Remove multiple indices from array

Question

I have an array and I want to remove a bunch of indices

var arr = [0,1,2,3,4,5,6]
var rmIndices = [1,4,5]

What is the best way to remove in

Answer 1

Remove elements using indexes array:

1. Array of Strings and indexes

   let animals = ["cats", "dogs", "chimps", "moose", "squarrel", "cow"]
   let indexAnimals = [0, 3, 4]
   let arrayRemainingAnimals = animals
       .enumerated()
       .filter { !indexAnimals.contains($0.offset) }
       .map { $0.element }
   
   print(arrayRemainingAnimals)
   
   //result - ["dogs", "chimps", "cow"]
   

2. Array of Integers and indexes

   var numbers = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]
   let indexesToRemove = [3, 5, 8, 12]
   
   numbers = numbers
       .enumerated()
       .filter { !indexesToRemove.contains($0.offset) }
       .map { $0.element }
   
   print(numbers)
   
   //result - [0, 1, 2, 4, 6, 7, 9, 10, 11]
   

Remove elements using element value of another array

3. Arrays of integers

   let arrayResult = numbers.filter { element in
       return !indexesToRemove.contains(element)
   }
   print(arrayResult)
   
   //result - [0, 1, 2, 4, 6, 7, 9, 10, 11]
   

4. Arrays of strings

   let arrayLetters = ["a", "b", "c", "d", "e", "f", "g", "h", "i"]
   let arrayRemoveLetters = ["a", "e", "g", "h"]
   let arrayRemainingLetters = arrayLetters.filter {
       !arrayRemoveLetters.contains($0)
   }
   
   print(arrayRemainingLetters)
   
   //result - ["b", "c", "d", "f", "i"]
   

Answer 2

The problem with flatmap is that it gives incorrect results if your array contains optionals.

The following is much faster than the functional style solutions provided and works with optionals. You just have to make sure rmIndices is sorted and unique. It's also fairly language agnostic.

 var numRemoved: Int = 0
 for index in rmIndices {
     let indexToRemove = index - numRemoved
     arr.remove(at: indexToRemove)
     numRemoved += 1
 }

If you need to make sure rmIndices is sorted and unique:

 rmIndices = Set(rmIndices).sorted()

Using XCTest to remove 500 elements (including the operation to ensure uniqueness and sorted):

0.006 sec

vs.

arr.enumerated().filter({ !rmIndices.contains($0.0) }).map { $0.1 }:

0.206 sec

I use this as an extension on Array

extension Array {

    mutating func remove(at indices: [Int]) {
        let rmIndices = Set(indices).sorted()
        var numRemoved: Int = 0
        for index in rmIndices {
            let indexToRemove = index - numRemoved
            self.remove(at: indexToRemove)
            numRemoved += 1
        }
    }

}