C++ Program to Evaluate an Expression using Stacks

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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.

  1. /*
  2.  * C++ Program to Evaluate an Expression using Stacks
  3.  */
  4. #include <iostream>
  5. #include <conio.h>
  6. #include <string.h>
  7. using namespace std;
  8. struct node
  9. {
  10.     int data;
  11.     node *next;
  12. }*p = NULL, *top = NULL, *save = NULL, *ptr;
  13. void push(int x)
  14. {
  15.     p = new node;
  16.     p->data = x;
  17.     p->next = NULL;
  18.     if (top == NULL)
  19.     {
  20.         top = p;
  21.     }
  22.     else
  23.     {
  24.         save = top; 
  25.         top = p;
  26.         p->next = save;
  27.     }
  28. }
  29. char pop()
  30. {
  31.     if (top == NULL)
  32.     {
  33.         cout<<"underflow!!";
  34.     }
  35.     else
  36.     {
  37.         ptr = top;
  38.         top = top->next;
  39.         return(ptr->data);
  40.         delete ptr;
  41.     }
  42. }
  43. int main()
  44. {
  45.     char x[30];
  46.     int a, b;
  47.     cout<<"enter the balanced expression\n";
  48.     cin>>x;
  49.     for (int i = 0; i < strlen(x); i++)
  50.     {
  51.         if (x[i] >= 48 && x[i] <= 57)
  52.             push(x[i]-'0');
  53.         else if (x[i] >= 42 && x[i] <= 47)
  54.         {
  55.             a=pop();
  56.             b=pop();
  57.             switch(x[i])
  58.             {
  59.             case '+':
  60.                 push(a+b);
  61.                 break;
  62.             case '-':
  63.                 push(a-b);
  64.                 break;
  65.             case '*':
  66.                 push(a*b);
  67.                 break;
  68.             case '/':      
  69.                 push(a/b);
  70.                 break;
  71.             }
  72.         }
  73.     }
  74.     cout<<"ans is "<<pop()<<endl;
  75.     getch();
  76. }

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

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Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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