Comparing two Doubles in Swift: "Type of expression is ambiguous without more context"

Question

I have a function that must check if a value (realOperand) is bigger than other value (realValue). These values are numeric, but they come in string, and I parse them to Double:

return Double(realOperand) > Double(realValue)

I can't understand why, but that line is giving this error:

Type of expression is ambiguous without more context

Answer 1

The function

Double.init(_ string: String) returns an Optional (Type Double?).

It would be like you wrote this code:

var a: Double? = nil
var b: Double? = 7

if a > b {
    print("A is greater")
}

Is nil greater than or less than 7? It's undefined. Optionals are not Comparable.

You need to decide what to do if either or both strings can't be converted to Double:

guard let operand = Double(realOperand),
  value = Double(realValue) else {
    // Crash on purpose. Not a great option for user-entered Strings!
    fatalError("Could not convert strings to Doubles")
}

return operand > value