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。