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iOS Build a Simple iPhone App with Swift 2.0 Getting Started with iOS Development Swift Recap Part 2

Caleb Kleveter
MOD
Caleb Kleveter
Treehouse Moderator 37,862 Points

Swift re-cap part 2.

Can someone explain this challenge to me? I am thoroughly confused over it. Great job Pasan :).

Here are the instructions:

In the editor you've been provided with two classes - Point to represent a coordinate point and Machine. The machine has a move method that doesn't do anything because most machines are motionless.

Your task is to subclass Machine and create a new class named Robot. In the Robot class, override the move method and provide the following implementation. If you enter the string "Up" the y coordinate of the Robot's location increases by 1. "Down" decreases it by 1. If you enter "Left", the x coordinate of the location property decreases by 1 while "Right" increases it by 1.

Note: If you use a switch statement you can use the break statement in the default clause to exit the current iteration.

classes.swift
class Point {
    var x: Int
    var y: Int

    init(x: Int, y: Int){
        self.x = x
        self.y = y
    }
}


class Machine {
    var location: Point

    init() {
        self.location = Point(x: 0, y: 0)
    }

    func move(direction: String) {
        print("Do nothing! I'm a machine!")
    }
}

// Enter your code below

Here is the error I got:

swift_lint.swift:31:12: error: binary operator '~=' cannot be applied to operands of type 'String' and 'Point'
      case "Up": y += 1
           ^~~~
swift_lint.swift:31:18: error: use of unresolved identifier 'y'
      case "Up": y += 1
                 ^
swift_lint.swift:32:12: error: binary operator '~=' cannot be applied to operands of type 'String' and 'Point'
      case "Down": y -= 1
           ^~~~~~
swift_lint.swift:32:20: error: use of unresolved identifier 'y'
      case "Down": y -= 1
                   ^
swift_lint.swift:33:12: error: binary operator '~=' cannot be applied to operands of type 'String' and 'Point'
      case "Left": x -= 1
           ^~~~~~
swift_lint.swift:33:20: error: use of unresolved identifier 'x'
      case "Left": x -= 1
                   ^
swift_lint.swift:34:12: error: binary operator '~=' cannot be applied to operands of type 'String' and 'Point'
      case "Right": x += 1
           ^~~~~~~
swift_lint.swift:34:21: error: use of unresolved identifier 'x'
      case "Right": x += 1
                    ^

This is the code:

class Robot: Machine {
  func move() { 
    switch location {
      case "Up": y += 1
      case "Down": y -= 1
      case "Left": x -= 1
      case "Right": x += 1
      default: break
    }
  }
}

9 Answers

Chase Marchione
Chase Marchione
155,055 Points

Fully accounting for:

  • The overriding of the Machine class's move function (with the direction String as a parameter.)
  • The switch statement being named for the data we are investigating (the direction String data), rather than location.
  • x and y being of the location Point.
class Robot: Machine {
  override func move(direction: String) { 
    switch direction {
      case "Up": location.y += 1
      case "Down": location.y -= 1
      case "Left": location.x -= 1
      case "Right": location.x += 1
      default: break
    }
  }
}

Hope this helps!

Christoph Hellmuth
Christoph Hellmuth
11,604 Points

Good job :) Was literally coming to the same solution just 5 minutes before you posted it. Took me a while but was worth the learnings.

Here is my solution:

class Point {
  var x: Int
  var y: Int

  init(x: Int, y: Int) {
    self.x = x
    self.y = y
  }
}

class Machine {
  var location: Point

  init() {
    self.location = Point(x: 0, y: 0)
  }

  func move(_ direction: String) {
    print("Do nothing! I am a machine!")
  }
}

// Enter your code below
class Robot: Machine {

  override func move(_ direction: String) {
    switch direction {
      case "Up": return location.y += 1
      case "Down": return location.y -= 1
      case "Left": return location.x -= 1
      case "Right": return location.x += 1
      default: break
    }
  }

}

Can someone explain why there is no init - super init needed with creating the Robot sub-class? Thank you

You want to create a new class, Robot, that inherits from the Machine class and overrides the move method that comes with the Machine class.

so basically:

class Robot: Machine {
    override func move() {
        //whatever
    }
}

check out the following link for some clear examples of subclassing and overriding: https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Inheritance.html

My immediate suspicion is scoping issues. Try self.x in the switch case.

Almost sure

I see a couple of things that should help you. Make sure you properly override the move function:

func override move(direction: String) {
}

Your switch control expression should be "direction" rather than "location" since "Up, Down, Right, and Left" are the directions.

Also try location.y instead of just y.

Hopefully this helps you.

Christoph Hellmuth
Christoph Hellmuth
11,604 Points

func override move(direction: String) { }

Didn't work for me but: override func move(direction: String) {} did.

Also thank you for the direction tip.

Robert Cadorette thats a good point

i think there need to be some changes to the given code as well as the new

-------------given code needs-------------

class Machine {
  var location: Point
// var direction: String ///

  init() {
    self.location = Point(x: 0, y: 0)
 // not sure what code would in be used here//
  }

  func move(_ direction: String) {
    print("Do nothing! I am a machine!")
  }
}

--------------------new code--------------------

/// the code need to do///
///direction = location(move)///
/// for example "Up" = (location(x: 0, y: 0) + y: 1)///
/// the switch case would give the value the class would change to///

Using a switch statement is probably a good way to go for this challenge; however, I'm providing an alternative answer below for those wanting to solve this using an 'if' statement:

class Point {
  var x: Int
  var y: Int

  init(x: Int, y: Int) {
    self.x = x
    self.y = y
  }
}

class Machine {
  var location: Point

  init() {
    self.location = Point(x: 0, y: 0)
  }

  func move(_ direction: String) {
    print("Do nothing! I am a machine!")
  }
}

class Robot: Machine {

  override func move(_ direction: String) {
    if direction == "Up" {
            self.location.y += 1
    } else if direction == "Down" {
            self.location.y -= 1
    } else if direction == "Left" {
            self.location.x -= 1
    } else if direction == "Right" {
            self.location.x += 1
    } else {
            self.location = location
    }
  }
}
alexander88
alexander88
10,824 Points

Random question, why is it that when I put

''' case "Up": y += 1 '''

versus

''' case "Up": location.y += 1'''

The first fails while the second succeeds? is it related to scope? The only reasoning I can really piece together is that x and y exist as variables within point, but inside the class Machine they only exist within location and Point, therefore they cannot be altered as individual variables. However, if we tried to alter them within the class Point, or simply outside any sort of object it would work? Trying to get a handle on scope and classes in general, any explanation or help is appreciated. Thanks in advanced.

Caleb Kleveter
Caleb Kleveter
Treehouse Moderator 37,862 Points

Hey Alexander, this should be posted as a new question instead of as an answer. You'll have a better chance of getting an answer and it keeps this thread clean. Thanks!