Swift sort array by Bool value and TimeStamp

Question

I have an array of messages that I want to sort by whether they are unread or not, and their time stamp. These messages will be displayed in a TableView.

All unread messages should be displayed at the top, and then sorted by their timestamp.

Below is what I have right now; but what seems to be happening is that the items are displayed only by time stamp, without sorting them by unread at the top and then unread messages after.

let channelsSortedByUnreadFirst = dataStore.messages.sorted { $0.isUnread == true && $1.isUnread == false }
let timeSortedItems = channelsSortedByUnreadFirst.sorted(by: { $0.timeStamp > $1.timeStamp })
messagesTableViewSection.items = timeSortedItems

How do I sort these messages by both isUnread and timeStamp?

Leo Dabus · Accepted Answer · 2021-05-12 12:36:49Z

3

Bool does not conform to Comparable. What you need is to extend Bool type and convert its value to a type that conforms to Comparable like integer 0 or 1 and sort it using its value:

extension Bool {
    var value: Int { self ? 1 : 0 }
}

true.value    // 1
false.value   // 0

Now you can use the boolean value when sorting your collection:

let channelsSorted = dataStore.messages.sorted { ($0.isUnread.value, $0.timeStamp) > ($1.isUnread.value, $1.timeStamp) }

edited May 12, 2021 at 12:36

answered May 12, 2021 at 0:55

Leo Dabus

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3 Comments

Duncan C Over a year ago

Wait, what? You can compare tuples and it evaluates the first item as a primary sort key and the 2nd sort key?!?

Leo Dabus Over a year ago

@DuncanC Actually since Swift 2.2 github.com/apple/swift-evolution/blob/master/proposals/…

Duncan C Over a year ago

Very cool. I somehow missed that. (Voted)

AnderCover · Accepted Answer · 2021-05-12 01:31:08Z

1

Another way using Swift Algorithms stablePartition(by:):

import Algorithms

var messages = dataStore.messages
messages.sort { $0.timeStamp > $1.timeStamp }
let firstReadIndex = messages.stablePartition { !$0.isUnread }

answered May 12, 2021 at 1:31

AnderCover

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Comments

ScottyBlades · Accepted Answer · 2021-05-12 19:49:41Z

1

Another Way that might be more intuitive but uses multiple passes.

print(messages.filter(\.isUnread).sorted { $0.timeStamp > $1.timeStamp } + messages.filter { !$0.isUnread })

edited May 12, 2021 at 19:49

answered May 12, 2021 at 19:34

ScottyBlades

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3 Comments

AnderCover Over a year ago

I think read messages should also be sorted

ScottyBlades Over a year ago

"All unread messages should be displayed at the top, and then sorted by their timestamp." Its kind of ambiguous language. meh

AnderCover Over a year ago

I agree, and it would be easy to add should OP feel the need :)

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