Postfix evaluation - c++

Question

I am writing a code that evaluates a given Postfix expression. Each operand and operator is separated by a blank space and the last operator is followed by a blank space and an 'x'.
Example:
Infix expression: (2*3+4)*(4*3+2)
Postfix expression: 2 3 * 4 + 4 3 * 2 + * x
"x" implies the end of expression.
The input (Postfix expression) is given as a string from by another function that converts an infix expression to a postfix expression.
The function for postfix evaluation is:
int pfeval(string input)
{
int answer, operand1, operand2, i=0;
char const* ch = input.c_str();
node *utility, *top;
utility = new node;
utility -> number = 0;
utility -> next = NULL;
top = new node;
top -> number = 0;
top -> next = utility;
while((ch[i] != ' ')&&(ch[i+1] != 'x'))
{
int operand = 0;
if(ch[i] == ' ') //to skip a blank space
i++;
if((ch[i] >= '0')&&(ch[i] <= '9')) //to gather all digits of a number
{
while(ch[i] != ' ')
{
operand = operand*10 + (ch[i]-48);
i++;
}
top = push(top, operand);
}
else
{
top = pop(top, operand1);
top = pop(top, operand2);
switch(ch[i])
{
case '+': answer = operand2 + operand1;
break;
case '-': answer = operand2 - operand1;
break;
case '*': answer = operand2 * operand1;
break;
case '/': answer = operand2 / operand1;
break;
case '^': answer = pow(operand2, operand1);
break;
}
top = push(top, answer);
}
i++;
}
pop(top, answer);
cout << "\nAnswer: " << answer << endl;
return 0;
}
The output for the example I've given should be "140" but what I get is "6". Please help me find the error.
The push and pop methods are as follows (in case somebody wants to review):
class node
{
public:
int number;
node *next;
};
node* push(node *stack, int data)
{
node *utility;
utility = new node;
utility -> number = data;
utility -> next = stack;
return utility;
}
node* pop(node *stack, int &data)
{
node *temp;
if (stack != NULL)
{
temp = stack;
data = stack -> number;
stack = stack -> next;
delete temp;
}
else cout << "\nERROR: Empty stack.\n";
return stack;
}

Answer 1

while((ch[i] != ' ')&&(ch[i+1] != 'x'))
You drop out of this loop as soon as a) the current character is a space, or b) the next character is 'x'. The current character becomes a space pretty early in the process; you only process a small portion of the string.

Answer 2

Try comparing with the following code.
#include<iostream>
using namespace std;
#include<conio.h>
#include<string.h>
#include<math.h>
class A
{
char p[30],ch;
int i,top,s[30],y1,y2,x,y,r;
public:
A()
{
top=-1;
i=0;
}
void input();
char getsymbol();
void push(int);
int pop();
void evaluation();
};
void A :: input()
{
cout<<"enter postfix expression\n";
gets(p);
}
char A :: getsymbol()
{
return p[i++];
}
void A :: push(int ch)
{
if(top==29)
cout<<"stack overflow\n";
else
s[++top]=ch;
}
int A :: pop()
{
if(top==-1)
{
cout<<"stack underflow\n";
return 0;
}
else
return s[top--];
}
void A :: evaluation()
{
ch=getsymbol();
while(ch!='\0')
{
if(ch>='a'&&ch<='z'||ch>='A'&&ch<='Z')
{
cout<<"enter the value for "<<ch;
cin>>x;
push(x);
}
if(ch=='+'||ch=='-'||ch=='*'||ch=='/'||ch=='^')
{
y2=pop();
y1=pop();
if(ch=='+')
{
y=y1+y2;
push(y);
}
else if(ch=='-')
{
y=y1-y2;
push(y);
}
else if(ch=='^')
{
y=y1^y2;
push(y);
}
else if(ch=='*')
{
y=y1*y2;
push(y);
}
else if(ch=='/')
{
y=y1/y2;
push(y);
}
else
{
cout<<"entered operator has no value\n";
}
}
ch=getsymbol();
}
if(ch=='\0')
{
r=pop();
cout<<"the result is "<<r;
}
}
int main()
{
A a;
int m=0;
while(m==0)
{
a.input();
a.evaluation();
cout<<"enter 0 to continue 1 to exit\n";
cin>>m;
}
getch();
return 0;
}