Removing elements from array during enumeration in Swift 5.1 without reversing

Question

I want to remove an element within a for-in loop if it fails a certain condition. Currently, there's a solution here, but it's old and seems complicated for all I'm trying to do.

In Java, I can just reference the index like so:

/* Program that removes negative numbers from an array */
//Make an array list and add some numbers to it.
ArrayList<Int> a = new ArrayList<Int>;
for (int i = 0; i < 10; i++){
    a.add( (int) ((Math.random - 0.5) * 100) //Add 10 numbers that are pos/neg.

//Remove all the negative numbers.
for (int i = 0; i < a.size; i++){
    if (a.get(i) < 0){
        a.remove(i); //Removes the element. 
    }
}

But in Swift, to my understanding, I have to use a for-each loop equivalent:

    var array = [-4, -2, 0, 4, 6, 7, 2, 5, 7]
    //Iterate looking for numbers < 0.
    for item in array{
        if (item < 0){
            array.removeAt(item) //ERROR 
        }
        else{
            //Right now, I just copy every value into a new array.
            //But I shouldn't even need this w/ the Arrays collection.
    }

How can I do this more simply? Sorry I'm a newbie here; thanks for your help!

Edit | Clarification June 14, 2020 at 11:44 PM:

Someone suggested filter which is a stellar response, but not what I was looking for. (My bad, sorry). I used this as an example for a different challenge I'm working on here while I learn Swift.

Please help me look for solutions that remove an element. Thank you!!

You can just remove it. It’s ok to mutate the sequence you are iterating, because it’s a copy. — matt, Jun 15 '20 at 03:48
possible duplicate of https://stackoverflow.com/a/52067229/2303865 — Leo Dabus, Jun 15 '20 at 04:09
As it stands, your question is a duplicate. If `filter` or `removeAll(where:)` does not what you are looking for then please [edit] your question and updated with a clear description of the problem. — Martin R, Jun 15 '20 at 06:38

Frankenstein · Answer 1 · 2020-06-15T03:43:57.507

1

Use the removeAll(where:) method like this:

array.removeAll(where: { $0 < 0 })

or:

array.removeAll { $0 < 0 }

edited Jun 15 '20 at 03:43

answered Jun 15 '20 at 03:30

Frankenstein

15,732
4
22
47

Not new: https://stackoverflow.com/a/52067229/1187415. – Martin R Jun 15 '20 at 06:39

score 1 · Answer 2 · answered Jun 15 '20 at 04:24

Expanding on the possible duplicate question link I have commented above, you can extend SignedInteger protocol and create a property to return a Boolean value based on a condition. Then you can use it as a keyPath on RangeReplaceableCollection method that removes all the elements that satisfy the given predicate.

extension SignedInteger {
    var isNegative: Bool { self < 0 }
    var isPositive: Bool { self > 0 }
}

extension Numeric {
    var isZero: Bool { self == .zero }
}

var array = [-4, -2, 0, 4, 6, 7, 2, 5, 7]

array.removeAll(where: \.isNegative)
array  // [0, 4, 6, 7, 2, 5, 7]

array.removeAll(where: \.isZero)
array  // [4, 6, 7, 2, 5, 7]

array.removeAll(where: \.isPositive)
array  // []

Removing elements from array during enumeration in Swift 5.1 without reversing

2 Answers2