一个词法分析器

正在学习编译原理，试着写了一个词法分析器

  1 #include<iostream>
  2 #include<string>
  3 #include <ctype.h>
  4 using namespace std;
  5 void scaner();      //扫描子程序
  6 char getch();       //读入字符函数
  7 char check();       //检查读入字符是否为空格或注释
  8 int synvalue(int n);//求关键字种别码函数
  9 char prog[80],token[8];
 10 char ch;
 11 int syn,p=0,m,n,sum;//p是缓冲区prog指针，m是token指针
 12 char *rwtab[6]={"begin","if","then","while","do","end"};
 13 int main()
 14 {
 15     p=0;
 16     cout<<"please input string:\n";
 17     do
 18     {
 19         ch=cin.get();
 20         prog[p++]=ch;
 21     }while(ch!='#');
 22     p=0;
 23     do
 24     {
 25         scaner();
 26         switch(syn)
 27         {
 28         case 11:cout<<"("<<syn<<","<<sum<<")"<<endl;
 29             break;
 30         case -1:cout<<"have error."<<endl;
 31             break;
 32         case 10:cout<<"("<<syn<<","<<"'"<<token<<"'"<<")"<<endl;
 33             break;
 34         default:cout<<"("<<syn<<","<<token<<")"<<endl;
 35         }
 36     }while(syn!=0);
 37     return 0;
 38 }
 39 void scaner()
 40 {
 41     for(n=0;n<8;n++)
 42         token[n]=NULL;
 43     ch=getch();//读入下一个字符
 44     check();//检查读入字符是否是空格或注释
 45     m=0;
 46     if(isalpha(ch))
 47     {
 48         while(isalpha(ch)||isdigit(ch))
 49         {
 50             token[m++]=ch;
 51             ch=getch();
 52         }
 53         token[m++]='\0';
 54         p--;//回退一个字符
 55         syn=10;
 56         for(n=0;n<6;n++)
 57         {
 58             if(strcmp(token,rwtab[n])==0)
 59             {
 60                 syn=synvalue(n);//返回种别码syn
 61                 break;
 62             }
 63         }
 64     }
 65     else
 66         if(isdigit(ch))
 67         {sum=0;
 68             while(isdigit(ch))
 69             {
 70                 
 71                 sum=sum*10+ch-'0';
 72                 ch=getch();
 73             }
 74             p--;
 75             syn=11;
 76         }
 77     else
 78     switch(ch)
 79     {
 80       case'<': m=0;
 81                token[m++]=ch;
 82                ch=getch();
 83                if(ch=='>')
 84                {
 85                   syn=21;
 86                   token[m++]=ch;
 87                }
 88                else if(ch=='=')
 89                {
 90                   syn=22;
 91                   token[m++]=ch;
 92                }
 93                else 
 94                {
 95                   syn=20;
 96                  p--;
 97                }
 98                break;
 99       case'>':token[m++]=ch;
100              ch=getch();
101              if(ch=='=')
102              {
103                  syn=24;
104                 token[m++]=ch; 
105              }
106              else
107              {
108                  syn=23;
109                  p--;
110              }
111              break;
112       case':':token[m++]=ch;
113               ch=getch();
114               if(ch=='=')
115               {
116                   syn=18;
117                 token[m++]=ch; 
118               }
119               else
120               {
121                   syn=17;
122                  p--;
123               }
124               break;
125       case'+':syn=13;token[0]=ch;break;
126       case'-':syn=14;token[0]=ch;break;
127       case'*':syn=15;token[0]=ch;break;
128       case'/':syn=16;token[0]=ch;break;
129       case'=':syn=25;token[0]=ch;break;
130       case';':syn=26;token[0]=ch;break;
131       case'(':syn=27;token[0]=ch;break;
132       case')':syn=28;token[0]=ch;break;
133       case'#':syn=0;token[0]=ch;break;
134       default:syn=-1;
135           
136    }
137 }
138 
139 char getch()
140 {
141     return prog[p++];
142 }
143 int synvalue(int n)
144 {
145     return n+1;
146 }
147 char check()
148 {
149     if(ch==' ')
150     {
151         do
152         {
153             ch=getch();
154         }while(ch==' ');
155     return ch;
156     }
157     else if(ch == '/')
158     {
159         ch=getch();
160         if(ch=='*')
161         {
162             do
163             {
164                 ch=getch();
165                 while(ch!='*')
166                     ch=getch();
167                 ch=getch();
168                 while(ch=='*')
169                     ch=getch();
170             }while(ch!='/');
171             ch=getch();
172             return ch;    
173         }
174         else
175         {
176            p--;
177            ch='/';
178            return ch;
179         }
180     }
181     else
182         return ch;
183 
184 }