C++ 递归实现简单语言解释器

程序可以解释赋值语句，完成的功能如下：

• 判断它是够包含合法的赋值语句
• 求所有表达式的值
  输入输出模版如下：
  
  递归包含直接递归和间接递归。例如：一个项可以是一个因子，或者一个因子后面跟着称号“*”（或“/”）以及另一个因子。而因子可能是一个标识符，一个数，圆括号中的表达式，或者带有负号的因子。
  调用链为term()->factor()->expression()->term()；
  代码如下：

/***********************interpreter.h***********************/#ifndef _INTERPRETER_H_#define _INTERPRETER_H_#include <iostream>#include <list>#include <algorithm>#include <cstring>using namespace std;class IdNode{private:    char *id;    double value;    friend class Statement;  //友元类    friend ostream &operator<<(ostream &out, const IdNode &node);public:    IdNode(char *s = "", double e = 0){        id = strdup(s); //申请空间并复制字符串        value = e;    }    bool operator==(const IdNode& node) const{        return strcmp(id, node.id) == 0;    }};class Statement{private:    list<IdNode> idList;    char ch;    double factor();    double term();    double expression();    void readId(char *);    void issueError(char *s){        cerr << s << endl;        exit(1);    }    double findValue(char *);    void processNode(char *id, double e);    friend ostream & operator<<(ostream &out, const Statement &s);public:    Statement(){ }    void getStatement();};#endif // _INTERPRETER_H_

/***********************interpreter.cpp***********************/#include <cctype>#include "Interpreter.h"using namespace std;double Statement::findValue(char *id)   //返回对应的值{    IdNode tmp(id);    list<IdNode>::iterator i = find(idList.begin(), idList.end(), tmp);    if(i != idList.end())        return i->value;    else        issueError("unknown variable");    return 0;}void Statement::processNode(char *id, double e) //更新值或者加到链表中去{    IdNode tmp(id, e);    list<IdNode>::iterator i = find(idList.begin(), idList.end(), tmp);    if(i != idList.end())        i ->value = e;    else        idList.push_front(tmp);}void Statement::readId(char *id)  //读输入到一个数组中{    int i = 0;    if(isspace(ch))        cin >> ch;    if(isalpha(ch)){ //是字母的话就将后续的数字或者字母都保存        while(isalnum(ch)){            id[i++] = ch;            cin.get(ch);        }        id[i] = '\0';    }    else        issueError("Identifier expected");}double Statement::factor()  //因子的处理{    double var, minus = 1.0;    static char id[200];    cin >> ch;    while(ch == '+' || ch == '-'){ //是数字前的符号        if(ch == '-')   //确定正负            minus *= -1.0;        cin >> ch;    }    if(isdigit(ch) || ch == '.'){   //数字        cin.putback(ch);        cin >> var >> ch; //保存数字    }    else if(ch == '('){ //等于左括号就处理括号内的表达式        var = expression(); //递归处理括号内的表达式        if(ch == ')')   //右括号就继续处理ch            cin >> ch;        else            issueError("right paren left out");    }    else{        readId(id);        if(isspace(ch))            cin >> ch;        var = findValue(id);    }    return minus * var;}double Statement::term(){    double f = factor();    while(true){        switch(ch){        case '*':            f *= factor();            break;        case '/':            f /= factor();            break;        default:            return f;        }    }}double Statement::expression(){    double t = term();    while(true){        switch(ch){        case '+':            t += term();            break;        case '-':            t -= term();            break;        default:            return t;        }    }}void Statement::getStatement(){    char id[20];    char command[20];    double e;    cout << "enter a statement: ";    cin >> ch;    readId(id);    strupr(strcpy(command, id));    if(strcmp(command, "STATUS") == 0)        cout << *this;    if(strcmp(command, "PRINT") == 0){        readId(id);        cout << id << " = " << findValue(id) << endl;    }    else if(strcmp(command, "END") == 0)        exit(0);    else{        if(isspace(ch))            cin >> ch;        if(ch == '='){            e = expression();            if(ch != ';')                issueError("there are some extras in the statement. ");            else                processNode(id, e);        }        else            issueError("'=' is missing");    }}ostream &operator<<(ostream &out, const Statement &s){    list<IdNode>::const_iterator i = s.idList.begin();    for( ; i != s.idList.end(); i++)        out << *i;    out << endl;    return out;}ostream &operator<<(ostream &out, const IdNode &r){    out << r.id << " = " << r.value << endl;    return out;}int main(){    Statement statement;    cout << "the program processes statements  of the following format:\n"         << "\t<id> = <expr>;\n\tprint <id>\n\tstatus\n\tend\n\n";         while(true)            statement.getStatement();         return 0;}