住房贷款和摊销计划的每月分期付款(EMI)

Question

对我的密码有什么评论吗？

数据模型

public class LoanRepaymentPlan
{
    public decimal TotalRepaymentAmount { get; private set; }
    public decimal CurrentOutstandingAmount { get; private set; }
    public decimal MonthlyRepaymentInterestAmount { get; private set; }
    public decimal MonthlyRepaymentAmount { get; private set; }
    public DateTime RepaymentDate { get; private set; }

    private LoanRepaymentPlan(decimal TotalRepaymentAmount, decimal CurrentOutstandingAmount, decimal MonthlyRepaymentAmount)
    {
        this.TotalRepaymentAmount = TotalRepaymentAmount;
        this.CurrentOutstandingAmount = CurrentOutstandingAmount;
        this.MonthlyRepaymentAmount = MonthlyRepaymentAmount;
    }

    public static LoanRepaymentPlan Create(decimal TotalRepaymentAmount, decimal CurrentOutstandingAmount, decimal MonthlyRepaymentAmount)
    {
        return new LoanRepaymentPlan(TotalRepaymentAmount, CurrentOutstandingAmount, MonthlyRepaymentAmount);
    }
}

主计算

static void Main(string[] args)
{
    Console.WriteLine("Equated Monthly Installment (EMI) for Home Loan");

    Console.WriteLine("Total Loan Amount: ");
    var principal = Convert.ToDouble(Console.ReadLine());

    Console.WriteLine("Interest Rate Per Annum (%): ");
    var interestRatePerAnnum = Convert.ToDouble(Console.ReadLine()) / 100;

    Console.WriteLine("Loan Period (in years): ");
    var years = Convert.ToDouble(Console.ReadLine());

    Console.WriteLine("------------------------------------------------");

    Console.WriteLine("Monthly installment: " + HousingLoanInterest(principal, interestRatePerAnnum, years).ToString("C2"));

    decimal totalRepaymentAmount = 0;
    var monthlyRepaymentAmount = HousingLoanInterest(principal, interestRatePerAnnum, years);
    var tenure = years * 12;
    var principalAndInterest = CompoundInterest(principal, interestRatePerAnnum, 12, years);
    List<LoanRepaymentPlan> repaymentPlans = new List<LoanRepaymentPlan>();

    while (tenure >= 0)
    {
        totalRepaymentAmount = totalRepaymentAmount + (decimal)monthlyRepaymentAmount;
        tenure = tenure - 1;

        repaymentPlans.Add(LoanRepaymentPlan.Create(totalRepaymentAmount,
            (decimal)principalAndInterest - totalRepaymentAmount, (decimal)monthlyRepaymentAmount));
    }

    var table = new ConsoleTable("Current Outstanding Amount", "Monthly Repayment Amount", "Total Repayment Amount");
    foreach (var item in repaymentPlans)
    {
        table.AddRow(item.CurrentOutstandingAmount, item.MonthlyRepaymentAmount, item.TotalRepaymentAmount);
    }
    table.Write();
}

帮助方法

static double CompoundInterest(double principal, double interestRate, int timesPerYear, double years)
{
    // (1 + r/n)
    double body = 1 + (interestRate / timesPerYear);

    // nt
    double exponent = timesPerYear * years;

    // P(1 + r/n)^nt
    return principal * Math.Pow(body, exponent);
}

static double HousingLoanInterest(double loanAmount, double interestRate, double years)
{
    return (loanAmount * Math.Pow((interestRate / 12) + 1,
         (years * 12)) * interestRate / 12) / (Math.Pow
         (interestRate / 12 + 1, (years * 12)) - 1);
}

这一计算有些不准确，但我以后会设法弄清楚的。困扰我的一件事是Math.Pow，它使用double。我已尝试将计算转换为decimal，但目前无法这样做。

Answer 1

var

特别是考虑到您有准确性问题，对于您自己理解代码在以下声明中所做的事情非常重要：

    var monthlyRepaymentAmount = HousingLoanInterest(principal, interestRatePerAnnum, years);
    var tenure = years * 12;
    var principalAndInterest = CompoundInterest(principal, interestRatePerAnnum, 12, years);

仅从这些行，我就不知道它们是否应该是双数，整数，小数等等，只需拼出实际的类型。

就地操作

        totalRepaymentAmount = totalRepaymentAmount + (decimal)monthlyRepaymentAmount;
        tenure = tenure - 1;

