Cannot extract the string value of the Dictionary<hashable,String>

Question

I have declared following dictionary

var locations:Dictionary<String,String> = ["loc1":"Los Angeles","loc2":"San Francisco"];

then trying to simply assign it to a variable rather than a constant

var location:String = locations["loc1"];

but then the compiler is complaining about : Value of optional type 'String?' not unwrapped; did you mean to use '!' or '?'?

If I change var location:String = locations["loc1"]; to var location:String? = locations["loc1"]; ,which means the String is optional, compiler error goes away.

but as you can see I haven't defined my dictionary to be optional e.g. Dictionary<String,String?>. So just wondering why does Swift convert my value type to be optional string(String?) behind the scene?

Tim · Accepted Answer · 2014-06-03 22:22:24Z

5

The code that pulls the value of the dictionary does not know whether or not locations["loc1"] will return a value - it could also return nil. Thus, it cannot be assigned to a non-optional String, as that would be a guarantee that the String is not nil, which is not the case.

However, if you want location as a non-optional String, you can initialize it, then use if let to unwrap the value from the dictionary:

var location : String = "Unknown"
if let value = locations["loc1"] {
     location = value
}

Slightly more verbose, but guarantees that location contains either a value or "Unknown" and cannot be nil.

answered Jun 3, 2014 at 22:22

Tim

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Connor · Accepted Answer · 2014-06-03 22:17:27Z

1

It's because your dictionary may not contain a value for that key so it returns a String optional just in case.

answered Jun 3, 2014 at 22:17

Connor

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