为什么我的 GPA Double 值在 Swift 中不能正常工作？

Question

为什么当我第一次使用 addA 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，而当它应该保持在 4.0 时。

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 中并在它下面添加与按钮中相同的代码并且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

我认为你问题的根本原因是你做的工作太多了。如果您删除 addGrades 函数（据我所知，它并没有真正添加任何功能）并用以下内容替换您的 AddA 和 SubtractA 函数，那么它将作为预期：

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

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

为了进一步开发，我建议创建一个子类数组，该数组具有加减成绩和 return GPA 的功能。这应该会使事情更具可读性和可重用性。

Answer 2

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。此外，您可能希望在您的程序中有更多的结构。将 gradesQuantity 的位置值关联到字母等级是可以的，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))

为什么我的 GPA Double 值在 Swift 中不能正常工作？

Why is my GPA Double value not working correctly in Swift?

iphone

ios

swift

ios9

swift2