词法分析.cpp

简单的词法分析程序

#include<stdio.h>
#include<ctype.h>
#include<string.h>
#define BEGIN 1
#define END 2
#define IF 3
#define WHILE 4
#define THEN 5
#define ELSE 6
#define ID  7
#define INT 8
#define LT 9
#define LE 10
#define EQ 11
#define NE 12
#define GT 13
#define GE 14       
char TOKEN[20];
extern int lookup(char *);
extern void out (int,char*);
extern report_error(void);
void scanner_example(FILE*fp)
{
	char ch;
	int i,c;
	ch=fgetc(fp);
	if(isalpha(ch))/*it must be a identifier!*/
	{
		TOKEN[0]=ch;
		ch=fgetc(fp);i=1;
		while(isalnum(ch))
		{
			TOKEN[i]=ch;i++;
			ch=fgetc(fp);
		}
		TOKEN[i]='\n';
		fseek(fp,-1,1);/* retract*/
		c=iookup(TOKEN);
		if(c==0)out(ID,TOKEN);
		else out(c,"");
	}
	else
		if(isdigit(ch))
		{
			TOKEN[0]=ch;
			ch=fgetc(fp);i=1;
			while(isdigit(ch))
			{
				TOKEN[i]=ch;i++;
				ch=fgetc(fp);
			}
			TOKEN[i]='\0';
			fseek(fp,-1,-1);
			out(INT,TOKEN);
		}
		else
			switch(ch)
		{
			case'<':ch=fgetc(fp);
				if(ch=='=')out(LE,"");
				else if(ch=='>')out(NE,"");
				     else
					 {
						 fseek(fp,-1,1);
						 out(LT,"");
					 }
					 break;
			case'=':out(EQ,"");break;
			case'>':ch=fgetc(fp);
				if(ch=='=')out(GE,"");
				else
				{
					fseek(fp,-1,1);
					out(GT,"");
				}
				break;
			default:report_error();
				break;
		}
		return;
}