Question

当运算符是变量字符时，执行数学计算的正确方法是什么。换句话说，在编译时不知道该操作。需要在运行时动态选择操作。

例如，假设我需要计算+的总和，但Char运算符存储在Dim operator As Char = "+"c Dim result As Long = 10 operator 20变量中，如下所示：

package hgp.pkg13.pkg16;

import java.util.Scanner;

public class HGP1316 {
    //Declaration of variables
    static int Running_total = 0;
    static double Average = 0.0;
    static int User_input = 0;
    static double counter = 0.0 ;
    static int Highest_input = -1;
    static int Lowest_input = -1; 

    public static void main(String[] args) {
        //Intro Program (Optional)
    {
        System.out.println("This program will give you the highest, lowest, and average of the integers you \n"
          + "enter. To end enter a negative number ");       
    }

    //2. Ask user to enter a positive integer (or a negative integer to stop)
    //3. Record the user's input
    //4. Test the input >= 0
    while (User_input >=0){
        Scanner user_input = new Scanner( System.in );
        Integer User_input;
        System.out.print("Please enter an integer: ");
        User_input = user_input.nextInt();
        //4a. If it is true that input >= 0
        if (User_input >= 0)
        {

            //4a1Add the input to "running total"
            Running_total = Running_total + User_input;
            //4a2. Increment "number of inputs counter"
            counter = counter + 1;  
            //4a3. Test if "number of inputs counter" is equal to 1
            if (counter == 1)
            {
                //4a31. If true, replace both "highest input" and "lowest input" with the input
                Highest_input = User_input;
                Lowest_input = User_input;
            }

            //4a5. If the input > "highest input" then replace "highest input" with input
           if (User_input > Highest_input)
           {
               Highest_input = User_input;
           }
           //4a6. If the input < "lowest input" then replace "lowest input" with input
           if (User_input < Lowest_input)
           {
               Lowest_input = User_input;
           }
           //4b. If false
           //4b1. Goto step 5
           else;
           {
               //5. Calculate average (formula: "running total" /"number of inputs counter" )
                Average = (Running_total / counter);
                //6. Display/communicate "highest input", "lowest input" and average
                System.out.println ("The Highest value entered was : "+ Highest_input);
                System.out.println ("The Lowest value entered was : " + Lowest_input);
                System.out.println("The Average of enteries was : "+ Average);
                //7. Terminate
                }
            }
        }
    }
}

Answer 1

简单的方法是使用Select Case语句使用基于输入字符的正确操作来执行计算。例如：

' Binary, as in not unary nor ternary
Public Function PerformBinaryOperation(operand1 As Long, operator As Char, operand2 As Long)
    Select Case operator
        Case "+"c
            Return operand2 + operand2
        Case "-"c
            Return operand2 - operand2
        ' ...
    End Select
End Function

更高级的解决方案是编写表达式解析器来分析表达式字符串，例如"10 + 20"，并将其转换为expression tree。如果您不想编写自己的解析器，可以使用其中一个库来执行此操作，例如NCalc。

表达式树解析的另一种替代方法是生成一段.NET代码，然后在运行时动态构建和执行该代码。例如，如果你做了这样的事情：

Dim operator As Char = "+"c
Dim codeBuilder As New StringBuilder()
codeBuilder.AppendLine("Public Class MyOperation")
codeBuilder.AppendLine("    Public Function Calculate() As Long")
codeBuilder.AppendLine("         Return 10 " & operator & " 20")
codeBuilder.AppendLine("    End Function")
codeBuilder.AppendLine("End Class")
Dim myOperation As Object = ' ... compile code and instantiate object dynamically
Dim result As Long = myOperation.Calculate()

传统上，您可以使用CodeDom类来执行动态编译，但现在使用.NET 4.6中的新Roslyn编译器，您可以使用新的Emit功能。 Here是一篇介绍各种动态编译选项的文章。

当operator是自定义字符时执行数学计算

1 个答案: