lexer.cpp

编译原理实验将简单中缀算术表达式变换成后缀形式

//lexer.c
#include "stdafx.h"
#include "global.h"

char lexbuf[BSIZE];
int lineno=1;
int tokenval=NONE;

int lexan()
{
	int t;
	while(1)
	{
		t=fgetc(fp1);
		if(t==' '||t=='\t')
			;
		//else if(t=='\n')
			//lineno=lineno+1;
		else if(isdigit(t))
		{
			ungetc(t,fp1);
			fscanf(fp1,"%d",&tokenval);
			return NUM;
		}
		else if(isalpha(t))
		{
			int p,b=0;
			while(isalnum(t))
			{
				lexbuf[b]=t;
				t=fgetc(fp1);
				b=b+1;
				if(b>=BSIZE)
					error2("compiler error");
			}
			lexbuf[b]=EOS;
			if(t!=EOF)
				ungetc(t,fp1);
			p=lookup(lexbuf);
			if(p==0)
				p=insert(lexbuf,ID);
			tokenval=p;
			return symtable[p].token;
		}
		else if(t==EOF)
			return DONE;
		else
		{
			tokenval=NONE;
			return t;
		}
	}
}