#include<stdio.h>     //standard input output functions
       //console functions
#include<string.h>       //string functions
#define MAX 50              //max size defined
int stack[MAX];             //a global stack
char post[MAX];             //a global postfix stack
int top=-1;                  //initializing top to -1
void pushstack(char a);       //push function
void pushstack2(char a);
void evaluate(char c);          //calculate function
int main()
{
    int i,l;
    //clrscr();
    printf("Insert a postfix notation :: ");
    gets(post);                    //getting a postfix expression
    l=strlen(post);               //string length
    for(i=0;i<l;i++)
    {
        if(post[i] == ' '){
            continue;
        }
        if (post[i] == '-' && post[i+1]>='0' && post[i+1]<='9'){
            pushstack2 (post [i+1]);
            i++;
        }
        else if(post[i]>='0' && post[i]<='9')
        {
            pushstack(post[i]);             //if the element is a number push it
        }
        else if(post[i]=='+' || post[i]=='-' || post[i]=='*' ||
        post[i]=='/' || post[i]=='^')       //if element is an operator
        {
            evaluate(post[i]);             //pass it to the evaluate
        }

    }                      //print the result from the top
    printf("\n\nResult :: %d",stack[top]);
   return 0;
}

void pushstack2(char a)          //definiton for push
{
   top++;                              //incrementing top
   stack[top]=(int) a - 48;    //type casting the string to its integer value
   stack[top]=-stack[top];
}

void pushstack(char a)          //definiton for push
{
   top++;                              //incrementing top
   stack[top]=(int) a - 48;    //type casting the string to its integer value
}

void evaluate(char c)       //evaluate function
{
   int a,b,ans;        //variables used
   a=stack[top];       //a takes the value stored in the top
   stack[top]='\0';     //make the stack top NULL as its a string
   top--;                //decrement top's value
   b=stack[top];         //put the value at new top to b
   stack[top]='\0';      //make it NULL
   top--;                //decrement top
   switch(c)     //check operator been passed to evaluate
   {
      case '+':          //addition
          ans=b+a;
          break;
      case '-':           //subtraction
          ans=b-a;
          break;
      case '*':            //multiplication
          ans=b*a;
          break;
      case '/':           //division
          ans=b/a;
          break;
      case '^':      //power
          ans=b^a;
          break;
      default:
          ans=0;      //else 0
   }
   top++;             //increment top
   stack[top]=ans;        //store the answer at top
}