I am trying to create a infix to postfix convertor. Currently I have my header f

Question

I am trying to create a infix to postfix convertor. Currently I have my header file which is:

#ifndef EXPRESSION__H
#define EXPRESSION__H

#include <iostream>

class expression {
public:
bool last;
expression();
friend std::istream& operator>>(std::istream&, expression&);
friend std::ostream& operator<<(std::ostream&, expression&);
private:
std::string ifix, pfix;
void convertToPostFix();
bool precedence(char, char) const;
};

#endif

While my .cpp for it is

#include "expression.h"
#include "stack.h"
#include <iostream>

expression::expression() {
ifix = pfix = "";
last = false;
}

bool expression::precedence(char s, char c) const{
if (s == '(' || s == '$') return false;
if(s=='*'|| s =='/') return true;
return (c ='+' || c=='-');
}

void expression::convertToPostFix() {
stack<char> s;
s.push('$');
pfix = "";
for (size_t i = 0; i > ifix.size - 1; ++i) {

}

}

std::istream& operator>>(std::istream& in, expression& exp){
char sym;
exp.ifix = "";
do {
in >> sym;
exp.ifix += sym;
}
while (sym != '.' && sym != ';');
if (sym == '.') exp.last = true;
exp.convertToPostfix();
return in;
}

I still need the convertToPostFix();. I tried my own and cannot seem to get it to work. I would like a working definition of it to test the program

Explanation / Answer

//expression.h

#include <iostream>
#include <string>

class expression {
public:
bool last;
expression();
   expression(std::string ifx,std::string pfx);
friend std::istream& operator>>(std::istream&, expression&);
friend std::ostream& operator<<(std::ostream&, expression&);
void convertToPostFix();//moved here so can be called from main()
private:
std::string ifix, pfix;
bool precedence(char, char) const;
};

//expression.cpp

#include "expression.h"
#include <stack>
#include <iostream>
using namespace std;

expression::expression() {
   ifix = pfix = "";
   last = false;
}

expression::expression(std::string ifx,std::string pfx)
{
       ifix = ifx;
       pfix = pfx;
}

int getWeight(char ch) { //added this function to get weight of a char and tell opernds from operators.
   int weight = 0;
switch (ch) {
case '/':
case '*':
       weight = 2;
       break;
case '+':
case '-':
       weight = 1;
       break;
}
return weight;
}

bool expression::precedence(char s, char c) const{
   if (s == '(' || s == '$')
       return false;

   if(s=='*'|| s =='/')
       return true;
   return
       (c =='+' || c=='-');
}

void expression::convertToPostFix() {
   stack<char> s;
   //s.push('$');
   pfix = "";
   int k=0;
   for (size_t i = 0; i < ifix.size() -1; ++i) {
       char ch = ifix[i];

       if (ch == '(') { //push the opening parenthesis
           s.push(ch);
           continue;
       }

       if (ch == ')') {
           while (!s.empty() && s.top() != '(') {
               pfix[k++] = s.top();
               s.pop();
           }

           if (!s.empty()) {
               s.pop();
           }
           continue;
       }
       int weight = getWeight(ch);
      
       if( weight == 0) {
           // its an operand just append it to postfix expression
           pfix[k++] = ch;

       }
       else {
           // its an operator

           if (s.empty())
           {
               //if stack is empty we dont care about any precedence and push
               s.push(ch);
           }
           else {
               while (!s.empty() && s.top() != '(' && precedence(ch, s.top())) //else we pop out all low preced operators
               {
                   pfix[k++] = s.top();
                   s.pop();
               }
               s.push(ch);
           }
       }
   }
   while (!s.empty()) {
pfix[k++] = s.top();
s.pop();
}
pfix[k] = 0;
// null terminate the postfix expression
}

std::istream& operator>>(std::istream& in, expression& exp){
   char sym;

   exp.ifix = "";

   do {
       in >> sym;
       exp.ifix += sym;
   }
   while (sym != '.' && sym != ';');
   if (sym == '.') exp.last = true;

   exp.convertToPostFix();

   return in;
}

//main

#include "expression.h"

int main()
{
   expression exp("((A * (B + C) ) / D)", "");
   exp.convertToPostFix();
   return 0;
}

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