How to Convert Infix to Postfix Expression in Stack C++ - c++

Question

I am building a program in OOP C++ that will convert an Infix to Postfix expression using predefined Stackitem and Stack classes ( these two classes work fine). But I have some problems in the implementation of conversion algorithm. I cannot get the expected output for some inputs.
I have tried to implement the following algorithm:
Basic steps, while parsing the infix expression:
if the item is an operand, output immediately
if the item is a left parenthesis, push onto the stack
if the item is a right parenthesis pop the stack and output the contents until a left parenthesis (parenthesis is popped but not output)
if the item is an operator, pop and output all the operators with higher or equal precendence, then push the item onto the stack
just push the item onto the stack, if the stack is empty or the top element is a left parenthesis or if the top element has a lower precendence.
Stackitem class implementation which helps in decesiding wheather the item is operator, operand, and its precedence (by giving an integer)
#include "StackItem.h"
StackItem::StackItem(bool isOp, int i) {
init(isOp, i);
next = 0;
}
StackItem::StackItem(string item) {
if(item.compare("+") == 0)
init(true, OPERATOR_PLUS);
else if(item.compare("-") == 0)
init(true, OPERATOR_MINUS);
else if(item.compare("*") == 0)
init(true, OPERATOR_MULTIPLICATION);
else if(item.compare("/") == 0)
init(true, OPERATOR_DIVISION);
else if(item.compare("(") == 0)
init(true, OPERATOR_LEFTPAR);
else if(item.compare(")") == 0)
init(true, OPERATOR_RIGHTPAR);
else
init(false, atoi(item.c_str()));
}
void StackItem::init(bool isOp, int i) {
isOperator = isOp;
if(isOp)
op = i;
else
n = i;
}
string StackItem::toString() {
stringstream ss;
if(!isOperator) {
ss << n;
} else {
switch(op) {
case OPERATOR_MINUS:
ss << "-";
break;
case OPERATOR_PLUS:
ss << "+";
break;
case OPERATOR_DIVISION:
ss << "/";
break;
case OPERATOR_MULTIPLICATION:
ss << "*";
break;
case OPERATOR_LEFTPAR:
ss << "(";
break;
case OPERATOR_RIGHTPAR:
ss << ")";
break;
}
}
return ss.str();
}
And here is the problematic code ( for conversion ). I am suspecting that the problem is with my ( fifth and sixth point of the algorithm steps)
#include "Calculator.h"
#include <iostream>
Calculator::Calculator( string expression ) {
infixExpression = expression;
stack = new Stack();
istringstream iss( expression );
string token;
iss >> token;
postfixExpression = "";
while ( token.compare( ";" ) != 0 ) {
cout << "token:" << token << endl;
StackItem *item = new StackItem( token );
if ( !item->isOperator ) {
postfixExpression += item->toString() + " ";
} else {
if ( item->op == OPERATOR_LEFTPAR )
stack->push( item );
else if ( item->op == OPERATOR_RIGHTPAR ) {
while ( !stack->isEmpty() && stack->top() != OPERATOR_LEFTPAR ) {
string s = stack->top()->toString();
delete stack->pop();
postfixExpression += s + " ";
}
string s = stack->top()->toString();
delete stack->pop();
} else {
while ( !stack->isEmpty() && ( item->op <= stack->top()->op ) ) {
if ( stack->top()->isOperator ) {
string s = stack->top()->toString();
delete stack->pop();
postfixExpression += s + " ";
}
break;
}
while ( ( stack->isEmpty() ) || ( stack->top()->op == OPERATOR_LEFTPAR ) || ( stack->top()->op < item->op ) ) {
stack->push( item );
}
}
}
iss >> token;
}
while ( !stack->isEmpty() ) {
string s = stack->top()->toString();
delete stack->pop();
postfixExpression += s + " ";
}
postfixExpression += ";";
}
string Calculator::getPostfix() {
return postfixExpression;
}
The required inputs and outputs has to be parsed according to ";" at the end and a white space has to be left between each input and output part.
Example:
Input: 1 + 2 + 3 ;
Output: 1 2 + 3 + ; ( this works fine)
However,
Input : ( 1 + 2 ) ;
Output : Nothing ( it gives return some memory address)
Also for:
Input : 10 + 10 * 40 - 45 / 5 ;
Output : 10 10 4 * 45 5 / + ;
Which is a wrong order!
Edit: The operator defines are as follows:
#define OPERATOR_LEFTPAR 0
#define OPERATOR_RIGHTPAR 1
#define OPERATOR_MINUS 2
#define OPERATOR_PLUS 3
#define OPERATOR_DIVISION 4
#define OPERATOR_MULTIPLICATION 5
Edit: This new code solved many problems with basic and short inputs, still very large and complicated expressions crash in the output.
#include "Calculator.h"
#include <iostream>
Calculator::Calculator(string expression)
{
infixExpression=expression;
stack = new Stack();
istringstream iss(expression);
string token;
iss >> token;
postfixExpression="";
while(token.compare(";") != 0)
{
//cout << "token:"<<token << endl;
StackItem* item=new StackItem(token);
if(!item->isOperator){
postfixExpression += item->toString() + " ";
}
else
{
if(item->op == OPERATOR_LEFTPAR)
stack->push(item);
else if(item->op == OPERATOR_RIGHTPAR)
{
while(!stack->isEmpty()&& stack->top()->op != OPERATOR_LEFTPAR)
{
string s = stack->top()->toString();
delete stack->pop();
postfixExpression +=s+" ";
}
string s = stack->top()->toString();
delete stack->pop();
}
else
{
while((!stack->isEmpty()) && item->op <= stack->top()->op)
{
string s = stack->top()->toString();
delete stack->pop();
postfixExpression +=s+" ";
}
while((stack->isEmpty()) || item->op > stack->top()->op || stack->top()->op==OPERATOR_LEFTPAR)
{
stack->push(item);
}
}
}
iss >> token;
}
while(!stack->isEmpty())
{
string s = stack->top()->toString();
delete stack->pop();
postfixExpression +=s+" ";
}
postfixExpression += ";";
}
string Calculator::getPostfix()
{
return postfixExpression;
}