简单词法分析器的实现

#include <iostream>

#include <string.h>

#include <ctype.h>

char prog[80],token[8];

char ch;

int syn,p,m,n,sum=0;

char *rwtab[6]={"begin","if","then","while","do","end"};

void scanner()

{

     m=0;

     for(n=0;n<8;n++)

         token[n]=NULL;

     ch=prog[p++];

     while(ch==‘ ‘)ch=prog[p++];

     if(isalpha(ch))//字母字符

     {

         while(isdigit(ch)||isalpha(ch))//字母或数字字符

         {

              token[m++]=ch;

              ch=prog[p++];

         }

         token[m++]=‘\0‘;p--;syn=10;

         for(n=0;n<6;n++)

              if(strcmp(token,rwtab[n])==0)

              {

                   syn=n+1;

                   break;

              }

     }

     else if(isdigit(ch))//数字字符

     {

         sum=0;

         while(isdigit(ch))//shuzi

         {

              sum=sum*10+ch-‘0‘;

              ch=prog[p++];

         }

         p--;

         syn=11;

     }

     else   

         switch(ch)

     {

         case ‘<‘: m=0;

              token[m++]=ch;

              ch=prog[p++];

              if(ch==‘>‘)

              {

                   syn=22;

                   token[m++]=ch;

              }

              else   if(ch==‘=‘)

              {

                   syn=22;

                   token[m++]=ch;

              }

              else

              {

                   syn=20;

                   p--;

              }

              break;

         case ‘>‘:token[m++]=ch;

              ch=prog[p++];

              if(ch==‘=‘)

              {

                   syn=24;

                   token[m++]=ch;

              }

              else

              {

                   syn=23;

                   p--;

              }

              break;

         case ‘:‘:token[m++]=ch;

              ch=prog[p++];

              if(ch==‘=‘)

              {

                   syn=18;

                   token[m++]=ch;

              }

              else

              {

                   syn=17;

                   p--;

              }

              break;

         case ‘+‘:syn=13;token[0]=ch;break;

         case ‘-‘:syn=14;token[0]=ch;break;

         case ‘*‘:syn=15;token[0]=ch;break;

         case ‘/‘:syn=16;token[0]=ch;break;

         case ‘=‘:syn=25;token[0]=ch;break;

         case ‘;‘:syn=26;token[0]=ch;break;

         case ‘(‘:syn=27;token[0]=ch;break;

         case ‘)‘:syn=28;token[0]=ch;break;

         case ‘#‘:syn=0;token[0]=ch;break;

         default:syn=-1;

     }

}

void main()

{

     using namespace std;

     p=0;

     int i;

     printf("\n   please input string:\n");

         do{

              ch=getchar();

              prog[p++]=ch;

         }while(ch!=‘#‘);

         p=0;

         do{

              scanner();

              switch(syn)

              {

              case   11:printf("(%d,%d)",syn,sum);

                            break;

              case   -1:printf("ERROR!");

                   break;

              default:

                   printf("(%d,",syn);

                   for(i=0;i<strlen(token);i++)

                   {

                       printf("%c",token[i]);

                   }

                   printf(")");

              }

         }while(syn!=0);

}