Swift - How to check if var is nil? - ios

Question

I need to check if the last element in a dataArray is nil or not, so I am running this code:
if(dataArray.last == nil){
print("dataArray.last == nil");
} else {
print("dataArray.last != nil , last = " + String(dataArray.last));
}
The weird thing is that it is printing this:
dataArray.last != nil , last = Optional(nil)
It is saying that the last element is nil despite the if condition not being met. How is this possible?
Additionally, if I print String(dataArray) this is what is returned:
[itemData,itemData , itemData , nil , nil , nil].

Answer 1

first and last methods of array return optional value, that contains nil or element of array. Going forward, element of array can be nil by itself. Let me show easy example:
let dataArray: [String?] = ["a", nil]
let lastElement = dataArray.last
You think that lastElement have type String?, but it have type String?? instead. When you check dataArray.last for nil, you only confirm that array is not empty. I offer you to use optional binding instead:
if let lastElement = dataArray.last, _ = lastElement{
print("dataArray.last != nil , last = " + String(dataArray.last));
}else{
print("dataArray is empty OR last element of dataArray is nil");
}

Answer 2

As also explained in the other answers, the last property of a
collection of Element has the type Element?, and returns nil
if the collection is empty. In your case, the element type itself is
an optional type Item?, so there are three cases possible:
The array is empty: dataArray.last is nil aka .None.
The array is not empty, and the last element is nil:
dataArray.last is .Some(nil).
The array is not empty, and the last element is not nil:
dataArray.last is .Some(.Some(element).
If you want do distinguish all three cases, you have to unwrap
twice:
let dataArray: [String?] = [ "a", "b", "c", nil ] // As an example
if let lastElement = dataArray.last {
if let lastUnwrapped = lastElement {
print("Last element is", lastUnwrapped)
} else {
print("Last element is nil")
}
} else {
print("Array is empty")
}
This can be shorted with a switch and the optional pattern:
switch dataArray.last {
case let element??:
print("Last element is", element)
case let element?:
print("Last element is nil")
case nil:
print("Array is empty")
}
If you are only interested if the last element of the array is not nil,
then you can use if case with an optional pattern:
if case let element?? = dataArray.last {
print("Last element is", element)
} else {
print("Last element is nil, or array is empty")
}
Here element?? is a shortcut for .Some(.Some(element)) which
matches case #3.
If you want to re-implement the last method for arrays then it
would simply be
extension Array {
func lastMe() -> Element? {
if isEmpty {
return nil
} else {
return self[count - 1]
}
}
}
If the element type is an optional Item?, then the return type
becomes Item??.

Answer 3

Why are you getting that result?
Let's look at this code
let dataArray: [Int?] = [1, nil]
if let last = dataArray.last {
print(last)
}
It does print
nil
It does happen because dataArray is defined as an array of optional Int (aka Int?). So the last property does return a value that is an Optional containing nil.
So the if clause is satisfied and the body of the if is executed.
This is the correct syntax
let dataArray: [Int?] = [1, nil ,3]
if let lastOptional = dataArray.last, last = lastOptional {
print("dataArray.last != nil, last = \(last)")
} else {
print("dataArray.last == nil")
}
Output:
dataArray.last != nil, last = 3
Explanation
Let's look at the if. It is made of 2 clauses and both must be satisfied for the if to be resolved as true.
The #1 clause
let lastOptional = dataArray.last
does succeed if the last element does exist. It will be an Int? so an optional value containing nil or an Int. In both case this clause is satisfied.
Basically the first clause is satisfied when the array is NOT empty.
The #2 clause
last = lastOptional
does succeed if the optional last (Int?) element is successfully unwrapped.
This clause is satisfied when the last element does exists and is not an Optional containing nil.
Wrap-up
Inside the body of the if you can use last and you can be sure that it will be the unwrapped value. Infact the type of last is Int.

Answer 4

Try this
if let lastObject = dataArray.last{
print("dataArray.last != nil", + String(lastObject));
}else{
print("dataArray.last == nil );
}

Answer 5

Array have optional function
last
Optional(nil) is not equal to nil
if you print out dataArray.last then Optional(nil) will be printed
try this
if let _ = dataArray.last {
print("dataArray.last != nil , last = " + String(dataArray.last));
}else{
print("dataArray.last == nil");
}