C++ Program to Evaluate an Expression using Stacks

This C++ program, using a stack data strucure, computes value of postfix expression which pushes operands and pops these values on encountering an operator.

Here is the source code of the C++ program to display the value of the postfix expression given as input. This C++ program is successfully compiled and run on DevCpp, a C++ compiler. The program output is given below.

```
/*
```

 * C++ Program to Evaluate an Expression using Stacks

```
 */
```
```
#include <iostream>
```
```
#include <conio.h>
```
```
#include <string.h>
```
```
using namespace std;
```
```
struct node
```
```
{
```
```
    int data;
```
```
    node *next;
```

}*p = NULL, *top = NULL, *save = NULL, *ptr;

```
void push(int x)
```
```
{
```
```
    p = new node;
```
```
    p->data = x;
```
```
    p->next = NULL;
```
```
    if (top == NULL)
```
```
    {
```
```
        top = p;
```
```
    }
```
```
    else
```
```
    {
```
```
        save = top; 
```
```
        top = p;
```
```
        p->next = save;
```
```
    }
```
```
}
```
```
char pop()
```
```
{
```
```
    if (top == NULL)
```
```
    {
```
```
        cout<<"underflow!!";
```
```
    }
```
```
    else
```
```
    {
```
```
        ptr = top;
```
```
        top = top->next;
```
```
        return(ptr->data);
```
```
        delete ptr;
```
```
    }
```
```
}
```
```
int main()
```
```
{
```
```
    char x[30];
```
```
    int a, b;
```

    cout<<"enter the balanced expression\n";

```
    cin>>x;
```
```
    for (int i = 0; i < strlen(x); i++)
```
```
    {
```
```
        if (x[i] >= 48 && x[i] <= 57)
```
```
            push(x[i]-'0');
```

        else if (x[i] >= 42 && x[i] <= 47)

```
        {
```
```
            a=pop();
```
```
            b=pop();
```
```
            switch(x[i])
```
```
            {
```
```
            case '+':
```
```
                push(a+b);
```
```
                break;
```
```
            case '-':
```
```
                push(a-b);
```
```
                break;
```
```
            case '*':
```
```
                push(a*b);
```
```
                break;
```
```
            case '/':      
```
```
                push(a/b);
```
```
                break;
```
```
            }
```
```
        }
```
```
    }
```
```
    cout<<"ans is "<<pop()<<endl;
```
```
    getch();
```
```
}
```

Output
enter the balanced expression
567+8-/
ans is -1

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