How to throw an error for non existing keys in dictionary?

Using Swift
1

Hi,

I'd like to know how to handle errors when dealing with non-existing keys in a dictionary.
In the example below I have a function called complementOfDNA, where I've placed a check if nil, throw (EDITED) but it's obviously wrong, as I get Value of optional type 'String?' must be unwrapped to a value of type 'String'. So, I'd like to learn how to handle errors in this context, if someone can point me in the right direction? I apologise but I did check Error Handling — The Swift Programming Language (Swift 5.7)

struct Nucleotide {
    var RNATranscription = [
        "G": "C",
        "C": "G",
        "T": "A",
        "A": "U"
    ]
    var DNAStrand: String
    enum TranscriptionError: Error {
        case invalidNucleotide(message: String)
    }
    
    init (_ DNAStrand: String) {
        self.DNAStrand = DNAStrand
    }

    func complementOfDNA () throws -> [String] {
     let split = self.splitter(self.DNAStrand)
     var transcribed: [String] = []
     for char in split {
        let x = self.RNATranscription[char]
        if x != nil {
          transcribed.append(x)
        } else {
          throw TranscriptionError.invalidNucleotide(message: "\(char) is not a valid Nucleotide")
        }
     }
     return transcribed
    }
    
    func splitter (_ text: String) -> [String] {
        let arr = text.map {
            String($0)
        }
        return arr
    }
}
2

Are you looking for if let?

3

Thanks for looking @Nevin, that's it?! Ok, I'll try:

if let x = self.RNATranscription[char] {
  transcribed.append(x)
} else {
  throw TranscriptionError.invalidNucleotide(message: "\(char) is not a valid Nucleotide")
}

Worked fine! thanks a lot for your time!

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