How do I write a for-loop in Swift 3 for an array that I modify during the for loop?

Question

So, I have a for-loop that looks similar to this:

for var i = 0; i < results.count ; i += 1 {
   if (results[i] < 5) {
      results.removeAtIndex(i)
      i -= 1
   }
}

This used to work. But when I changed it to the preferred Swift 3.0 syntax:

for var i in 0..<results.count {
   if (results[i] < 5) {
      results.removeAtIndex(i)
      i -= 1
   }
}

I get an array IOOBE exception because it doesn't re-check the count and continues on until the original results.count.

How do I fix this? It works now, but I don't want to get into trouble in the future.

it makes no sense why apple made the for loop so complex, i never remember having an issue with it for the last 25 year — Klajd Deda, Oct 19 '17 at 22:44

score 84 · Accepted Answer · edited Dec 27 '21 at 10:03

84

While the solution making use of filter is a fine solution and it's more Swift-ly, there is another way, if making use of for-in is, nonetheless, still desired:

func removeBelow(value: Int) {
    var results = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

    for i in (0 ..< results.count).reversed() {
        if (results[i] < value) {
            results.remove(at: i)
        }
    }

    print(results)
}

removeBelow(value: 5)

Result:

[5, 6, 7, 8, 9, 10]

The problem with removeAtIndex within the loop is that it will not cause the array to re-index itself in-place and thus causing an array out of bounds exception due to count not being updated.

By traversing backwards, the out of bounds exception can thus be avoided.

edited Dec 27 '21 at 10:03

DeyaEldeen

10,847
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answered Jun 03 '16 at 23:40

Unheilig

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2

Oh god... that's really smart. Yeah I know that the filter way would be more *swiftly* but I think this way applies better, gonna use it. thanks! – JoeVictor Jun 04 '16 at 07:51
hooray, an editor of an OP that actually provides a well received answer, not just editing to increase rep. rare. – aremvee Mar 29 '17 at 11:39
@Unheiling - .reverse() this function not supported in swift3. Anybody can more help on same question? – New-Learner Jan 31 '18 at 11:28
You should remove the `var` keyword in `for var i in (0.. – atulkhatri Mar 13 '18 at 09:53

score 19 · Answer 2 · edited Dec 27 '21 at 10:05

19

Could you use a filter instead?

let numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
let greaterThan4 = numbers.filter{$0 >= 5}
print(greaterThan4)

edited Dec 27 '21 at 10:05

DeyaEldeen

10,847
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42
75

answered Jun 02 '16 at 15:29

Brandon Shega

769
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The new array should be named `greaterThan4` ;) Anyway this is the correct way to solve the problem. – Luca Angeletti Jun 02 '16 at 16:36
Oh yeah right! the function that calls this is called "filterArray". I never made the connection haha. But I also need the filtered results to be appended to the end. Any suggestions? – JoeVictor Jun 02 '16 at 23:07

score 15 · Answer 3 · answered Jun 02 '16 at 15:33

15

If you want to continue using a for-loop, you can enumerate over both index and element using enumerate:

for (index, element) in results.enumerate() {
   if (element < 5) {
     results.removeAtIndex(index)
   }
}

Although depending on what you're doing in your loop, the filter method might be a better idea.

answered Jun 02 '16 at 15:33

Luka Jacobowitz

22,795
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That's a bad idea, you will run into the out-of range trap under certain circumstances, for example try the array `[6, 5, 4, 3, 2, 1]`. The reason is that the array is modified in the loop but the index is not adjusted. – vadian Jun 02 '16 at 15:40

Miguel Herrero · Answer 4 · 2017-08-18T09:37:33.537

1

If your loop goes forward...

for var i in (0..<results.count) where results.indices.contains(i) { 

//if the index doesn't exist, the loop will be stopped.

if (results[i] < 5) {
        results.removeAtIndex(i) 
    }

}

edited Aug 18 '17 at 09:37

answered Aug 18 '17 at 09:23

Miguel Herrero

39
2

How do I write a for-loop in Swift 3 for an array that I modify during the for loop?

4 Answers4

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