【编译原理】c++实现自上而下语法分析器

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　　使用递归下降子程序实现的PL/0语言的算术表达式的自上而下语法分析。该语言的其他语法实现思想与此一致，故不赘述。

/*
this code was first initiated by TZ,COI,HZAU
contact email:[email protected]
personal website:wnm1503303791.github.io
personal blogs:www.cnblogs.com/acm-icpcer/
this code has been posted on my personal blog,checking url:www.cnblogs.com/acm-icpcer/p/8964342.html
Copyright 2018/4/27 TZ.
All Rights Reserved.
*/

#include<cstdio>  
#include<cstring>  
#include<algorithm>  
#include<iostream>  
#include<string>  
#include<vector>  
#include<stack>  
#include<bitset>  
#include<cstdlib>  
#include<cmath>  
#include<set>  
#include<list>  
#include<deque>  
#include<map>  
#include<queue>  
#include<fstream>
using namespace std;

/*
S->X(AX)*|AX(AX)*
X->Y(MY)*
Y->I|N|(S)
A->+|-
M->*|/
C->=|#|<|<=|>|>= 
*/
char buffer[1024];
bool x(fstream &f);

bool preproccess(char *a,char *b)
{
    int i1=0,i2=1;
    memset(b,1024,'\0');
    while(a[i2]!=',')
    {
        b[i1]=a[i2];
           ++i1,++i2;
    }
    return true;
}

bool a(fstream &f)
{
    f.getline(buffer,1024);
    char t[1024];//存放字符标志 
    preproccess(buffer,t);
    cout<<buffer<<","<<t<<endl;
    
    if((!strcmp(t,"plus"))||(!strcmp(t,"minus")))
    {
        return true;
    }
    else 
    {
        cout<<"add operator ERROR"<<endl;
        return false;
    }
}

bool m(fstream &f)
{
    f.getline(buffer,1024);
    char t[1024];//存放字符标志 
    preproccess(buffer,t);
    cout<<buffer<<","<<t<<endl;
    
    if((!strcmp(t,"times"))||(!strcmp(t,"slash")))
    {
        return true;
    }
    else 
    {
        cout<<"times operator ERROR"<<endl;
        return false;
    }
}

bool c(fstream &f)
{
    f.getline(buffer,1024);
    char t[1024];//存放字符标志 
    preproccess(buffer,t);
    cout<<buffer<<","<<t<<endl;
    
    if    (    (!strcmp(t,"eql"))
            ||(!strcmp(t,"lss"))
            ||(!strcmp(t,"leq"))
            ||(!strcmp(t,"gtr"))
            ||(!strcmp(t,"geq"))
          )
    {
        return true;
    }
    else 
    {
        cout<<"compare operator ERROR"<<endl;
        return false;
    }
}

bool s(fstream &f)
{
    a(f);
    x(f);
}

bool y(fstream &f)
{
    f.getline(buffer,1024);
    char t[1024];//存放字符标志 
    preproccess(buffer,t);
    cout<<buffer<<","<<t<<endl;
    
    if(!strcmp(t,"ident"))
    {
        return true;
    }
    else if(!strcmp(t,"number"))
    {
        return true;
    }
    else if(!strcmp(t,"lparen"))
    {
        return true;
    }
    else 
    {
        cout<<"yingzi operator ERROR"<<endl;
        return false;
    }
}

bool x(fstream &f)
{
    y(f);
    m(f);
    y(f);
}

int main()
{
    fstream f;
    f.open("lexical.txt", ios::in);//打开文件，供读
    
    bool result=s(f);//start the grammar detection
       
    if(result)
        cout<<"ACCEPTED!"<<endl;
    else
        cout<<"ERROR!"<<endl;
    
    f.close();
    return 0;
}

tz@COI HZAU

2018/4/27