1

I'm having problems with the following code. The particular function I'm having problems with is charCountRec. The error I'm getting is type of expression is ambiguous without more context.

func merging(_ dict1: [Character: Int], with dict2: [Character: Int]) -> [Character: Int] {
    var newDict : [Character: Int ] = [:]
    for (key,value) in dict1 {
        newDict[key] = value
    } 
    for (key,value) in dict2 {
        newDict[key] = value
    }
    return newDict
    
}

extension Dictionary {
    /// Creates a new dictionary from the key-value pairs in the given sequence.
    ///
    /// - Parameter keysAndValues: A sequence of key-value pairs to use for
    ///   the new dictionary. Every key in `keysAndValues` must be unique.
    /// - Returns: A new dictionary initialized with the elements of `keysAndValues`.
    /// - Precondition: The sequence must not have duplicate keys.
    /// - Note: Differs from the initializer in the standard library, which doesn't allow labeled tuple elements.
    ///     This can't support *all* labels, but it does support `(key:value:)` specifically,
    ///     which `Dictionary` and `KeyValuePairs` use for their elements.
    init<Elements: Sequence>(uniqueKeysWithValues keysAndValues: Elements)
    where Elements.Element == Element {
      self.init(
        uniqueKeysWithValues: keysAndValues.map { ($0.key, $0.value) }
      )
    }
  }

func charCountRec( str: String, result: inout [Character: Int]) -> [Character: Int] {
    var res : [Character: Int] = [:]
    var firstIndex = str.startIndex
    if str.count == 1 {
        res[str[firstIndex]] = 1
        return res
    } else if str.count > 1 {
        firstIndex = str.startIndex
        result[str[firstIndex]]! += 1
        var partialResult : [Character: Int] = [:]
        for _ in 1...result.count {
            result.dropLast()
        }
        partialResult = charCountRec(str: str, result: &Dictionary(result.dropFirst()))
        // Append the two dictionaries together
        return merging( result, with: partialResult)
    }
}
2

There are a couple inconsistencies in the charCountRec function. Below, I added "// <1>" and "// <2>" to refer to specific lines:

func charCountRec(str: String, result: inout [Character: Int]) -> [Character: Int] {
  var res: [Character: Int] = [:]
  var firstIndex = str.startIndex
  if str.count == 1 {
    res[str[firstIndex]] = 1
    return res
  } else if str.count > 1 {
    firstIndex = str.startIndex
    result[str[firstIndex]]! += 1
    var partialResult: [Character: Int] = [:]
    for _ in 1...result.count {
      _ = result.dropLast()
    }
    partialResult = charCountRec(str: str, result: &Dictionary(result.dropFirst())) // <1>
    // Append the two dictionaries together
    return merging(result, with: partialResult)
  }
  // <2>
}
  • at line <1> you're using Dictionary(result.dropFirst()), you're missing the uniqueKeysWithValues: label.
  • at line <1> you're passing the constant Dictionary(result.dropFirst()) as an inout parameter, but since it's a constant it cannot be mutated. You should declare it as a variable.
  • at line <2> you don't return, so your function returns a dictionary only when str.count is greater or equal to 1. You should return something also when str.count is 0.

The compiler should correctly show these 3 errors. Which version of Xcode are you using?