/* This C program is designed to evaluate a postfix expression*/

#include <stdio.h>
#include <string.h>
#include <math.h>
#include <ctype.h>

#define SIZE 15 //initializing a common size for all arrays

int stack[SIZE];
char user[SIZE];
int top=0;

int pop()
{
    return stack[--top];//This sentence returns a value back to where the function is called
}

void push(int n)
{
    stack[top++]=n;//Enters an element into the stack
}

int main()
{
    start:int i=0,cn=0,pop1=0,pop2=0;
    char input;
    top=0;
    printf("Please enter a valid postfix expression, each token separated by a space:-\n");
    gets(user);//getting the string from the user

    while(user[i]!='\0')//Reading the string token wise
    {
        int result=0,l=0;
        input=user[i];
        if((input>=48)&&(input<=57))//checking for an number (ASCII 48 to 57)
        {
            for(int k=0;user[i+k]!=' ';k++)//calculating decimal value of number
            {
                result=(result*10)+user[i+k]-48;
                l++;
            }
            i+=l;
            push(result);
        }

        else if ((top!=0)&&((top%2==0)||((top-cn)%2==0))&&((input=='+')||(input=='-')||(input=='*')||(input=='/')||(input=='$')||(input=='^')))
        {
            pop1=pop();
            pop2=pop();
            switch(input)
            {
            case '+'://Adding
                result=pop2+pop1;
                break;
            case '-'://subtracting
                result=pop2-pop1;
                break;
            case '*'://multiplying
                result=pop2*pop1;
                break;
            case '/':
                if(pop1!=0)//dividing
                result=pop2/pop1;
                else
                printf("Can't divide by zero");
                break;

            }

           push(result);
           cn++;//calculating position of computed result in stack
    }
    else if(input==' ');
    else
    {
    printf("Invalid expression\n");
    goto start;
    }
            i=i+1;//incrementing i to traverse the input array
}
    if(top==1)//print result only if a single result exists
    printf("Result=%d",pop());
    else
    {
        printf("Invalid expression\n");
        goto start;
    }
    return 0;
}