How to throw error in map for converting string to int array for number format exception - swift2

Question

I have a string
var str = "1 2 3 4"
and I want to convert it into [Int]. It can be done as follow
let intArray = str.characters.split {$0 == " "}.map(String.init).map { Int($0)!}
Now what if my string is
var invalid = " 1 a 4"
Then, the program will crash with
fatal error: unexpectedly found nil while unwrapping an Optional value
I need to be able to check number and throw number format error in map.

Answer 1

You can use throws, try - throw, do - try - catch and guard (if) for that. Here is the code
var invalid = " 1 a 4"
let intArray: [Int]
do {
intArray = try getIntArray(invalid, delimiter: " ")
}catch let error as NSError {
print(error)
intArray = []
}
func getIntArray(input:String, delimiter:Character ) throws -> [Int] {
let strArray = input.characters.split {$0 == delimiter}.map(String.init)
let intArray = try strArray.map {
(int:String)->Int in
guard Int(int) != nil else {
throw NSError.init(domain: " \(int) is not digit", code: -99, userInfo: nil)
}
return Int(int)!
}
return intArray
}
In getIntArray function, We first convert the input string to string array.
Then when we are converting string array to int array, we are expanding the map closure function parameter to include number format checking and throwing error using "guard".
"guard" can be replaced with "if" too if it is not available
if Int(int) == nil {
throw NSError.init(domain: " \(int) is not digit", code: -99, userInfo: nil)
}

Answer 2

Rather than throwing NSError types, you can create your own Swift native enum conforming to ErrorType where your enumeration contains the error case you would like to explicitly handle. E.g.
enum MyErrors : ErrorType {
case NumberFormatError(String)
}
/* throwing function attempting to initialize an
integer given a string (if failure: throw error) */
func strToInt(str: String) throws -> Int {
guard let myInt = Int(str) else { throw MyErrors.NumberFormatError(str) }
return myInt
}
Example usage within a do-try-catch construct:
func strAsIntArr(str: String) -> [Int]? {
var intArr: [Int] = []
do {
intArr = try str.characters.split {$0 == " "}
.map(String.init)
.map { try strToInt($0) }
} catch MyErrors.NumberFormatError(let faultyString) {
print("Format error: '\(faultyString)' is not number convertible.")
// naturally you could rethrow here to propagate the error
} catch {
print("Unknown error.")
}
return intArr
}
/* successful example */
let myStringA = "1 2 3 4"
let intArrA = strAsIntArr(myStringA)
/*[1, 2, 3, 4] */
/* error throwing example */
let myStringB = "1 b 3 4"
let intArrB = strAsIntArr(myStringB)
/* [], Format error: 'b' is not number convertible. */