数据结构作业—表达式求值

嘛...就是转换成反波兰式嘛......

#include <iostream>
#include <stack>
#include <cmath>
#include <string>
#include <sstream>
#include <algorithm>

using namespace std;

string changeToReversePolish(string& expression);
double calculateByReversePolish(string reversePolish);
// return true if stackTop is higher or equal than readIn
bool isHigh(char stackTop, char readIn);
bool isOperator(char readIn);

int main()
{
string expression = "";
int num = 0;
cin >> num;

int count = 0;

while (count < num)
{
cin >> expression;
cout << calculateByReversePolish(changeToReversePolish(expression)) << endl;

count++;
}

return 0;
}

bool isHigh(char stackTop, char readIn)
{
if ((stackTop == '+') && (readIn == '+'))
return true;
if ((stackTop == '+') && (readIn == '-'))
return true;
if ((stackTop == '-') && (readIn == '+'))
return true;
if ((stackTop == '-') && (readIn == '-'))
return true;
if ((stackTop == '*') && (readIn == '*'))
return true;
if ((stackTop == '*') && (readIn == '/'))
return true;
if ((stackTop == '/') && (readIn == '*'))
return true;
if ((stackTop == '/') && (readIn == '/'))
return true;
if (readIn == ')')
return true;

return false;
}

bool isOperator(char readIn)
{
if (readIn == '+' || readIn == '-' || readIn == '*' || readIn == '/')
return true;
return false;
}

string changeToReversePolish(string& expression)
{
stack<char> operandStack;
string result;
char operand = NULL;
istringstream iss(expression);

while (iss >> operand)
{
if (isdigit(operand) || operand == '.')
{
result.push_back(operand);
}
if (operand == '(')
{
operandStack.push(operand);
}
else
{
if (isOperator(operand))
{
result.push_back(' ');
if (operandStack.empty())
{
operandStack.push(operand);
}
else if (operandStack.top() != '(' && isHigh(operandStack.top(), operand))
{
result.push_back(operandStack.top());
operandStack.pop();
}
else
{
operandStack.push(operand);
}
}
else if (operand == ')')
{
while (!operandStack.empty() && operandStack.top() != '(')
{
result.push_back(operandStack.top());
operandStack.pop();
}
if (operandStack.top() == '(')
operandStack.pop();
}

}
}

while (!operandStack.empty())
{
result.push_back(operandStack.top());
operandStack.pop();
}

return result;
}

double calculateByReversePolish(string reversePolish)
{
stack<double> digitStack;
string tempDigit = "";

for (int i = 0; i < reversePolish.length(); i++)
{
if (isdigit(reversePolish[i]))
{
tempDigit.push_back(reversePolish[i++]);
while ((isdigit(reversePolish[i]) || reversePolish[i] == '.') && i < reversePolish.length())
{
tempDigit.push_back(reversePolish[i]);
i++;
}
i--;
double digit = strtod(tempDigit.c_str(), NULL);
digitStack.push(digit);
tempDigit.clear();
}
else if(reversePolish[i] == '+' && !digitStack.empty())
{
double rightOperand = digitStack.top();
digitStack.pop();

double leftOperand = digitStack.top();
digitStack.pop();

double result = leftOperand + rightOperand;
digitStack.push(result);
}
else if(reversePolish[i] == '-' && !digitStack.empty())
{
double rightOperand = digitStack.top();
digitStack.pop();

double leftOperand = digitStack.top();
digitStack.pop();

double result = leftOperand - rightOperand;
digitStack.push(result);
}
else if(reversePolish[i] == '*' && !digitStack.empty())
{
double rightOperand = digitStack.top();
digitStack.pop();

double leftOperand = digitStack.top();
digitStack.pop();

double result = leftOperand * rightOperand;
digitStack.push(result);
}
else if(reversePolish[i] == '/' && !digitStack.empty())
{
double rightOperand = digitStack.top();
digitStack.pop();

double leftOperand = digitStack.top();
digitStack.pop();

double result = leftOperand / rightOperand;
digitStack.push(result);
}

}

return digitStack.top();

}