自上向下.cpp

编制一个递归下降分析程序

#include <fstream.h>
#include <stdio.h>
#include <iostream.h>
#include <string.h>
char prog[80],token[8];
char ch;
int syn, p, m, n,sum,kk=0;
char *rwtab[6]={"begin","if","then","while","do","end"};
void scaner();
int Isparser();
int yucu();
int statement();
int expression();
int term();
int factor();

void main()
{
	p=0;
    
  printf("Please input :\n");
  do
  {
     scanf("%c",&ch);
     prog[p++] = ch;

  }while(ch != '#');
	p=0;
	scaner();
    Isparser();

}
void Isparser()//递归下降分析程序
{
	if(syn==1)
	{
		scaner();
		yucu();
		if(syn==6)
		{
			scaner();
			if(syn==0&&kk==0)
				cout<<"success"<<endl;
		}
		else
		{
			if(kk!=1)
				cout<<"error:缺end"<<endl;
			    kk=1;
		}
	}
	else
	{
		cout<<"begin error"<<endl;//错误的开始
		kk=1;
	}

}
void yucu()
{
	statement();
	while(syn==26)
	{
		scaner();
		statement();
	}

}
void statement()
{
	if(syn==10)
	{
		scaner();
		if(syn==18)
		{
			scaner();
			expression();
		}
		else {cout<<"the wrong value"<<endl;kk=1;}
	}
	else{cout<<"wrong sentence"<<endl;kk=1;}

}
void expression()
{
	term();
	while(syn==13||syn==14)
	{
		scaner();
		term();
	}
	
}
void term()
{
	factor();
	while(syn==15||syn==16)
	{
		scaner();
		factor();
	}

}
void factor()
{
	if(syn==10||syn==11)
		scaner();
	else if(syn==27)
	{
		scaner();
		expression();
		if(syn==28)
			scaner();
		else {cout<<") error"<<endl;kk=1;}//')'错误
	}
	else{cout<<"error expression"<<endl;kk=1;}//表达式错误

}

void scaner()
{   	
	for(int k=0;k<8;k++) token[k]=NULL;
	ch=prog[p++]; m=0;
		while(ch==' ') ch=prog[p++];
		if(ch>='a'&& ch<='z'||ch>='A' &&ch<='Z')
			{   
				while(ch>='a'&&ch<='z'||ch>='0' &&ch<='9'||ch>='A' &&ch<='Z')
				{    
					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(ch>='0' &&ch<='9')
				{
					while(ch>='0' &&ch<='9')
					{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=21;
						 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;
					}
				}
}