inserting a fake row#indexPath, iOS - ios

Question

Im creating a new table, and within it, Im inserting a new row with fake text, but I get the crash report
Terminating app due to uncaught exception
'NSInternalInconsistencyException', reason: 'attempt to insert row 5
into section 0, but there are only 5 rows in section 0 after the
update
after some fiddling around, I changed my code from:
#IBAction func add() {
let newRow = ChecklistItems(text: "Im the new Row", checked: false)
items.append(newRow)
let index = items.count
let indexPath = NSIndexPath(forRow: index, inSection: 0)
tableView.insertRowsAtIndexPaths([indexPath], withRowAnimation: .Left)
}
TO THIS:
#IBAction func add() {
let index = items.count
let newRow = ChecklistItems(text: "Im the new Row", checked: false)
items.append(newRow)
let indexPath = NSIndexPath(forRow: index, inSection: 0)
tableView.insertRowsAtIndexPaths([indexPath], withRowAnimation: .Left)
}
can someone explain to me the actual details why this switch made the code work?
thank you so much~

Answer 1

Welcome to SO.
In your first version, you add an item to your items array, then create an indexPath which points to the count of the array (So if the array contains 5 items, you create an indexPath value of row:5, section:0)
However, arrays are zero-based. If an array contains 5 elements, valid array indexes are 0 - 4. The same goes for indexPath values. The highest valid row value is totalRows - 1. If the array contains 5 elements, 5 is not a valid array index.
In the second version of your code, index = the item count BEFORE adding an item. So if the row contains 4 items, count = 4, you add an item, and now items contains 5 items and 4 is a valid index into the array, so the insert does not refer to an item that's beyond the end of your items array.

Answer 2

Because the indexPath is beginning with 0, so it's max value is the items.count - 1

Answer 3

Because count is starting from 1, but NSIndexPath and also array index starting from 0.
so lets say you have an array of ("a", "b", "c"), array count is 3. but the index of it is 0, 1, 2.
so with your first code, you add new item and choose the index based on its count which is beyond its array index. for example you add new item "d" to example array above, the count will be changed to 4. But the max index is 3, which is why it throws error.
with your second code, you get the index based on count first before adding. So you set index for item d with 3, because the array only contain a, b, c and then add d to array.
it is working for you right now, but it will potentially create a mismatch between your array index and your tableView index.
A good way to retrieve object index is using array function indexOf.