Converting infix to postfix notation

Question

I am doing infix to postfix notation. My program compiles, although, for some reason it will not take in any infix expressions, only postfix expressions. Which is the opposite I wanted to do. Here is my program:

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

string infixexpr (istream&  in)
{
    //Holds value in computation
    stack<string> postfixstack;
    //used to to read in characters from the expression
    char ch;
    // used to read in numbers from expression
    int num;
    // Used to remove infix expressions from stack
    string lexpr, rexpr;
    ch = in.peek();
    while ( ch != EOF)
    {
        //If we have a whitespace character skip it and continue with
        // the end of the loop.
        if(isspace(ch))
        {
            ch = in.get();
            ch =in.peek();
            continue;
        }

        //nonspace character is next to input stream
        // if the next character is a number read it and convert it
        // to string then put the string onto the postfix stack
        if (isdigit (ch))
        {
            in >> num;
            // use to convert string
            ostringstream numberstr;
            // convert to number using sstream
            numberstr << num;
            // Push the representing string onto stack0
            postfixstack.push(numberstr.str());
            ch = in.peek();
            continue;
        }

        // if operator pop the two postfix expressions
        // stored on the stack, put the operator after
        postfixstack.pop();
        lexpr = postfixstack.top();
        postfixstack.pop();

        if (ch == '+' || ch == '-' || + ch == '*' || ch == '/' || ch == '%')
            postfixstack.push(rexpr + " " + lexpr + " " + ch);
        else
        {
            cout << "Error in input expression" << endl;
            exit(1);
        }
        ch = in.get();
        ch = in.peek();
    }
    return postfixstack.top();
}

int main()
{
    string input;
    cout <<  "Enter a infix expression to convert to postfix,"
         << " \nor a blank line to quit the program:";
    getline(cin,input);

    while (input.size() != 0 )
    {
        //convert string to a string stream
        istringstream inputExpr(input);
        cout << "the infix equavilent is: "
             << infixexpr(inputExpr) << endl;
            cout << "Enter a infix Expression to evaluate: ";
            getline(cin,input);
    }

    return 0;
}

For example the program runs like this:

• if I enter 56 2 + (add spaces after each number or operator)
• I'll get back 56 2 +, which is what I want. But if I enter
• 56 + 2, the program will crash.

If you like to see my stack class and header, if that is the problem please let me know. I can post it under reply.

Answer 1

Oh my, where to begin. You should probably take this to the Code Review Stack Exchange instead , but let's get into it:

• You don't describe the "crash", so we don't know what failure you're observing.
• Your " if the next character is a number read it and convert it to string then put the string onto the postfix stack " code pushes the operand on the stack. But in Dijkstra's shunting-yard algorithm , which you appear to be trying to implement, only operators go on the stack.
• Your " if operator pop the two postfix expressions stored on the stack, put the operator after " code doesn't guard against popping items off an empty stack.
• That same code also pops two items off the stack, but you only pushed one on - the "56" in "56 + 2". Since you're forcing me to guess, I'm guessing this is where the program crashes.
• That same code also pushes the result onto the stack, another example of how not to implement the shunting-yard algorithm.