Basic RPN expression evaluator

This is a discussion on Basic RPN expression evaluator within the C++ Programming forums, part of the General Programming Boards category; Works perfectly!!! Got it within 2 tries...but had to put 'p' and 'm' for '+' and '-' respectively.....Any idea why ...

  1. #1
    Registered User manasij7479's Avatar
    Join Date
    Feb 2011
    Posts
    2,599

    Smile Basic RPN expression evaluator

    Works perfectly!!! Got it within 2 tries...but had to put 'p' and 'm' for '+' and '-' respectively.....Any idea why '+' and ' - ' characters are not accepted by the program? TIA
    If possible please compile the code and see if '+' and '-' do the job in your case..

    Code:
    #include<string>
    #include<sstream>
    #include<iostream>
    #include<deque>
    using namespace std;
    
    int main()
    {
        string input;
        getline(cin,input);
        deque<double> stack;
        istringstream in;
        in.str(input);
        double opn;
        char opt;
        for(;;)
        {
            if(in>>opn)
            {
                stack.push_back(opn);
    
                continue;
            }
            else
            {
                in.clear();
                if (in>>opt)
                {
    
                switch (opt)
                {
                    case 'p':
                    {
                        opn = stack.back();
                        stack.pop_back();
                        opn += stack.back();
                        stack.pop_back();
                        stack.push_back(opn);
                        break;
                    }
                    case 'm':
                    {
                        opn = stack.back();
                        stack.pop_back();
                        opn=stack.back()-opn;
                        stack.pop_back();
                        stack.push_back(opn);
                        break;
                    }
                    case '*':
                    {
                        opn = stack.back();
                        stack.pop_back();
                        opn *= stack.back();
                        stack.pop_back();
                        stack.push_back(opn);
                        break;
                    }
                    case '/':
                    {
                        opn = stack.back();
                        stack.pop_back();
                        opn=stack.back() / opn;
                        stack.pop_back();
                        stack.push_back(opn);
                        break;
                    }
                    default: cout<<"sorry._ :( _.";
    
    
                }
                }
                else break;
            }
    
        }
        opn = stack.back();
        stack.pop_back();
        if(stack.empty())
        cout<<opn;
        else
        cout<<"More Operands than Operators";
        cin.get();
        return 0;
    
    }

    Also searching for a better way to implement the operator selection than the cumbersome switch-case which would grow in complexity with increase in operators .

  2. #2
    C++ Witch laserlight's Avatar
    Join Date
    Oct 2003
    Location
    Singapore
    Posts
    23,803
    Quote Originally Posted by manasij7479
    Also searching for a better way to implement the operator selection than the cumbersome switch-case which would grow in complexity with increase in operators .
    It would not really grow in complexity (or become more complicated, if that is what you mean). It would just become longer, but that is not necessarily a problem. In fact, with what you have now, you can move this:
    Code:
    opn = stack.back();
    stack.pop_back();
    to before the switch and move this:
    Code:
    stack.pop_back();
    stack.push_back(opn);
    to after the switch. As for what I had in mind as an alternative: map strings to function pointers/objects. This way, if you want to add a new operation, you just modify the creation of the map, and add the definition of the function (or function object class).
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

  3. #3
    Registered User manasij7479's Avatar
    Join Date
    Feb 2011
    Posts
    2,599
    After some unary operations or (implementations of a algebraic library ) are added, it could potentially become become more complex.
    How would I fit things like "sin" or "log" inside the switch?
    Is using goto a very bad idea for implementing the unary operators? (to jump over the 2nd pop_back statement)

  4. #4
    C++ Witch laserlight's Avatar
    Join Date
    Oct 2003
    Location
    Singapore
    Posts
    23,803
    Quote Originally Posted by manasij7479
    How would I fit things like "sin" or "log" inside the switch?
    It is similiar to a unary operator.

