Cannot assign value of type 'String?' to type 'String'

Question

class C{
 var b:[String]?
 subscript(i:Int)->String?{
  get{
   return b?[i]
  }
  set{
   b?[i] = newValue! // notice the unwrapped “!” here
  }
 }
}

In the code, I put a exclamation mark "!" to unwrap the newValue, Because newValue is the same type of the return value of subscript which is String?. If I don't put the exclamation mark "!" it will report error: "

error: cannot assign value of type 'String?' to type 'String' b?[i] = newValue

" Question: b?[i] is obviously an optional String value String?, how come it is of type String. I wonder it was a bad error code

totiDev · Accepted Answer · 2017-01-19 12:14:36Z

2

Your var b: [String]? is an optional array. However the string inside is not optional. If you changed it to: var b: [String?]? then it would work.

answered Jan 19, 2017 at 12:14

totiDev

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Comments

dfrib · Accepted Answer · 2017-01-19 12:13:58Z

You property b is indeed an optional array, but its member elements are not optionals. Hence, when attempting to assign a new value to an existing member, the new value must be of concrete type String, and not the optional type String?.

// this is an Optional array, with non-optional elements
var foo: [String]? = ["foo"]

let optStr: String? = "bar"
let nonOptStr = "bar"

// ok, nonOptStr is not an optional
foo?[0] = nonOptStr

// as for 'nonOptStr', we need to unwrap
// it prior to attempting to assign it as a 
// replacement member of 'foo'
if let str = optStr {
    foo?[0] = str
}

Also, avoid explicit unwrapping, and consider validating the index prior to using it. E.g.:

class C {
    var b: [String]?
    subscript(i: Int) -> String? {
        get {
            if b?.indices.contains(i) ?? false { return b?[i] }
            return nil            
        }
        set {
            if b?.indices.contains(i) ?? false, let newValue = newValue {
                b?[i] = newValue
            }
        }
    }
}

user579980 · Accepted Answer · 2019-05-16 00:44:58Z

More generally, I found myself with this exact error after doing a split on a string and ended up printing out both variables. That clarified that I was trying to assign a two element variable to an indexed array that contained a string. So, that is the source of the error, which is essentially saying your trying to assign two elements of an array into a string inside your other array. Well, that makes sense out of the error.
Algorithmically it is: 1) array1[0] = array2Item1, array2Item2

That's why this was fine: 2) array1 = array2Item1, array2Item2

but the above was not.

So, the correct thing to do is 2) and then assign it by index:

array1 = array2Item1, array2Item2

array3 = array1[0] to do what I thought I was doing in 1).

Just another explanation for any others who venture here.

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