infix到后缀

Question

程序编译成功，但是当我试图转换以下infix表达式'A *B+C/ D‘时，我会得到以下错误消息:分段错误(内核转储)。它适用于像‘a+b-c*d/e’这样的表达式，但我不知道它是否正确。

#include<iostream>
#include<stack>
#include<string>
using namespace std;

string infix;  // infix exression string
string operand;
string operate;
stack <string> mystack; // this stack is used to push the operator
string postfix;  // postfix string where the operand is appended

//....................................................................
// this function read the infix expression from user 
string input()
{
 cout<<" Enter the damn infix expression: "<<endl; 
 getline(cin, infix);
 return infix;
}
//......................................................................
// this function checks for operator precedency in the stack
int precedence(string e)
{
int f;
if(e == "*" || e== "/" || e =="%")
 f = 2;
else
{
if(e == "+" || e == "-")
 f = 1;
}

if(e=="."){
f=0;
}

return f;

}




//....................................................................
// This function converts infix to postfix
string convert()
{ 
 for(int i=0; i<infix.length(); i++)
 { 

 switch(infix[i]){

 // operate case start  
 case '+': case'-': case'*': case'/': case'^': case '(': case ')': case'.':

 operate=infix[i];
 {
  if(mystack.empty() || precedence(operate)>= precedence(mystack.top()))
  {
   mystack.push(operate);    
   break;
  }       

  else 
  {
   while(precedence(operate)< precedence(mystack.top()))
   {
    postfix.append(mystack.top());
    mystack.pop();
   }

   mystack.push(operate); 


  }
 }//operate case closed

 default:        //when operand string char is parsed
  {
                        operand=infix[i];
                        postfix.append(operand);
                        break;

  } // default case closed

 }//switch closed

 }// for loop close

 while(!mystack.empty())
 {
  postfix.append(mystack.top());
  mystack.pop();
 }

return postfix;
cout<<postfix;

} // function close braces



//...................................................................
int main()
{

input();

convert();
cout<<"postfix is "<<postfix<<endl;
} 

Answer 1

你有：

while(precedence(operate)< precedence(mystack.top())) { // SEGV here.
  postfix.append(mystack.top());
  mystack.pop();
}

在看到顶部的元素或pop之前，您需要检查堆栈中是否有元素。

改变

while(precedence(operate)< precedence(mystack.top()))

至

while(!mystack.empty() && precedence(operate)< precedence(mystack.top()))

起作用了。