    Quote Originally Posted by manasij7479
    Is using goto a very bad idea for implementing the unary operators?
    Yes, avoid goto.
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

  5. #5
    Registered User manasij7479's Avatar
    Join Date
    Feb 2011
    Posts
    2,599
    Quote Originally Posted by laserlight View Post
    ...... and move this:
    Code:
    stack.pop_back();
    stack.push_back(opn);
    to after the switch.
    just after that I've put
    Code:
              stack.pop_back();
                    if(in>>opt_s)
                    {
                        if(opt_s=="sin")stack.push_back(sin(opn));
                    }
    for the similar unary functions...(.as the switch didn't accept them)
    but it shows a segmentation fault at runtime.....Tried putting up cout 's to determine the source with no success...

  6. #6
    and the Hat of Guessing tabstop's Avatar
    Join Date
    Nov 2007
    Posts
    14,185
    As to why + and - are giving you fits: you are trying to read in a double. A double can start with + and with -, so those symbols are taken as the start of a double rather than falling through to the character parser. I would suggest this is why trying to read doubles-and-somethimes-characters vs. reading character at a time is a pretty bad idea.

  7. #7
    C++ Witch laserlight's Avatar
    Join Date
    Oct 2003
    Location
    Singapore
    Posts
    23,803
    Quote Originally Posted by manasij7479
    but it shows a segmentation fault at runtime.....Tried putting up cout 's to determine the source with no success...
    Use a debugger.
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

  8. #8
    Registered User manasij7479's Avatar
    Join Date
    Feb 2011
    Posts
    2,599
    I would suggest this is why trying to read doubles-and-somethimes-characters vs. reading character at a time is a pretty bad idea.
    Then I have to put the character portion of the code before trying to play with the doubles...
    Use a debugger.
    I tried to put off learning to do so as long as possible...!..Would the Code::blocks debugger be a better option than the built in gnu debugger?

  9. #9
    and the Hat of Guessing tabstop's Avatar
    Join Date
    Nov 2007
    Posts
    14,185
    Quote Originally Posted by manasij7479 View Post
    I tried to put off learning to do so as long as possible...!..Would the Code::blocks debugger be a better option than the built in gnu debugger?
    C::B has nothing of its own (no compiler, no debugger, no anything). If you have the gnu debugger, then you can use that. C::B has a front end for that debugger, but I've not used it much.

  10. #10
    Registered User manasij7479's Avatar
    Join Date
    Feb 2011
    Posts
    2,599
    I thought the debugger was a standalone plugin of c::B unlike the compiler....!.........So, theoretically any any debugger would work well with it..
    What is the most used debugger in a linux envionment ?

  11. #11
    a guy with long hair Xupicor's Avatar
    Join Date
    Sep 2010
    Location
    Poland
    Posts
    103
    I'd say GDB - you most probably already have it, and it should work with C::B. I don't know what other debuggers does the C::B frontend support, to be honest.

    As for what I had in mind as an alternative: map strings to function pointers/objects. This way, if you want to add a new operation, you just modify the creation of the map, and add the definition of the function (or function object class).
    I concur. Actually, this is exactly what I did (well, it differed in implementation details, irrelevant at this point ).
    Last edited by Xupicor; 02-03-2011 at 04:45 PM.

Popular pages Recent additions subscribe to a feed

Similar Threads

  1. Screwy Linker Error - VC2005
    By Tonto in forum C++ Programming
    Replies: 5
    Last Post: 06-19-2007, 02:39 PM
  2. recursion error
    By cchallenged in forum C Programming
    Replies: 2
    Last Post: 12-18-2006, 08:15 AM
  3. Please Help - Problem with Compilers
    By toonlover in forum C++ Programming
    Replies: 5
    Last Post: 07-23-2005, 10:03 AM
  4. Expression Evaluator Contest
    By Stack Overflow in forum Contests Board
    Replies: 20
    Last Post: 03-29-2005, 09:34 AM

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21