在字典内的数组中附加int

Question

I use this line to append indexPath.row in array in dictionary.

var downloadQ = [Int: [Int]]()
var id = 1

override func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) {

    downloadQ[id]?.append(indexPath.row)
    print("downloadQ:\(downloadQ)")

}

And this line to check.

print("downloadQ:\(downloadQ)")

But my array not append indexPath.row in console I get this downloadQ:[:]

How to fix it?

Answer 1

eg

downloadQ[1] = [1,2]
downloadQ[2] = [1,2]

var arr = downloadQ[2]
arr?.append(3)
downloadQ[2] = arr
print(downloadQ)

What you doing is just appending your new item into array, but you have to replace that array in your dictionary. so, after appending your array item assign your array in dictionary.

downloadQ[2] = arr
print(downloadQ)

That's it.

Answer 2

When you create your dictionary it is initially empty, that is, it doesn't contain any arrays for any keys.

When you say downloadQ[id]?.append(indexPath.row) , downloadQ[id] is nil as you have never stored an array for key id . The append is then ignored since you have conditionally unwrapped downloadQ[id] .

You need to handle the case where there is no array for the key. The nil coalescing operator is a nice way to do this. Something like

var theArray = downloadQ[id] ?? [Int]()
theArray.append(indexPath.row)
downloadQ[id] = theArray