20155237 第十一周java课堂程序

20155237 第十一周java课堂程序

内容一：后缀表达式

abcde/-f+

内容二：实现Linux下dc的功能，计算后缀表达式的值

填充下列代码：

import java.util.Scanner;

           public class MyDCTester  {

            public static void main (String[] args) {

            String expression, again;

             int result;

             try
             {
                   Scanner in = new Scanner(System.in);

                   do
                       {
                         MyDC evaluator = new MyDC();
                         System.out.println ("Enter a valid postfix expression: ");
                         expression = in.nextLine();

                         result = evaluator.evaluate (expression);
                         System.out.println();
                         System.out.println ("That expression equals " + result);

                         System.out.print ("Evaluate another expression [Y/N]? ");
                         again = in.nextLine();
                         System.out.println();
                       }
                   while (again.equalsIgnoreCase("y"));
             }
             catch (Exception IOException)
             {
                   System.out.println("Input exception reported");
                 }
           }
  }
  import java.util.StringTokenizer;
   import java.util.Stack;
           public class MyDC
  {
             /** constant for addition symbol */
             private final char ADD = '+';
           /** constant for subtraction symbol */
           private final char SUBTRACT = '-';
           /** constant for multiplication symbol */
           private final char MULTIPLY = '*';
           /** constant for division symbol */
           private final char DIVIDE = '/';
           /** the stack */
           private Stack<Integer> stack;

           public MyDC() {
             stack = new Stack<Integer>();
           }

           public int evaluate (String expr)
           {
             int op1, op2, result = 0;
             String token;
             StringTokenizer tokenizer = new StringTokenizer (expr);

             while (tokenizer.hasMoreTokens())
             {
               token = tokenizer.nextToken();

              **//如果是运算符，调用isOperator
               if ()
               {
                   //从栈中弹出操作数2
                  //从栈中弹出操作数1
                 //根据运算符和两个操作数调用evalSingleOp计算result;
                   //计算result入栈;
               }
               else//如果是操作数
                   //操作数入栈;
             }**

             return result;
           }

           private boolean isOperator (String token)
           {
             return ( token.equals("+") || token.equals("-") ||
                      token.equals("*") || token.equals("/") );
           }

           private int evalSingleOp (char operation, int op1, int op2)
           {
             int result = 0;

             switch (operation)
                    {
               case ADD:
                 result = op1 + op2;
                 break;
               case SUBTRACT:
                 result = op1 - op2;
                 break;
               case MULTIPLY:
                 result = op1 * op2;
                 break;
               case DIVIDE:
                 result = op1 / op2;
             }

             return result;
           }
  }

填写的代码是：
//如果是运算符，调用isOperator
if ()
{
//从栈中弹出操作数2
//从栈中弹出操作数1
//根据运算符和两个操作数调用evalSingleOp计算result;
//计算result入栈;
}
else//如果是操作数
//操作数入栈;
}

//当输入是运算符
调用isOperator方法片段所输入的是否为运算符，是运算符则为true，不是则为false，当是运算符的时候进行出栈、运算、将结果进栈的操作

根据提示信息和栈的基础操作，从栈中弹出一个数字为stack.pop()，并将此值赋给int a,

根据所给的evalSingleOp对前两个操作数进行加减乘除，并用charAt(0)方法取得字符串的运算符号，进行相应的运算。

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