What is the best way to print Float and Double from embedded Swift?

Using Swift
1

Today I found printing Double or Float from embedded swift on ESP32 is kinda impossible: undefined reference to 'swift_float32ToString'. I even start to ask ChatGuPoT, but he had some hallucination, or I could not understand him.

2

I made something like this:

extension BinaryFloatingPoint {
    func string(_ decimal: Int) -> String {             // -1234,5678
        let isNeg = self < 0
        var val = abs(self)                       //  1234,5678
        var round = val.rounded(.down)            //  1234.0
        var rest = val - round                    //     0.5678
        var result = "\(isNeg ? "-" : "")\(Int(val))." // "-1234."

        for _ in 0..<decimal {                  
            val = rest * 10                             // 5.678
            round = val.rounded(.down)                  // 5.0
            rest = val - round                          // 0.678
            result = "\(result)\(Int(round))"           // "1234.5"               
        }
       
       return result
    }
}

works.

3

You should be able to get close to what you would expect from "normal" Swift by adding something like this

extension String.StringInterpolation {
    mutating func appendInterpolation(_ value: Double) {
        self.appendLiteral(value.description) // or your value.string(x) implementation
    }
}
extension Double {
    var description: String {
     // add Double to String logic here
    }
}

That should make print("value: \(double))" work as expected.

Depending on your embedded setup you might offload the value to string conversion to the C standard library. I am using snprintf for that through a C wrapper to get around the CVarArg not supported issue.