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cl1p.net/postfix

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#include<stdio.h>
#include<stdlib.h>
#include<ctype.h>
typedef struct stack
{
  int s[100];
  int top;
}STACK;
void evaluation(char *,STACK *);
int pop(STACK *);
void push(STACK *,float ele);
int main(void)
{
  STACK *S1;
  S1=(STACK *)malloc(sizeof(STACK));
  S1->top = -1;
  char postfix[100];
  printf("Enter postfix expression\n");
  scanf("%[^\n]c", postfix);
  evaluation(postfix, S1);
}

void evaluation(char *postfix, STACK *S1)
{
  char ch;
  while(*postfix!='\0')
  {
    ch= *postfix;
    if(isdigit(ch))
    {
      push(S1, ch-'0');
    }
    else
    {
      int op2 = pop(S1);
      int op1 = pop(S1);
      switch(ch)
      {
        case '+' : push(S1, op1+op2);
                   break;
        case '-' : push(S1, op1-op2);
                   break;
        case '*' : push(S1, op1*op2);
                   break;
        case '/' : push(S1, op1/op2);
                   break;
        case '%' : push(S1, op1%op2);
                   break;
      }
    }
     postfix++;
  }
   printf("Value of entered expression is %d\n", S1->S[S1->top]);
}
  
void push(STACK *S1, float ele)
{
  S1->top++;
  S1->S[S->top] = ele;
}
 
int pop(STACK *S1);
{
  if(S1->top==-1)
  {
    printf("Entered exp. is wrong\n");
    exit(1);
  }
  int ele;
  ele = S1->S[S1->top];
  S1->top--;
  return ele;
}