d_postfixeval.cpp

后辍表达式的计算

#include "d_postfixEval.h"

void postfixEval::getOperands(double& left, double& right)
{
	if (operandStack.empty())
		throw  expression_error("postfixEval: 太多操作数");

	right = operandStack.top();
	operandStack.pop();

	if (operandStack.empty())
		throw  expression_error("postfixEval: 太多运算符");

	left = operandStack.top();
	operandStack.pop();
}

double postfixEval::compute(double left, double right, char command) const
{
	double value;

	switch(command)
	{
		case '+':	value = left + right;
						break;

		case '-':	value = left - right;
						break;

		case '*':	value = left * right;
						break;


		case '/':	if (right == 0)
							throw
								expression_error("postfixEval: 除数不能为0");
						value = left / right;

	}

	return value;
}

bool postfixEval::isCommand(char command) const
{
	return command == '+' || command == '-' || command == '*' || command== '/';
}


postfixEval::postfixEval()
{}

string postfixEval::getPostfix() const
{
	return postfixExpression;
}

void postfixEval::setPostfix(const string& postfix)
{
	postfixExpression = postfix;
	while(!operandStack.empty()) 
		operandStack.pop();
}


double postfixEval::evaluate()
{
	double left, right, expValue;
	char ch;


    
	for (unsigned int i=0; i < postfixExpression.length(); i++)
	{
		
		ch = postfixExpression[i];

		if (isdigit(ch))
	
			operandStack.push(ch - '0');

		else if (isCommand(ch))
		{

			getOperands(left, right);

			operandStack.push(compute(left, right, ch));
		}

		else if(!isspace(ch))
			throw expression_error("postfixEval: 非法的算式");
		
	}

	expValue = operandStack.top();
	operandStack.pop(); 

	if (!operandStack.empty())
		throw expression_error("postfixEval: 非法的算式");

	return expValue;
	
}