with the answer presented by @Sulthan because its a lot cleaner and more swifty.

Simba Over a year ago

No problem - I also prefer Sulthan's answer!

user652038 · Accepted Answer · 2020-04-26 10:04:32Z

0

Sulthan's answer is good, but your guard statement makes it so that his set is overcomplicated. Instead, just put the more simply-constructed set into the capture list.

let newAreas = allAreas.map { [usersAreaNames = Set( usersAreas.map(\.name) )] in
  Area(
    name: $0.name,
    isSelected: usersAreaNames.contains($0.name)
  )
}

answered Apr 26, 2020 at 10:04

user652038

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