用于提取数学的类或结构（Swift）

Question

hi all i'm very new to programming and it's turning into a bit of a mission.

I'm trying to create a class with function to do some basic maths. However I'm still learning how to extract the new value and maybe either pass in on within my class/func or pass into another.

here is what I have so far

class fuel {
var rate = 3.7
var laps: Double = 13.0
var totalFuel: Double

init (rate: Double, laps: Double, totalFuel: Double){
    self.rate = rate
    self.laps = laps
    self.totalFuel = totalFuel

    }
func fuelMaths (fuel: Double) -> Double {
    self.totalFuel = (rate * Double(laps))
    print(self.totalFuel)

    }

} fuelMaths(self.totalFuel)

I'm getting an unresolved identifier error when I call "fuelMaths" or am I meaning to of created a struct?

Thanks in advance.

Answer 1

Regarding class vs. struct, this seems like a struct is a better choice because it only contains a few stored properties which are all value types.

Even so, here are the issues:

1) Your fuelMaths instance method is not returning a value. Your implementation should resemble this:

func fuelMaths(fuel:Double) -> Double {
    // "someValue" contains your result
    return someValue
}

2) The reason you are getting an unresolved identifier error is because you need to first declare an instance of fuel , and then call fuelMaths on that instance:

// someRate, someLaps, someTotal, and someFuelValue previously defined

let myFuel = fuel(rate: someRate, laps: someLaps, totalFuel: someTotal)
let myReturnedValue = myFuel.fuelMaths(someFuelValue)

Finally, it appears that totalFuel is a good candidate for a computed property instead of a stored property. This eliminates your need for the fuelMaths method. Just perform the calculations that are in fuelMaths inside the body of your computed property instead. It also means that your initializer only needs to provide two arguments for rate and laps :

var totalFuel: Double {
    return rate * laps
}

init(rate:Double, laps:Double) { //... }

One more thing, the convention is to capitalize the names of your classes and structs.

EDIT:

The class version of your code might look like this:

class Fuel {
    var rate = 3.7  // default value tells compiler this is a Double
    var laps = 13.0
    var totalFuel: Double {
        return rate * laps
    }

    init(rate:Double, laps:Double) {
        self.rate = rate
        self.laps = laps
    }
}

The struct version of your code might look like this (notice that it doesn't require you to explicitly define an initializer, unlike a class):

struct Fuel {
    var rate = 3.7
    var laps = 13.0
    var totalFuel: Double {
        return rate * laps
    }
}