How to sort outer array based on inner array value?

Question

I have an array of tuple, tuple contains array of struct Bill

let billDetails = [(name:String, bills:[Bill])]()

struct Bill {
    var date: Date
}

The inner array bills is already sorted by the date property. Now I want to sort the outer array based on the inner array's first object's date property.

How to do this without force unwrapping?

billDetails = billDetails.sorted(by: { $0.bills.first!.date < $1.bills.first!.date })

Need to check $0.bills.first != nil && $.bills.first != nil isn't? — arun siva
– arun siva, Commented May 11, 2019 at 12:01
@MartinR If it was Int, I would have used Int.min and Int.max. Didn't know about distantPast and distantFuture. Thanks — arun siva
– arun siva, Commented May 11, 2019 at 12:17

Sweeper · Accepted Answer · 2019-05-11 12:10:58Z

2

You need to decide where to put an item if its bills is empty. There is really only 2 options, either put them at the start of the array or at the back of the array.

You can use ?? to provide a default value of either distantFuture or distantPast depending on where you want the empty bills to go.

// empty bills last
billDetails.sort(by: { $0.bills.first?.date ?? Date.distantFuture < $1.bills.first?.date ?? Date.distantFuture })
// or empty bills first
billDetails.sort(by: { $0.bills.first?.date ?? Date.distantPast < $1.bills.first?.date ?? Date.distantPast })

edited May 11, 2019 at 12:10

answered May 11, 2019 at 12:06

Sweeper

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2 Comments

arun siva Over a year ago

Can you give titles for your answers, which puts empty bills at first and which puts last. If I want to remove the items when its bills is empty, It it possible to do without using another filter function?

Sweeper Over a year ago

@arunsiva edited. You can’t remove stuff from an array with just a sort.

alanpaivaa · Accepted Answer · 2019-05-11 12:16:27Z

1

How about the following?

billDetails = billDetails.sorted(by: { lhs, rhs in
  guard !lhs.bills.isEmpty || !rhs.bills.isEmpty else {
    return false
  }

  if !lhs.bills.isEmpty, rhs.bills.isEmpty {
    return false
  }

  if lhs.bills.isEmpty, !rhs.bills.isEmpty {
    return true
  }

  return lhs.bills[0].date < rhs.bills[0].date
})

edited May 11, 2019 at 12:16

answered May 11, 2019 at 11:57

alanpaivaa

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4 Comments

arun siva Over a year ago

What happens when one element in billDetails contains empty bills? That one will be the first element after sorting?

alanpaivaa Over a year ago

Please check my edit. It should achieve your desired behavior. But you can easily change it if you want to empty bills in the end.

arun siva Over a year ago

Thanks for your input. Giving default value seems more simple and readable

alanpaivaa Over a year ago

Agreed. But in the case you want to handle the case where both are empty and sort by a secondary field, just edit in the first guard. Anyways, I'm glad you could find your answer! :)

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