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

Sanfoundry Global Education & Learning Series – 1000 C++ Programs.

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Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He is Linux Kernel Developer & SAN Architect and is passionate about competency developments in these areas. He lives in Bangalore and delivers focused training sessions to IT professionals in Linux Kernel, Linux Debugging, Linux Device Drivers, Linux Networking, Linux Storage, Advanced C Programming, SAN Storage Technologies, SCSI Internals & Storage Protocols such as iSCSI & Fiber Channel. Stay connected with him @ LinkedIn