Thread: Stack operations from switch statement elements

I have a program that compiles with no errors or warnings, but the logic is flawed. It is a postfix calculator...I have a default in the switch statement that is supposed to match non-operator input and post it.

The switch is called as a result of a function to define isoperator conditions.

Unfortunately, the input is read, and each char input, space, and \n is printf'd back out as the invalid input default and a random math total is displayed.

Am I misinterpreting the logic contained within my switch statement?

Code:

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <ctype.h>
#define STACKSIZE 256
#define TRUE 1
#define FALSE 0

/* stack type definition */

typedef struct {
   int top;
   float items[STACKSIZE];
} stack_type;

/* function prototypes */
int is_empty(stack_type*);
void push(stack_type*, float);
float pop(stack_type*);


int main(void)
{
   char buffer[256];
   char *input[STACKSIZE];
   stack_type stk;
   int i=0;
   stk.top = -1;            /* Initialize the stack to empty */
   int sum;
   printf("This program will evaluate postfix expressions using a\n");
   printf("good implementation of stacks.\n");
   printf("You may enter up to a 50-char expression.\n");
   printf("Please enter a postfix/reverse-polish expression for me.\n");
   fgets(*input, sizeof(*input), stdin);
   while(1) {
   for (i=0; i<strlen(*input); i++) {
   int operator = is_operator(input[i]);
   if(operator)
   {
      switch(operator) {

         case '+':
            sum = pop(&stk) + pop(&stk);
         break;
         case '-':
            sum = pop(&stk) - pop(&stk);
         break;
         case '*':
            sum = pop(&stk) * pop(&stk);
         break;
         case '/':
            sum = pop(&stk) / pop(&stk);
         break;
         case '%':
            sum = pop(&stk) % pop(&stk);
         break;
         default:
            printf("Epic Fail...invalid input.\n");
      }
         push(&stk, sum);
   }
   else if (isdigit(*input[i]))
      {
         push(&stk, (*input[i] - 48));
      }
   else if (strcmp(*input, "quit\n") == 0)
      {
         printf("Time to quit...goodbye!\n");
         exit(0);
      }
  }

printf("The total is %f\n", stk);
printf("Please enter another expression: \n");
fgets(*input, sizeof(*input), stdin);
}
return 0;
}


/* IS Empty */
int is_empty(stack_type* stk)
{
   if ( (*stk).top == -1 )
      return TRUE;
   else
      return FALSE;
}


/* Push */
void push(stack_type* stk, float element)
{
   if ( stk == NULL )
   {  printf("Warning! Stack pointer is set to NULL.\n");
      exit(1); /* exit the program */
   }

   /* check stack is not full */
   if ( (*stk).top == (STACKSIZE - 1) )
   {  printf("Stack Overflow!\n");
      exit(1);
   }

   /* increment stack top */
   (*stk).top++;

   /* add the new element to the top */
   (*stk).items[(*stk).top] = element;

   return;
}
/* Pop */
float pop(stack_type* stk)
{  float value=0;
   /* Check stack exists */
   if ( stk == NULL )
   {  printf("Warning!  Stack pointer is set to NULL.  Is it really there?\n");
      exit(1);
   }
   /* check that the stack is not empty */
   if (is_empty(stk))
   {  printf("Stack underflow!\n");
      exit(1);
   }
   /* get top item */
   value = (*stk).items[(*stk).top];
   /* decrement top */
   (*stk).top--;
   /* return popped item to calling f(x); */
   return value;
}
int is_operator(int x)
{
  if(x == '+' || x == '-' || x =='/' || x == '*' || x == '%')
    return;
}