Updating an item in an array passed by reference

Question

What's the easiest/right way to update an item in an array? I want the caller to have the updated array as well. So:

static func updateItem(updatedItem: Item, inout items: [Item]) -> Bool {
        var item = items.filter{ $0.id == updatedItem.id }.first
        if item != nil {
            item = updatedItem
            return true
        }

        return false
    }

I want the caller to have the updated items (with the updated item). I think the problem with the above code is that it only updates the local variable item. What's the best way to actually update the relevant item inside the items array?

What do you want to the `updatedItem` if there's no existing item with the same `id`? — Alexander, Dec 27 '16 at 20:22

matt · Accepted Answer · 2016-12-28T01:42:40.433

5

You do it the same way Superman gets into his tights — one leg at a time. Cycle through the incoming inout array and replace any items where the id matches:

func updateItem(updatedItem: Item, items: inout [Item]) -> Bool {
    var result = false
    for ix in items.indices {
        if items[ix].id == updatedItem.id {
            items[ix] = updatedItem
            result = true
        }
    }
    return result
}

Note that this is Swift 3 syntax where inout precedes the type, not the label.

You can write it a little more "Swiftily" by using map:

func updateItem(updatedItem: Item, items: inout [Item]) {
    items = items.map {
        $0.id == updatedItem.id ? updatedItem : $0
    }
}

... but that amounts to the same thing in the end.

edited Dec 28 '16 at 01:42

answered Dec 27 '16 at 21:25

matt

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I'm returning bool because the caller does some extra things depending on if item was found or not. How can I return false if item was not found in your "swiftily" approach? – Prabhu Dec 27 '16 at 22:49
I rewrote the first approach to return a Bool, and I suggest you use that. There is no savings in using the second approach — `map` is still a loop. – matt Dec 28 '16 at 01:43

score 2 · Answer 2 · edited Feb 26 '19 at 03:24

2

You are mutating item which is merely a copy of the instance in the array (if Item is a value type, such as a struct, tuple, or enum), or a reference to it (if Item is a reference type, such as a `class). In either case, the array will be unaffected.

You'll need to find the index of the instance within the array, then mutate the array at that index.

func updateItem(updatedItem: Item, inout items: [Item]) -> Bool {
    guard let index = items.index(where: { $0.id == updatedItem.id }) else {
        return false // No matching item found
    }

    items[index] = updatedItem
    return true
}

This is all rather clunky, though. It would be better if you used a dictionary instead, mapping the id to the instance with that id. This means you'll have fast, constant time look up, and it'll be way more convenient. Here's how that will look:

// Assuming the "id" is an Int
func updateItem(updatedItem: Item, items: inout [Int: Item]) -> Bool {
    return items.updateValue(updatedItem, forKey: updatedItem.id) != nil
}

edited Feb 26 '19 at 03:24

brandonscript

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answered Dec 27 '16 at 20:19

Alexander

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Thanks, with the dictionary how will the calling code look? – Prabhu Dec 27 '16 at 21:20
1

The same, you just pass in the new item and the dict of all items – Alexander Dec 27 '16 at 21:21
Hmm, it's saying value of type [Item] has no member index. – Prabhu Dec 27 '16 at 22:44
@Prabhu I don't think such a function exists in Swift 2. You can write an extension on Sequence to add it yourself, but really, just use Swift 3 – Alexander Dec 27 '16 at 22:46
Hmm yeah, the thing is we are not ready to migrate to Swift 3 yet, so will have to do the extension I guess. – Prabhu Dec 27 '16 at 22:47

Updating an item in an array passed by reference

2 Answers2