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

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");
}
}
}

测试结果截图