20155333 第十一周课堂练习

第十一周课堂内容

后缀表达式

ab*cde/-f*+

补充代码

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 = '/';
private Stack 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;
       }
    }

需要补充的部分

//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;
            }

调用isOperator方法判断所输入的是否为运算符

若输入运算符，则

//如果是运算符，调用isOperator
if (isOperator(token))

根据提示信息和栈的基础操作，从栈中弹出操作数stack.pop(),即

 op1 = stack.pop();//从栈中弹出操作数2
 op2 = stack.pop(); //从栈中弹出操作数1

调用evalSingleOp对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。

result = evalSingleOp(token.charAt(0),op1,op2);//根据运算符和两个操作数调用evalSingleOp计算result;

根据提示进行入栈操作

stack.push(result); //计算result入栈;

若输入的是操作数，则

stack.push(Integer.parseInt(token));//操作数入栈;

测试代码

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");
                 }
           }
  }