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iOS Error Handling in Swift 2.0 Error Handling Handling Errors

Darcy Phillips
Darcy Phillips
4,087 Points
Posted by Darcy Phillips
Darcy Phillips
4,087 Points

I don't fully understand the objective

Error Handling in Swift, course item no. 8. Here is the challenge:

"In the editor, you have a struct named Parser whose job is to deconstruct information from the web. The incoming data can be nil, keys may not exist, and values might be nil as well. In this task, complete the body of the parse function. If the data is nil, throw the EmptyDictionary error. If the key "someKey" doesn't exist throw the InvalidKey error. Hint: To get a list of keys in a dictionary you can use the keys property which returns an array. Use the contains method on it to check if the value exists in the array"

I don't quite understand what is meant by the "data" being nil. Does it mean that it would have a valid key, but the value would be nil (That wouldn't seem like an "empty dictionary" then)? Just not sure what I am to be checking for.

error.swift 
enum ParserError: ErrorType {
  case EmptyDictionary
  case InvalidKey
}

struct Parser {
  var data: [String : String?]?

  func parse() throws {

  }
}

let data: [String : String?]? = ["someKey": nil]
let parser = Parser(data: data)

3 Answers

jcorum
jcorum
71,830 Points
jcorum
jcorum
71,830 Points

Darcy, here you go:

    func parse() throws {
        if data == nil {
            throw ParserError.EmptyDictionary
        }

        if data!["someKey"] == nil {
            throw ParserError.InvalidKey
        }
    }

If data is nil then throw the EmptyDictionary error. If there's no value for "someKey" then throw the InvalidKey error.

Dominik Metzger
Dominik Metzger
4,959 Points
Dominik Metzger
Dominik Metzger
4,959 Points

but is a force-unwrap really that good?

Darcy Phillips
Darcy Phillips
4,087 Points
Darcy Phillips
Darcy Phillips
4,087 Points

Oh, do the instructions mean "if the variable "data" is nil"? My confusion comes from the expression "the data",

John Hendrix
John Hendrix
21,844 Points
John Hendrix
John Hendrix
21,844 Points

The above has an error with the capitols. This worked for me to pass challenge 1 of 3.

enum ParserError: Error {
  case emptyDictionary
  case invalidKey
}

struct Parser {
  var data: [String : String?]?

  func parse() throws {
    if data == nil {
            throw ParserError.emptyDictionary
        }

        if data!["someKey"] == nil {
            throw ParserError.invalidKey
        }
  }
}

let data: [String : String?]? = ["someKey": nil]
let parser = Parser(data: data)
jcorum
jcorum
71,830 Points
jcorum
jcorum
71,830 Points

Dominik, forced unwrapping is generally to be avoided, as the various TH instructors emphasize.

But note that here the first if checks to see if data is nil. So there's absolutely no way for the forced unwrapping to fail in this case.

Or, for a different example, would you say the forced unwrapping in IBOutlets is really not that good?

@IBOutlet weak var display: UILabel!