LL(1)文法--递归下降程序

递归下降程序

递归下降程序一般是针对某一个文法的。而递归下降的预测分析是为每一个非终结符号写一个分析过程，由于文法本身是递归的，所以这些过程也是递归的。

以上是前提。

Sample

1. 假如给的是正规式子，首先要做的是将其改为文法表示:
   
   \((int | float) id(,id)^*\)
2. 以上式子为例，将其改为文法表示
   
   \(D --> TL\)
   
   \(T-->int | float\)
   
   \(L--> L,id | id\)
3. 然后消除其左递归
   
   \(D --> TL\)
   
   \(T-->int | float\)
   
   \(L--> idR\)
   
   \(L-->, idR|ε\)
4. 求其\(FIRST表和FOLLOW表\)
   
   \(FIRST(D)=(int,float)\)
   
   \(FIRST(T)=(int,float)\)
   
   \(FIRST(L)=(id)\)
   
   \(FIRST(L)=(,ε)\)
   
   \(FOLLOW(D)=(\)$\()\)
   
   \(FOLLOW(L)=(\)$\()\)
   
   \(FOLLOW(T)=(id)\)
   
   \(FOLLOW(R)=(\)$\()\)
5. \(Code\)

//递归下降分析程序

#include<iostream>
#include<cstdio>
#include<sstream>
#include<cstring>
#include<string>
#include<cstdlib>
using namespace std;
string str;
size_t lookahead = 0;
string alphabet[] = {
	"int","float","(",")","id",","
};
//( int | float )id(,id)*
//正规式表达
void D();
void T();
void L();
void R();
void error();

void error() {
	cout<<"Syntax Error!"<<endl;
}
void D() {
	T();
	L();
}

void T() {
	string m1 = str.substr(lookahead, 3);
	string m2 = str.substr(lookahead, 5);
	if (m1 == "int")
	{
		lookahead += 3;
	}
	else if (m2 =="float") {
		lookahead += 5;
	}
	else
		error();
}

void L() {
	string m3 = str.substr(lookahead, 2);
	if (m3 == "id") {
		lookahead += 2;
		R();
	}
	else
		error();
}

void R() {
	if (str[lookahead] == ',') {
		lookahead += 1;
		string m4 = str.substr(lookahead, 2);
		if (m4 == "id") {
			lookahead += 2;
			R();
		}
		else
			error();
	}
}

int main(void) {
	int n;
	cout<<"Test Number:" << endl;
	cin >> n;
	while (n--)
	{
		cout << "Input:";
		cin >> str;
		str.append("$");
		str.append("\0");
		D();
		if (str[lookahead] == '$')
			cout << "Accepted" << endl;
		else
			error();
		lookahead= 0;
	}
	return 0;
}