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Question

Through doing a little research online, I've found that this is error is caused by trying to pop something off an empty stack. In my code, I thought I tried to make sure this exact thing wouldn't happen by storing the top of my stack in a temporary string called c , but for some reason the error keeps occurring. This is a project related to converting infix notation to postfix notation. When I input 1+2+3 , I should get 12+3+ , but instead this error occurs. The error occurs in the precedence while loop.

stack <string> InfixToPostfix(char * expr)
{
  // YOUR CODE STARTS HERE! 
 char* token;
 stack<string> s;
 string postfix;
 stack<string> p;
                                     // use stack s to manipulate the infix      to postfix
 s.push("(");
  token = strtok (expr," "); // get the first token
  while(!s.empty())
  {
          if(token=="(")
        {
            s.push(token);
          }
         else if(token==")")
      {
            string c=s.top();
            s.pop();
             while(c != "(")
            {
            postfix += c + " ";
            c = s.top();
            s.pop();
        }
  }
  else if(isNumber(token))
  {
      postfix+=token;
  }
  else 
  {
      while(precedence(s.top())>=precedence(token))
      {
          //put on bottom if need be

           string c = s.top();
            s.pop();
            postfix += c;
            postfix += " ";
      }
      s.push(token);
  }

token = strtok(NULL, " "); // use blanks as delimiter
  if (token == NULL) // no more tokens, exit the loop
     break;
  }
  p.push(postfix);
  return p;
}

Answer 1

Inside the if statements within the main while loop, you have some calls to pop() without checking if the deque is empty for example

 while(c != "(") {
     postfix += c + " ";
     c = s.top();
     s.pop();
 }

This should probably be:

while(c != "(" && !s.empty()) {

}

And the same below

while(precedence(s.top())>=precedence(token))
{
      //put on bottom if need be
      string c = s.top();
      s.pop();
      postfix += c;
      postfix += " ";
 }
 s.push(token);

These are quite likely sources of invalid access. In general you should try to use gdb or valgrind --tool=memcheck which will show you the source of error. Or any other debugger really...

Also if the token is a single character as I suspect it should be, make it a char not a char * . Then in your token comparisons use token == ')' (note the single quotes) which compares two characters and not what you have now which compares a char pointer with a string literal... This will probably apply to all the data structures that you maintain as well which should become, eg deque<char> ...

Finally pick up any decent introductory C++ book and read it. You will thank yourself later.