可以是

totalRepaymentAmount += monthlyRepaymentAmount;
tenure--;

格式

这可以从一些临时变量中受益，但即使没有这些变量，

    return (loanAmount * Math.Pow((interestRate / 12) + 1,
         (years * 12)) * interestRate / 12) / (Math.Pow
         (interestRate / 12 + 1, (years * 12)) - 1);

更易读懂

return (
    loanAmount * Math.Pow(
        interestRate / 12 + 1,
        years * 12
    ) * interestRate / 12
) / (
    Math.Pow(
        interestRate / 12 + 1,
        years * 12
    ) - 1
);

而这又相当于

double annual = interestRate / 12;
double pow = Math.Pow(
    annual + 1,
    years * 12
);
return loanAmount * pow * annual / (pow - 1);

更简单：

double annual = interestRate / 12,
       pow = Math.Pow(
           annual + 1,
           -12 * years
       );
return loanAmount * annual / (1 - pow);

请测试这个是否等价。

Answer 2

@Reinderien提到了一些很大的改进点，所以我不会重复它们。

还有其他领域可以微调。

LoanRepaymentPlan -属性

在这种情况下，您不需要显式地为属性指定private set。如果您只指定getter，那将是非常好的，因为ctors可以设置属性，即使它们不公开getter。

public class LoanRepaymentPlan
{
    public decimal TotalRepaymentAmount { get; }
    public decimal CurrentOutstandingAmount { get; }
    public decimal MonthlyRepaymentInterestAmount { get; }
    public decimal MonthlyRepaymentAmount { get; }
    public DateTime RepaymentDate { get; }

    public LoanRepaymentPlan(decimal TotalRepaymentAmount, decimal CurrentOutstandingAmount, decimal MonthlyRepaymentAmount)
    {
        this.TotalRepaymentAmount = TotalRepaymentAmount;
        this.CurrentOutstandingAmount = CurrentOutstandingAmount;
        this.MonthlyRepaymentAmount = MonthlyRepaymentAmount;
    }
}

使用这种方法，您已经创建了一个不可变的类(在初始化之后不能修改)。从内存优化的角度来看，将class更改为struct是有意义的。这样，它很有可能被分配到堆栈上(甚至在CPU寄存器中)，因此不需要为这个对象分配堆，也不需要gargabe集合。

还有另一个有趣的话题，目前还没有，但值得一提的是C# 9记录。在C# 9中，可以通过以下代码实现相同的目标：

public data class LoanRepaymentPlan
{
    public decimal TotalRepaymentAmount { get; init; }
    public decimal CurrentOutstandingAmount { get; init; }
    public decimal MonthlyRepaymentInterestAmount { get; }
    public decimal MonthlyRepaymentAmount { get; init; }
    public DateTime RepaymentDate { get; }    
}

LoanRepaymentPlan -静态构造函数

在我看来，这没有多大意义。您必须在每次初始化时多写两个字符，并且必须维护两个对象创建者函数。我不明白这个静电发生器的全部意义。

输入处理

您的代码在双小数类型之间跳来跳去。如果精度很重要，那么在任何地方都使用decimal，否则就使用double。

Convert.ToDouble(Console.ReadLine())确实容易出错。如果用户输入“一些字符串”怎么办？它会抛出一个FormatException。使用double.TryParse代替：

var userInput = Console.ReadLine();
if(!double.TryParse(userInput, out var principal))
{
  //TODO: handle that case
}

您还应该考虑根据有效范围验证用户输入。例如，利率应该在0到100之间。

while vs for循环

尽管在我看来，while循环工作得很好，但for循环可能更有表现力和简洁性。

for (var tenure = years * 12; tenure >= 0; tenure--)
{
    totalRepaymentAmount += (decimal)monthlyRepaymentAmount;

    repaymentPlans.Add(LoanRepaymentPlan.Create(totalRepaymentAmount,
        (decimal)principalAndInterest - totalRepaymentAmount, (decimal)monthlyRepaymentAmount));
}

迭代器变量在被修改的地方声明。

它提高了可读性和可维护性。

函数分解

我强烈鼓励你把你的主要职能分成几个小块，专门负责：

1. 收集用户输入以进行计算
2. 执行计算
3. 转储计算结果

它还将提高可测试性、可维护性和可读性。