diguixiajiang.txt

递归 下降分析!实现递归分析!下来看看希望对你门有帮助

实验内容：

1、使用递归下降分析算法分析表达式文法：

exp ::= exp addop term | term

addop ::= + | -

term ::= term mulop factor | factor

mulop ::= * | /

factor ::= (exp) | number

其中number可以是多位的十进制数字串（整数即可），因此这里还需要一个小的词法分析器来得

到number的值。

2、该词法分析器以子程序形式出现，当需要进行词法分析时进行调用；

3、能够识别正确和错误的表达式；

4、在进行语法分析的过程中，计算输入表达式的值。

代码如下：

package a;
import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
public class fenxi extends JFrame implements ActionListener
{
 int m;
 int right;
 int t=0;
 private JTextArea ta1;
 private JTextArea ta2;
 private JButton jb=new JButton("语法分析"); 
 private JButton jb1=new JButton("清空文本区");
 private JLabel  jl1=new JLabel("输入源代码：");
 private JLabel  jl2=new JLabel("分析结果：");
 char[] c=new char[50];
 char exp[]=new char[50];
 int result;
 public fenxi()
 {   
  super("语法分析器");  
  this.setSize(600,448);
  this.setLocation(200,150);
  this.setDefaultCloseOperation(EXIT_ON_CLOSE);
  this.setLayout(null);
  JPanel jp =new JPanel();
  JPanel jp1 =new JPanel();
  jp.setSize(300, 400);
  jp1.setSize(300, 400);
  jp.setLocation(0, 0);
  jp1.setLocation(300, 0);
  jp.setLayout(null);
  jp1.setLayout(null);
  ta1=new JTextArea(20,30);
  ta2=new JTextArea(20,30);
  ta2.setEditable(false);
     JScrollPane scrol1=new JScrollPane(ta1,JScrollPane.VERTICAL_SCROLLBAR_ALWAYS,
                JScrollPane.HORIZONTAL_SCROLLBAR_AS_NEEDED);
     JScrollPane scrol2=new JScrollPane(ta2,JScrollPane.VERTICAL_SCROLLBAR_ALWAYS,
             JScrollPane.HORIZONTAL_SCROLLBAR_AS_NEEDED);
     scrol1.setSize(260, 310);
     scrol1.setLocation(18, 40);
     scrol2.setSize(260, 310);
     scrol2.setLocation(18, 40);
        jp.add(scrol1);  
     jp1.add(scrol2);
     jb.setBounds(80, 370, 100, 25);
     jb1.setBounds(80, 370, 100, 25);
     jl1.setFont(new Font("楷书",Font.BOLD,12));
     jl1.setSize(140, 25);
     jl1.setLocation(0, 0);
     jl2.setFont(new Font("楷书",Font.BOLD,12));
     jl2.setSize(140, 25);
     jl2.setLocation(0, 0);
     jp.add(jb1);
     jp.add(jl1);
  jp1.add(jb);
  jp1.add(jl2);
  this.add(jp);
  this.add(jp1);
  jb.addActionListener(this);
  jb1.addActionListener(this);
  this.setVisible(true);
 }
 public void actionPerformed(ActionEvent e)
 {    
  if(e.getSource()==jb1)
     {
     int a1=JOptionPane.showConfirmDialog(null, "确定清空吗？","提示！",JOptionPane.YES_NO_OPTION);
     if( a1==JOptionPane.YES_OPTION)
     { 
      ta1.setText("");
         ta2.setText("");
         result=-1;
     }
     }
  if(e.getSource()==jb)
  {   
   int n=0;
   right=1;
   m=0;
   String jg;
   String a=ta1.getText();
   char[] b=new char[a.length()];
   for(int i=0;i<a.length();i++)
   {
       b[i]=a.charAt(i);
       c[i]=a.charAt(i);
         n=i;
   }
   for(int d=n+1;d<50;d++)
    c[d]='#';
   while(m<a.length()&&c[m]!='#'&&right==1)
   {
    if(Character.isDigit(c[m])||c[m]=='+'||c[m]=='-'||c[m]=='/'||c[m]=='*'||c[m]=='('||c[m]==')'||c[m]=='#')
    {
     e(c[m]);
    }
    else
    {
     ta2.setText("输入不正确，请重新输入！");
     return;
    }
    
   }
   if(c[m]=='#'&&right==1&&result!=-1)
    {
      ta2.setText("OK"+"\n");
     
         jg=String.valueOf(result);
         ta2.setText(ta2.getText()+jg);
       }
   else
    if(result!=-1)
    ta2.setText("Error"+"\n");
   
  }
 }
  int e(char i)
  {
   result = t (c[m]);
   if(result==-1)
    return -1;
    while (c[m] == '+' || c[m] == '-') 
    {
     if(c[m] == '+')
     {
      m++;
      result += t (c[m]); 
     }
      else 
       if(c[m] == '-')
       {
        m++;
        result -= t (c[m]);        
       }
    }
     return result;
  }

  int t(char i)
  {
   result = f (c[m]);
    while (c[m] == '*' || c[m] == '/') 
    {     
     if(c[m] == '*') 
     {
      m++;
      result *= f (c[m]); 
     
     }
      else 
       if(c[m] == '/')
       {
        m++;
        if(c[m]=='0')
        {
         c[m]='#';
         ta2.setText("除数不能为0，请重新输入！");
          return -1;
        }
        else
        result /= f (c[m]);
        
       }
    }
     return result;
  }
  
  int f(char i)
  {   
    if(c[m]=='(')
    {
     m++;
     t=e(c[m]);
     if(c[m]==')')
      m++;
     else
     {
      right=0;
     return right;
     }
     
    }
    else if(Character.isDigit(c[m]))
    {
     int n=0;
     
     char[] b=new char[5];
     b[0]=c[m];
     n=1;
     m++;
     while(Character.isDigit(c[m]))
     {
      b[n]=c[m];
      m++;
         n++;         
     }
     
     for(;n<5;n++)
      b[n]='#';
     t=(int)b[0]-48;
     n=1;
     while(n<5)
     {
        if(Character.isDigit(b[n]))
        {
         t=t*10+((int)b[n]-48);
            n++;
        }
        else
         n=5;
     }
     
    }
    else
    {
     right=0;
     return right;
    }
    
   return t;
   
  }
  
 
 public static void main(String args[])
 {
  new fenxi();
 }
}