Inheritance error

Cocoa Programming for OS X (5th Edition) 3. Structures and Classes
1

I have copied the code for Rocket class, however the compiler prints error: Designanted initializer cannot delegate(with ‘self.init’) on the first selected line and Cannot assign to ‘thrust’ in ‘self’. Could you please with this issue?
The code I have used(I copied it from the book without any changes):

class Rocket: Particle { let thrust: Double var thrustTimeRemaining: NSTimeInterval let direction = Vector(x: 0, y: 1) convenience init(thrust: Double, thrustTime: NSTimeInterval) { self.init(position: Vector(), thrust: thrust, thrustTime: thrustTime) } // First line of error init(position: Vector, thrust: Double, thrustTime: NSTimeInterval) { // Second line of error self.thrust = thrust self.thrustTimeRemaining = thrustTime self.init(position: position) } }
Somewhere above:

class Particle { var position: Vector var velocity: Vector var acceleration: Vector init(position: Vector) { self.position = position self.velocity = Vector() self.acceleration = Vector() } convenience init() { self.init(position: Vector()) } func tick(dt: NSTimeInterval) { velocity = velocity + acceleration * dt position = position + velocity * dt position.y = max(0, position.y) } }

2

Simple typo; just hard to see.

In this code:

// First line of error init(position: Vector, thrust: Double, thrustTime: NSTimeInterval) { // Second line of error self.thrust = thrust self.thrustTimeRemaining = thrustTime self.init(position: position) }
That last ‘self.init’ is the issue; it should be referencing the superclass via ‘[color=#FF0000]super[/color].init’:

// First line of error init(position: Vector, thrust: Double, thrustTime: NSTimeInterval) { // Second line of error self.thrust = thrust self.thrustTimeRemaining = thrustTime super.init(position: position) }
Fixing that takes care of everything.

3

I am still getting an error when I try to implement the Rocket Subclass.

I have checked and rechecked and I can’t see any variation from what is in the book. However I get an error

[code]class Rocket: Particle {

let thrust: Double
var thrustTimeRemaining: NSTimeInterval
let direction = Vector(x: 0, y: 1)

convenience init(thurst: Double, thrustTime: NSTimeInterval)
{
    self.init(position: Vector(), thrust: thrust, thrustTime: thrustTime)
}

init(position: Vector, thrust: Double, thrustTime: NSTimeInterval)
{
    self.thrust = thrust
    self.thrustTimeRemaining = thrustTime
    super.init(position: position)
}

override func tick(dt: NSTimeInterval) {
    if thrustTimeRemaining > 0.0 {
        let thrustTime = min (dt, thrustTimeRemaining)
        let thrustToApply = thrust * thrustTime
        let thrustForce = direction * thrustToApply
        acceleration = acceleration + thrustForce
        thrustTimeRemaining -= thrustTime
    }
    super.tick(dt)
}

}[/code]

4

I found the issue. Actually a friend found it. I had thrust misspelled in a couple of spots.

class Rocket: Particle {

    let thrust: Double
    var thrustTimeRemaining: NSTimeInterval
    let direction = Vector(x: 0, y: 1)

    convenience init(thrust: Double, thrustTime: NSTimeInterval)
    {
        self.init(position: Vector(), thrust: thrust, thrustTime: thrustTime)
    }
    
    init(position: Vector, thrust: Double, thrustTime: NSTimeInterval)
    {
        self.thrust = thrust
        self.thrustTimeRemaining = thrustTime
        super.init(position: position)
    }

    override func tick(dt: NSTimeInterval) {
        if thrustTimeRemaining > 0.0 {
            let thrustTime = min (dt, thrustTimeRemaining)
            let thrustToApply = thrust * thrustTime
            let thrustForce = direction * thrustToApply
            acceleration = acceleration + thrustForce
            thrustTimeRemaining -= thrustTime
        }
        super.tick(dt)
    }
}
5

By the way, I recommend skipping all the examples in Chapter 3, and instead read the section in the Swift language guide called Classes and Structures here:

developer.apple.com/library/ios … -CH13-ID82

The examples in Chapter 3 are confusing even if you know some physics, and if you don’t know any physics then they are a complete waste of time. In addition, the plot in the Playground is very misleading. Personally, I’ve abandoned using Playgrounds all together because I find using a Swift Command Line Tool a much better alternative.