Calculator

gistfile1.cpp

#include <iostream>
#include <stdexcept>
#include <string>
#include <sstream>
#include <cmath>
using namespace std;
class parse_error : exception {
  string msg;
public:
  parse_error(const char * str) : msg(str) { }
  virtual ~parse_error() throw () { }
  virtual const char * what() const throw () {
    return msg.c_str();
  }
};
void error(const char * info) {
  throw parse_error(info);
}
double T(istream & in);
double S(istream & in) {
  double val = T(in);
  while (in.peek() == '-' || in.peek() == '+')
    if (in.get() == '+')
      val += T(in);
    else
      val -= T(in);
  return val;
}
double V(istream & in) {
  double val;
  bool negative = false;
  while (in.peek() == '-' || in.peek() == '+')
    negative ^= in.get() == '-';
  if (in.peek() == '(') {
    in.get();
    val = S(in);
    if (in.get() != ')')
      error("Bracket doesn't match.");
  } else {
    if (!isdigit(in.peek()))
      error("Missing a operand.");
    in >> val;
  }
  return negative ? -val : val;
}
double U(istream & in) {
  double val = V(in), index, result;
  if (in.peek() == '^') {
    in.get();
    index = U(in);
    result = pow(val, index);
  } else
    result = val;
  return result;
}
double T(istream & in) {
  double val = U(in);
  while (in.peek() == '*' || in.peek() == '/')
    if (in.get() == '*')
      val *= U(in);
    else
      val /= U(in);
  return val;
}
int main(int argc, const char *argv[]) {
  // istringstream in("1.2+(9.6^2.5---3+9*9)");
  istream & in = cin;
  try {
    cout << S(in) << endl;
  } catch (parse_error e) {
    cerr << e.what() << endl;
  }
  return 0;
}

微软的奇葩定义……一直觉得微软的东西还是封装一下比较好。

加入了错误显示。还有对6-+-(67^5)这样表达式的支持。

还应该加入除0错误的提示

rule<phrase_scanner_t> factor, term, exp; 
factor = real_p | ('(' >> exp >> ')');  // real_p | !(ch_p('+')|ch_p('-')) >> ('(' >> exp >> ')') 注释部分是带正负号的规则
term   = factor >> *(('*' >> factor) | ('/' >> factor)); 
exp    = term >> *(('+' >> term) | ('-' >> term)); 

const char *szExp = "(2+3)"; 
parse_info<> rule = parse( szExp , exp, space_p); 
cout << "parse " << (rule.full?"success":"fail") << endl; 
我在网上找到的用boost::spirit对四则运算的验证，后面注释是我改写的可以在括号前面带正负号规则验证。

这倒不错，spirit十分方便，直接写cfg就行了，不过一直嫌boost配置太麻烦。