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，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:
#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 ""
#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 >= ()) {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 (() || () == "(") {(word);continue;}topElem = ();while (getOpPriority(topElem) > getOpPriority(word)) {outExpr << topElem << " ";();if (()) {break;}topElem = ();}(word);} else if (word == "(") {(word);} else if (word == ")") {while (true) {topElem = ();();if (topElem == "(") {break;}assert(!() && "[E]Expr error. Missing '('.");outExpr << topElem << " ";}} else if (word == "") {while (!()) {topElem = ();assert (topElem != "(" && "[E]Expr error. Redundant '('.");outExpr << topElem << " ";();}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(()[0]) || isdigit(()[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;
}以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持脚本之家。
![[C语言] 选择排序之直接选择排序的特性及实现](https://images2018.cnblogs.com/blog/1292139/201808/1292139-20180806203749936-461409019.png)
