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iOS Intermediate Swift Extensions and Protocols Extending a Native Type

Thomas Dobson
Thomas Dobson
7,511 Points
Posted by Thomas Dobson
Thomas Dobson
7,511 Points

Extensions: String and Int addition

**Challenge Task 1 of 1

Your task is to extend the String type and add a method that lets you add an integer value to the string. For example: "1".add(value: 2) If the operation is possible, return an integer value. If the string does not contain an integer and you cannot add to it then return nil.**

Can anyone explain to me what I am doing wrong here? This extension makes sense in my head, but the compiler is giving me grief. I cant seem to get around converting the original string to an Int value in order to add the value.

The error I am getting

Playground execution failed: error: WorkSPace.xcplaygroundpage:10:20: error: type of expression is ambiguous without more context
            return Int(self) += value
                   ^~~~~~~~~
types.swift 
// Enter your code below
extension String
{
    var isInt: Bool
    {
        return Int(self) != nil
    }
}


extension String 
{

    func add(value: Int) -> Int?
    {
        if self.isInt
        {
            return Int(self) += value
        }
        else
        {
            return nil
        }
    }

}

var testString = "2"
testString.add(value: 5)

2 Answers

Jeff McDivitt
Jeff McDivitt
23,970 Points
Jeff McDivitt
Jeff McDivitt
23,970 Points

Hi Thomas -

Here is what I did not sure if you have the extension in there twice in your actual code; that may be causing the problem

extension String {

    func add(value: Int) -> Int?{
        guard let convertedNumber = Int(self) else{
            return nil
        }

        return convertedNumber + value
    }

}

"1".add(value: 3)
Thomas Dobson
Thomas Dobson
7,511 Points
Thomas Dobson
Thomas Dobson
7,511 Points

The first extension is just a "Can this string be converted to an Integer Bool". I called it as the determining factor on my if statement.

Also, way to make use guard; never even crossed my mind.

I was able to make it work with: 

// Enter your code below
extension String
{
    var isInt: Bool
    {
        return Int(self) != nil
    }
}


extension String
{

    func add(value: Int) -> Int?
    {
        if self.isInt
        {
            return Int(self)! + value
        }
        else
        {
            return nil
        }
    }

}

var testString = "2"
testString.add(value: 5)

But I had to force unwrap Int(self) which I know to be a no no.

I also had a second solution that I think I like better overall ::

extension String {

    func add(value: Int) -> Int?
    {

        if let stringToInt = Int(self)
        {
            return stringToInt + value
        }
        else
        {
            return nil
        }
    }

}

var testString = "2"
testString.add(value: 5)
Clinton Johnson
Clinton Johnson
28,714 Points
Clinton Johnson
Clinton Johnson
28,714 Points 
extension String {                        
     func add(value: Int) -> Int? {  
          guard let instanceOfValueIntoStringEqualsItself = Int(self) else {
              return nil                        
          }
          return instanceOfValueIntoStringEqualsItself + value
     }
}
"1".add(value: 2)