Converting infix to postfix notation - c++

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.