You declared an array of String
var stringArray = [String]()
Then you appended one element
stringArray.append(forss as! String)
That means only index 0 is valid
stringArray[0]
and you get an out-of-range error for
stringArray[1]
stringArray[2]
etc.
Note:
The question is not particularly related to Swift 4. All versions of Swift exhibit this behavior.
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solved Swift 4: Array index out of Range