How to check if a variable is nil

Question

I have a variable

var a: [AnyObject? -> Void]

and I am adding data in to it by append method. Now I want to check if the variable is nil or not. I tried using [] but not working and also tried "", this also not working, can anyone tell what is the meaning of this variable and how to check if it is nil.

Maybe, you just want to check if the array is empty? – rintaro Feb 04 '15 at 09:24 — rintaro, Feb 04 '15 at 09:24
yes i want to check if it is empty. – user2413621 Feb 04 '15 at 09:29 — user2413621, Feb 04 '15 at 09:29

Vinzzz · Answer 1 · 2015-02-04T09:55:36.153

16

As far as I understand, var a is an Array of functions that take an optional Object of any type, and return void. So these functions's parameter IS optional, but the Array itself isn't : it cannot be nil, or it would be declared [AnyObject? -> Void]? , no?

EDIT : if, nevertheless, you declared this a as an optional (but WHY would you do that ?) - adding a ? - you check an optional existence with if let :

if let b = a {
// a not nil, do some stuff
} else {
// a is null
}

If you just want to check if the array is empty, use isEmpty method from Swift Array

edited Feb 04 '15 at 09:55

answered Feb 04 '15 at 09:14

Vinzzz

11,746
5
36
42

if i wanted to check if it is nil, what should i do? – user2413621 Feb 04 '15 at 09:16
As @Vinzzz already wrote, defined in that way *it cannot be `nil`* – DeFrenZ Feb 04 '15 at 12:24

Leo Dabus · Answer 2 · 2016-04-13T18:41:28.327

7

Update: Xcode 7.3 Swift 2.2

If you want to check if a variable is nil you should use if let to unwrap if for you. There is no need to create a second var.

let str = "123"
var a = Int(str)        
if let a = a {
    print(a)
}

Or

if let a = Int(str) {
    print(a)
}

edited Apr 13 '16 at 18:41

answered Feb 04 '15 at 14:52

Leo Dabus

229,809
59
489
571

score 2 · Answer 3 · answered Feb 04 '15 at 10:31

In Swift, nil is not a pointer—it is the absence of a value of a certain type. Optionals of any type can be set to nil, not just object types.

So, You can check it with below code:

let possibleNumber = "123"
let convertedNumber = possibleNumber.toInt()

if convertedNumber != nil {
    println("convertedNumber contains some integer value.")
}
// prints "convertedNumber contains some integer value."

Please refer this about nil for more information.

score 2 · Answer 4 · answered Apr 12 '17 at 07:36

2

In Swift 3.0

if let imageURL = dictObj["list_image"] as? String {
   print(imageURL)
}

answered Apr 12 '17 at 07:36

Aditya Sharma

585
5
18

You can improve the quality of this Answer by explaining how it works and thus how it improves the Questioner's situation. Also, you have not addressed part of the Question, namely where they ask "can anyone tell what is the meaning of this variable". – toonice Apr 12 '17 at 08:09
Please don't duplicate your answers: http://stackoverflow.com/a/43363148/2415822, http://stackoverflow.com/a/43363066/2415822 – JAL Apr 12 '17 at 20:02

score 1 · Answer 5 · answered Feb 05 '15 at 22:21

You can use if let. if let is a special structure in Swift that allows you to check if an Optional holds a value, and in case it does – do something with the unwrapped value.

var a:Int=0
if let b=a{
   println(a)
} else {
   println("Value - nil")
}

But for Strings you can also use .isEmpty() If you have initialized it to "".

var str:String=""
if !str.isEmpty(){
   println(str)
}

score 0 · Answer 6 · answered Jul 21 '15 at 20:57

For me none of the above solutions worked when I was using an AVFoundation object.

I would get Type 'AVCaptureDeviceInput does not conform to protocol 'BooleanType' when I tried if (audioDeviceInput) and I would get Binary operator '!=' cannot be applied to operands of type 'AVCaptureDeviceInput' and 'nil'.

Solution in my situation

if (audioDeviceInput.isEqual(nil))

nil is a pointer like any other and can be referenced as such, which is why this works.

How to check if a variable is nil

6 Answers6

Linked