Swift: Setting variables if not equalling nil

Question

I'm using Swift and I'm wondering how to achieve this in 1 line of code:

If a != nil, myVariable = a

else if a == nil, myVariable = b

I need to do this so it fits with

var myVariable = a {
    didSet{
        //Do something
    }
}

Is there a way to do this in 1 line so I don't have to change it elsewhere and trigger the didSet{}?

Something like this:

if var myVariable = a 
else myVariable = b {
    didSet{
        //Do something
    }
}

Or...

var if a != nil {myVariable = a}
else {myVariable = b} {
    didSet{
        //Do something
    }
}

I know this may take some time to realise what I mean, I tried my best to explain it.

EDIT ---> The compiler says:

Left side of nil coalescing operator '??' has a non-optional type '[[Bool]]', so the right side is never used

and

Treating a forced downcast to '[[Bool]]' as optional will never produce 'nil'

This is what it looks like using a coalescing operator:

var purchased =
UserDefaults.standard.array(forKey: "purchased") as! [[Bool]] ?? 
[[true, false], [true, false], [true, false], [true, false], [true, false]] { //a is left, b is right
    didSet {
        //Do something
    }
}

Any ideas would be greatly appreciated. Thank you.

Graham · Accepted Answer · 2017-09-22 18:58:40Z

5

How about

var myVariable = a ?? b

This sets myVariable to a except for the case when a == nil, then it sets myVariable to b. didSet would be called once with the respective value. It is called a "nil coalescing operator".

Your edited questions shows that a in your case cannot be nil because you force unwrap the array (with as!). Instead you should conditionally unwrap it (as?), so that your code can check for nil and use the hard-coded default instead.

edited Sep 22, 2017 at 18:58

Graham

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answered Jun 19, 2017 at 6:22

Gerriet

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8 Comments

J.Treutlein Over a year ago

Thanks for that. The problem with this now is that a is optional and b is not. So according to the compiler, b will never be used since it is a non-optional.

Gerriet Over a year ago

For me this compiles and works perfectly without complaints. The case in the documentation is the same (a is optional, b is not). If your context is more complex please add additional information to your question.

J.Treutlein Over a year ago

It says: "Left side of nil coalescing operator '??' has a non-optional type '[[Bool]]', so the right side is never used". Is a optional for you and b not? I will add my exact figures in the post now.

Gerriet Over a year ago

Left side is a. If a is non-optional, the question makes no sense, you can just do var myVariable = a (because it will never be nil).

J.Treutlein Over a year ago

a is optional, b is not.

|

impression7vx · Accepted Answer · 2017-06-19 06:31:25Z

0

//Sets my variable to a if a != nil, else b.
var myVaraiable = (a != nil ? a : b)

Since a is optional and b is not, it would be more like

var myVariable = (a != nil ? a! : b)

edited Jun 19, 2017 at 6:31

answered Jun 19, 2017 at 6:22

impression7vx

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