语法分析.cpp

编制一个递归下降分析程序

#include "stdio.h"
#include "string.h"
#include "conio.h"
#include "ctype.h"
char prog[80],
     token[8];                     /*存放构成单词符号的字符串*/
char ch;
int  syn,                           /*存放单词字符的种别码*/
      n,
     kk,
    sum,                           /*存放整数型单词*/
    m,p;                           /*p是缓冲区prog的指针，m是token的指针*/
char *rwtab[6]={"begin","if","then","while","do","end"}; /*关键字*/
void scaner();
void lrparser();
void yucu();
void statement();
void expression();
void factor();
void term();

void scaner()
{
    m=0;
    sum=0;
    for(n=0;n<8;n++) token[n]='\0';
    ch=prog[p++];
    while(ch==' ')  ch=prog[p++];
    if(isalpha(ch))    /*判断ch为字母字符*/
    {
        while(isalpha(ch)||isdigit(ch))    /*判断ch 为字母字符或者数字字符*/
		{
           token[m++]=ch;
           ch=prog[p++];}
        token[m++]='\0';
        ch=prog[p--];
        syn=10;
        for(n=0;n<6;n++)
            if(strcmp(token,rwtab[n])==0)    /*字符串的比较*/       
			{
                syn=n+1;
                break; 
			}
	}
    else
        if(isdigit(ch))    /*判断ch是数字字符*/
		{
            while(isdigit(ch))    /*判断ch是数字字符*/
			{
                sum=sum*10+ch-'0';
                ch=prog[p++];
			}
            ch=prog[p--];
            syn=11;}
        else
            switch(ch)
		{
               case'<':m=0;
                       token[m++]=ch;
                       ch=prog[p++];
                       if(ch=='>')
					   {
                           syn=21;
                           token[m++]=ch;
					   }
                       else if(ch=='=')
					   {
                           syn=22;
                           token[m++]=ch;
					   }
                            else
							{
                                syn=20;
                                ch=prog[p--];
							}
                        break;
                case'>':m=0;
                        token[m++]=ch;
                        ch=prog[p++];
                        if(ch=='=')
						{
                            syn=24;
                            token[m++]=ch;
						}

                        else
						{
                            syn=23;
                            ch=prog[p--];
						}
                        break;
             case':':m=0;
                     token[m++]=ch;
                     ch=prog[p++];
                     if(ch=='=')
					 {
                         syn=18;
                         token[m++]=ch;
					 }
                     else
					 {
                         syn=17;
                         ch=prog[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 lrparser()
{
    if(syn==1)
	{
        scaner();
        yucu();
        if(syn==6)
		{
            scaner();
            if(syn==0 && (kk==0))
                printf("success");
        }
    }
    else
	{
        printf("error");
        kk=1;
	}
    return;
}

void yucu()
{
    statement();
    while(syn==26)
	{
        scaner();
        statement();
	}
	return;
}
void statement()
{
    if(syn==10)
	{
        scaner();
        if(syn==18)
		{
            scaner();
            expression();
		}
        else
		{
            printf("\":=\" ERROR!");
            kk=1;
		}
	}
    else
	{
        printf("statement ERROR!");
        kk=1;
	}
    return;
}

void expression()
{
    term();
    while(syn==13 || syn==14)
	{
        scaner();
        term();
	}
    return;
}

void term()
{
    factor();
    while(syn==15 || syn==16)
	{
        scaner();
        factor();
	}
    return;
}

void factor()
{
    if(syn==10 || syn==11)
        scaner();
        else if(syn==27)
		{
                 scaner();
                 expression();
                 if(syn==28)
                     scaner();
                 else
				 {
                     printf("\')\' ERROR!");
				 }
		}
             else
			 {
                 printf("expressions ERROR!");
                 kk=1;
			 }
    return;
}
void main()
{
    kk=0;
    printf("When input a \'#\' at the end of an line,this programe will be over.\n");
    printf("And the programe will output the codes you inputed just now.\n");
    p=0;
    printf("\nplease input string:\n");
    do {
           ch=getchar();
           prog[p++]=ch;
       }while(ch!='#');
    p=0;
    scaner();
    lrparser();
    getch();
}