C++代码实现逆波兰式
100行以内C++代码实现逆波兰式
逆波兰式(Reverse Polish notation,RPN,或逆波兰记法),也叫后缀表达式(将运算符写在操作数之后)。
算术表达式转逆波兰式例子:
逆波兰式整体的算法流程图如下:
下面给出我基于C++ 语言对逆波兰式算法的实现代码,值得注意的是:
1、算法中对操作数,仅支持一个字符的字母或数字的操作数,如:x,y,j,k,3,7等;如果要支持多个字符的操作数,如:var1,3.14等。需要读者自己扩展对算术表达式操作数的分词部分的代码。
2、为了为了增加转换后的逆波兰表达式的可读性,我在每个操作数和操作符输出时后面追加了一个空格。
代码如下:
/// file: ReversePolishNotation.h #include <string> #include <stack> class ReversePolishNotation { private: std::string _expr; unsigned _idx; std::stack<std::string> _stk; public: ReversePolishNotation(const std::string &expr); std::string nextWord(); std::string operator()(); static int getOpPriority(const std::string &word); bool isWord(const std::string &word); bool isOperator(const std::string &word); };
/// file: ReversePolishNotation.cpp #include <iostream> #include <cassert> #include "ReversePolishNotation.h" #include <cctype> #include <sstream> using std::cout; using std::endl; ReversePolishNotation::ReversePolishNotation( const std::string &expr) : _idx(0), _expr(expr) {} std::string ReversePolishNotation::nextWord() { if (_idx >= _expr.length()) { return ""; } return _expr.substr(_idx++, 1); } std::string ReversePolishNotation::operator()() { std::stringstream outExpr; std::string word; std::string topElem; while (true) { word = nextWord(); if (isWord(word)) { outExpr << word << " "; } else if (isOperator(word)) { if (_stk.empty() || _stk.top() == "(") { _stk.push(word); continue; } topElem = _stk.top(); while (getOpPriority(topElem) > getOpPriority(word)) { outExpr << topElem << " "; _stk.pop(); if (_stk.empty()) { break; } topElem = _stk.top(); } _stk.push(word); } else if (word == "(") { _stk.push(word); } else if (word == ")") { while (true) { topElem = _stk.top(); _stk.pop(); if (topElem == "(") { break; } assert(!_stk.empty() && "[E]Expr error. Missing '('."); outExpr << topElem << " "; } } else if (word == "") { while (!_stk.empty()) { topElem = _stk.top(); assert (topElem != "(" && "[E]Expr error. Redundant '('."); outExpr << topElem << " "; _stk.pop(); } break; } else { assert(false && "[W]>>>Can not recognize this word"); } } return outExpr.str(); } int ReversePolishNotation::getOpPriority(const std::string &word) { if (word == "+") { return 1; } if (word == "-") { return 1; } if (word == "*") { return 2; } if (word == "/") { return 2; } return 0; } bool ReversePolishNotation::isWord(const std::string &word) { return isalpha(word.c_str()[0]) || isdigit(word.c_str()[0]); } bool ReversePolishNotation::isOperator(const std::string &word) { return word == "+" || word == "-" || word == "*" || word == "/"; } /// ---测试代码--- int main() { assert(ReversePolishNotation("a+b*c")() == "a b c * + "); assert(ReversePolishNotation("(a+b)*c-(a+b)/e")() == "a b + c * a b + e / - "); assert(ReversePolishNotation("3*(2-(5+1))")() == "3 2 5 1 + - * "); // assert(ReversePolishNotation("3*((2-(5+1))")() == "3 2 5 1 + - * "); // Failed Case: Redundant '(' // assert(ReversePolishNotation("3*(?2-(5+1))")() == "3 2 5 1 + - * "); // Failed Case: Can not recognize '?' return 0; }
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