编译原理实验 词法分析器的设计

• 实验目的

  设计词法分析器

• 设计方法

1. 写出该语言的词法规则。
2. 把词法规则转换为相应的状态转换图。
3. 把各转换图的初态连在一起，构成识别该语言的自动机
4. 设计扫描器
5. 把扫描器作为语法分析的一个过程，当语法分析需要一个单词时，就调用扫描器。
6. 扫描器从初态出发，当识别一个单词后便进入终态，送出二元式。
7. 实验代码：

   #include<bits/stdc++.h>
   #define KEYWORD         1
   #define BORDER          2
   #define OPERATOR        3
   #define IDENTIFIER      4
   #define CONSTINTEGRAL   5
   #define ELSE            0
   using namespace std;
   
   
   string keyWord[] = { "void", "int", "char", "main", "return", "if", "else"};
   string border[] = { "(", ")", "[", "]", "{", "}", ";", "," };
   string operator0[] = { "+", "-", "*", "/", "%", "=" };
   
   
   
   bool Letter(char c)
   {
       if (c >= 'A' && c <= 'Z' || c >= 'a' && c <= 'z')
       {
           return true;
       }
       return false;
   }
   
   
   bool KeyWord(string str)
   {
       for (int i=0;i<7;i++)
       {
           string s = keyWord[i];
           if (s == str)
               return true;
       }
       return false;
   }
   
   
   bool Border(string str)
   {
       for (int i=0;i<8;i++)
       {
           string s = border[i];
           if (s == str)
               return true;
       }
       return false;
   }
   
   
   bool Operator(string str)
   {
       for (int i=0;i<6;i++)
       {
           string s = operator0[i];
           if (s == str)
               return true;
       }
       return false;
   }
   
   
   bool Digit(char c)
   {
       if (c >= '0' && c <= '9')
       {
           return true;
       }
       return false;
   }
   
   int Code(string str)
   {
       int type = ELSE;
   
       if (KeyWord(str))
           type = KEYWORD;
   
       else if (Border(str))
           type = BORDER;
   
       else if (Operator(str))
           type = OPERATOR;
   
       else if (Digit(str[0]))
           type = CONSTINTEGRAL;
   
       // 标识符
       else if (str[0] == '_' || Letter(str[0]))
       {
           type = IDENTIFIER;
           {
               for (int i = 1; i < str.length(); i++)
               {
                   // 若不符合标识符规则，则type=0
                   if (!(str[i] == '_' || Letter(str[i]) || Digit(str[i])))
                   {
                       type = ELSE;
                       break;
                   }
               }
           }
       }
       else
           type = ELSE;
   
       return type;
   }
   
   int main()
   {
       string token="";
       cout << "请输入测试代码，以#结尾" << endl;
       char  ch='*';
       do{
           ch=getchar();
           if(ch==' '||ch=='\n')
               continue;
           token=token+ch;
       }while(ch!='#');
      token.erase(token.end() - 1);
   
       cout << "token = " << token << endl;
   
       string str = "";
       int i = 0;
       while(i < token.length())
       {
           str = "";
           // 数字开头
           if (Digit(token[i]))
           {
               while (Digit(token[i]))
                   str = str + token[i++];
               cout << "(" << Code(str) << ", " << "\"" << str << "\"" << ")" << endl;
           }
           // 下划线或者字母开头的单词，即标识符或者关键字
           else if (token[i] == '_' || Letter(token[i]))
           {
               while (token[i] == '_' || Letter(token[i]) || Digit(token[i]))
               {
                   str = str + token[i++];
                   // 如果是关键字，立即结束
                   if (Code(str) == KEYWORD)
                       break;
               }
               cout << "(" << Code(str) << ", " << "\"" << str << "\"" << ")" << endl;
           }
           // 运算符
           else {
               str = str + token[i++];
               cout << "(" << Code(str) << ", " << "\"" << str << "\"" << ")" << endl;
           }
       }
   }