# 加减乘除科学计算

2017/05/17 17:22

#include<iostream>
#include<stack>
#include <string>
#include <vector>

//判断 new_op > old_op
bool CompareOp(const std::string& new_op, const std::string& old_op)
{
if (new_op == "+" || new_op == "-")
{
if (old_op == "+" || old_op == "-" || old_op == "/" || old_op == "*")
{
return false;
}
else
{
return true;
}
}
else
{
if (old_op == "/" || old_op == "*")
{
return false;
}
else
{
return true;
}
}

}

//中缀转后缀
std::vector<std::string> InfixToPostfix(const std::vector<std::string>& infix_list)
{
std::vector<std::string> result;
std::stack<std::string> operator_stack;

for (const auto& input : infix_list)
{
if (input == "+" || input == "-" || input == "*" || input == "/")
{
while (true)
{
if (operator_stack.empty())
{
operator_stack.push(input);
break;
}
else
{
//比较当前运算符和栈顶的运算符的优先级，栈顶优先级大于等于当前的运算符时，写入结果中
const auto& top_operator = operator_stack.top();
if (CompareOp(input, top_operator))
{
operator_stack.push(input);
break;
}
else
{
result.push_back(top_operator);
operator_stack.pop();
}
}
}
}
else
{
//数字直接写入结果中
result.push_back(input);
}
}

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

return std::move(result);
}

//计算后缀表达式的结果
float CalcPostFix(std::vector<std::string> infix_list)
{
std::stack<float> number_stack;
for (auto input : infix_list)
{
if (input == "+" || input == "-" || input == "*" || input == "/")
{
//遇到运算符， 弹出栈顶的两个数字进行计算，并把计算结果入栈
//注意顺序，先弹出的应该是是右边的数字
auto num2 = number_stack.top();
number_stack.pop();

auto num1 = number_stack.top();
number_stack.pop();

if (input == "+")
{
number_stack.push(num1+num2);
}
else if (input == "-")
{
number_stack.push(num1 - num2);
}
else if (input == "*")
{
number_stack.push(num1 * num2);
}
else if (input == "/")
{
number_stack.push(num1/num2);
}
}
else
{
//数字直接入栈
char* end;
number_stack.push(std::strtof(input.c_str(), &end));
}
}

return std::move(number_stack.top());
}

int main()
{
const std::vector<std::string>  infix_list = { "4.2", "*", "5", "-", "6", "*", "2"};
auto result = InfixToPostfix(infix_list);
std::cout << CalcPostFix(result) << '\n';
return 0;
}


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