I thought to make the conversion shorter this way. It didn't work. The outputNumber doesn't update and doesn't make coversion when I prompt number into TextField. Why it doesn't work?

    @State private var inputNumber = ""
    @State private var inputUnit = 0
    @State private var outputUnit = 1

    let unitsList = ["meters", "kilometers", "feet", "yards", "miles"]

    var outputNumber: Measurement<UnitLength> {
        let inputValue = Double(inputNumber) ?? 0

        return Measurement(value: inputValue, unit: UnitLength.init(symbol: unitsList[inputUnit])).converted(to: UnitLength.init(symbol: unitsList[outputUnit]))

    }

Switch implemenatation that works

    var imperialUnitsInMeters: Measurement<UnitLength> {
        let inputValue = Double(inputNumber) ?? 0

        switch unitsList[inputUnit] {
        case "meters":
            return Measurement(value: inputValue, unit: UnitLength.meters)
        case "kilometers":
            return Measurement(value: inputValue, unit: UnitLength.kilometers).converted(to: UnitLength.meters)
        .........................................
        .........................................
        default:
            return Measurement(value: inputValue, unit: UnitLength.meters)
        }
    }

    var outputNumber: Measurement<UnitLength> {
        let inputValue = Double(imperialUnitsInMeters.value)

        switch unitsList[outputUnit] {
        case "meters":
            return Measurement(value: inputValue, unit: UnitLength.meters)
        case "kilometers":
            return Measurement(value: inputValue, unit: UnitLength.meters).converted(to: UnitLength.kilometers)
        .........................................
        .........................................
        default:
            return Measurement(value: inputValue, unit: UnitLength.meters)
        }
    }

I haven't used the init(symbol:) before, but if it works, then you should use the correct symbols in the array, so ["m", "km"] and so on...

But the recommendation is to use an array of UnitLength instead of String for your units.

@MarcusKay , I checked this out. Didn't work.

      let unitsList = ["m", "km", "ft", ...]

This shows the correct units; also, units change when switching the picker choice, but the value reamins 0

      Text("\(outputNumber.description)")

Well then your solution is to use an array of UnitLength:

let unitsList: [UnitLength] = [.meters, .kilometers, ...]

For your picker you can then use the symbol to display.

We discussed this in depth over on this thread if you want more information: https://www.hackingwithswift.com/forums/100-days-of-swiftui/day-19-challenge-is-there-a-better-way-for-this-code/2682