Postfix Calculator Using Stack, Linked Lists

Hello,

I've got a program I'm doing that does +, -, *, and / operations on positive integers in a postfix notation. The code I have at the moment compiles, but upon running the code, it stalls once I enter the expression. When I hit Enter, it should recognize the '\n' character and break the loop but it does not. Any help on fixing this error (and any others)?

Code:

---

#include <iostream>
#include <iomanip>
using namespace std;

class node
{
  public:
    float number;
    node *next;
};

node* push(node *stack, float data)
{
  node *utility;
  utility = new node;
  utility -> number = data;
  utility -> next = stack;
  return utility;
}

node* pop(node *stack, float &data)
{
  node *temp;
  if (stack != NULL)
  {
    temp = stack;
    data = stack -> number;
    stack = stack -> next;
    delete temp;
  }
  else cout << "\nERROR: Empty stack.\n";
  return stack;
}

int main()
{
  float answer, operand1, operand2;
  char ch;
  node *utility, *top;

  utility = new node;
  utility -> number = 0;
  utility -> next = NULL;
  top = new node;
  top -> number = 0;
  top -> next = utility;

  cout << "Postfix Calculator\n\n"
      << "Enter a legal positive integer postfix operation.\n\n";
  cin >> ch;
  while(ch != '\n')
    {
        cin >> ch;
        int operand = 0;
        while(ch == ' ')
          cin >> ch;
        if((ch >= '0')&&(ch <= '9')){
          while(ch != ' '){
            operand = operand*10 + (ch-48);
            cin >> ch;}
          top = push(top, operand);}
        else{
          pop(top, operand1);
          pop(top, operand2);
          switch(ch){
            case '+': answer = operand2 + operand1;
            case '-': answer = operand2 - operand1;
            case '*': answer = operand2 * operand1;
            case '/': answer = operand2 / operand1;}
          push(top, answer);}
    }
  pop(top, answer);
  cout << "\nAnswer: " << answer << endl;
}

---

The cin >> ch at the very top of the loop means you don't process the first number read in.

I also don't see why you need to seed the stack with two "blank" values.

Edit: And don't forget some breaks after your switch cases.

Edit edit: And the magic phrase is "cin >> noskipws".

I always forget those breaks... :D

However even with making those fixes it still doesn't advance once I hit Enter/Return...

You should only need to do noskipws once, somewhere -- I put it before any other reads in your code.

Code:

---

#include <iostream>
#include <iomanip>
using namespace std;

class node
{
  public:
    float number;
    node *next;
};

node* push(node *stack, float data)
{
  node *utility;
  utility = new node;
  utility -> number = data;
  utility -> next = stack;
  return utility;
}

node* pop(node *stack, float &data)
{
  node *temp;
  if (stack != NULL)
  {
    temp = stack;
    data = stack -> number;
    stack = stack -> next;
    delete temp;
  }
  else cout << "\nERROR: Empty stack.\n";
  return stack;
}

int main()
{
  float answer, operand1, operand2;
  char ch = ' ';                                    //just so that the while loop runs
  node *utility, *top;

  utility = new node;
  utility -> number = 0;
  utility -> next = NULL;
  top = new node;
  top -> number = 0;
  top -> next = utility;

  cout << "Postfix Calculator\n\n"
      << "Enter a legal positive integer postfix operation.\n\n";
 while(ch != '\n')
    {
        cin >> noskipws >> ch;
        int operand = 0;
        while(ch == ' ')
          cin >> ch;
        if((ch >= '0')&&(ch <= '9')){
          while(ch != ' '){
            operand = operand*10 + (ch-48);
            cin >> ch;}
          top = push(top, operand);}
        else{
          pop(top, operand1);
          pop(top, operand2);
          switch(ch){
            case '+': answer = operand2 + operand1;
            case '-': answer = operand2 - operand1;
            case '*': answer = operand2 * operand1;
            case '/': answer = operand2 / operand1;}
          push(top, answer);}
    }
  pop(top, answer);
  cout << "\nAnswer: " << answer << endl;
}

---

This is where I am now and the problem still hasn't righted itself... it doesn't recognize the new-line character at all.

EDIT: Never mind, the program gives me an answer... except it's not the right one. 2 3 45 + + gives me 90 instead of 50. Hmm...

You do realize pop is unable to change the value of its first parameter? Just like you have top = push, you should to top = pop.

I don't get what you mean. Are you meaning to put:

top = pop(top, operand1);
top = pop(top, operand2);

in the "else" part of the main function?

Yes. Otherwise top will still be pointing at 45, forever and ever.

I tried that as soon as I posted last, and it gave me:

ERROR: Empty stack.

ERROR: Empty stack.

Answer: 2

And you've got one stray push down at the bottom that needs to be top = push too.

Brilliant, that did the trick. Thanks. :)