ios - 为什么我的 GPA Double 值在 Swift 中不能正常工作？

Question

为什么当我第一次添加 A 时这段代码不起作用，但后来它变为 2.66 并从那里继续下降，而它应该保持在 4.0。

import UIKit

//Quantity of          A    A-   B+   B    B-   C+   C    C-   D+   D    D-   F
var gradesQuantity = [0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0]
var gradesAdded = 0.0
var gpa = 0.0

func addGrades(grades: [Double]) -> Double {
    for grade in grades {
        gradesAdded += grade
        if gradesQuantity[0] <= 0 {
            gradesQuantity[0] = 0
        }
    }

    return gradesAdded
}

func calcGPA(grades: [Double]) -> Double {
    gpa = (grades[0] * 4.0 + grades[1] * 3.7 + grades[2] * 3.3 + grades[3] * 3.0 + grades[4] * 2.7 + grades[5] * 2.3 + grades[6] * 2.0 + grades[7] * 1.7 + grades[8] * 1.3 + grades[9] * 1.0 + grades[10] * 0.7) / gradesAdded

    return gpa
}

class ViewController: UIViewController {

    @IBOutlet weak var GPALabel: UILabel!
    @IBOutlet weak var GPANumber: UILabel!
    @IBOutlet weak var AQuantity: UILabel!

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }

    @IBAction func AddA() {
        gradesQuantity[0] += 1.0
        addGrades(gradesQuantity)
        let x = calcGPA(gradesQuantity)
        GPANumber.text = String(x)
        AQuantity.text = String(Int(gradesQuantity[0]))
    }

    @IBAction func SubtractA() {
        gradesQuantity[0] -= 1.0
        addGrades(gradesQuantity)
        let x = calcGPA(gradesQuantity)
        GPANumber.text = String(x)
        AQuantity.text = String(Int(gradesQuantity[0]))
    }
}

有人可以帮我把它放在 Playground 上，并在它下面添加与按钮相同的代码，它可以工作，但在项目中它不起作用。这让我相信问题出在 addA 和 subtractA 按钮上。

Answer 1

您正在编写具有副作用的函数(修改外部变量)。尽量避免这种情况。另外，您可能希望程序中有更多的结构。将 gradesQuantity 的位置值与字母等级相关联是可以的，但您可以使用字典做得更好:

enum GradeLetter: Double {
    case A = 4.0
    case AMinus = 3.7
    case BPlus = 3.3
    case B = 3
    case BMinus = 2.7
    case CPlus = 2.3
    case C = 2
    case CMinus = 1.7
    case DPlus = 1.3
    case D = 1
    case DMinus = 0.7
    case F = 0
}

func calculateGPA(grades: [GradeLetter: Int]) -> Double {
    let courseCount = grades.reduce(0.0) { aggregate, grade in
        return aggregate + Double(grade.1)
    }
    let totalPoint = grades.reduce(0.0) { aggregate, grade in
        return aggregate + grade.0.rawValue * Double(grade.1)
    }

    return totalPoint / courseCount
}

假设您有 1 个 A、2 个 A- 和 3 个 B:

var grades = [GradeLetter: Int]()
grades[.A] = 1
grades[.AMinus] = 2
grades[.B] = 3

print(calculateGPA(grades))