STACK postfix evaluation problem

As it stands, your calculator recognizes a single digit as operand while it would need to recognize a number (multiple digits). That means you must allow a separator in your input expression (otherwise it wouldn't be able to distinguish between two input numbers), you have to handle it and, finally, you need to handle multiple digits as a single token (a number, the operand).
I have modified you code in order to work with such requirements.

You may input expressions like:

#include <stdio.h>
#include <math.h>
#define MAX 20
struct stack
{
int top;
float str[MAX];
} s;//stack
char postfix[MAX];//postfix
int operand;

void push(float);
float pop();


int isoperand(int n);
int get_operand(int n);
int skip_blanks(int n);

float operate(float,float,char);
int main()
{
  int i=0;
  printf("Enter Expression:");
  fgets(postfix, sizeof(postfix), stdin); // get a whole input line
  float ans,op1,op2;
  while(1)
  {
    i = skip_blanks(i);
    if ( postfix[i] == '\n')
      break;
    else if(isoperand(i))
    {
      i = get_operand(i);
      push(operand);
    }
    else
    {
     op1=pop();
     op2=pop();
     ans=operate(op1,op2, postfix[i]);
     push(ans);
     printf("%f %c %f = %f\n",op2,postfix[i],op1,ans);
     i++;
    }
  }
  printf("%f",s.str[s.top]);
  getchar();
  return 0;
}
int skip_blanks(int n)
{
  while (postfix[n] == ' ') n++;
  return n;
}

int get_operand(int n)
{
  operand = 0;
  while (postfix[n] >= '0' && postfix[n] <= '9')
  {
    operand *= 10;
    operand += postfix[n]-'0';
    n++;
  }
  return n;
}

int isoperand(int i)
{
  char x = postfix[i];
  if(x>='0' && x<='9')
  return 1;
  else
  return 0;
}
void push(float x)
{
   if(s.top==MAX-1)
    printf("Stack is full\nStack overflow\n");
   else
   {
    s.top++;
    s.str[s.top]=x;
    }
}
float pop()
{
   if(s.top==-1)
   {
         printf("Stack is emplty\nSTACK UNDERFLOW\n");
         getchar();
        return nan("?");
   }
   else
   {
     s.top--;
     return s.str[s.top+1];
   }
}
float operate(float op1,float op2,char a)
{
  switch(a)
  {
   case '+':return op2+op1;
   case '-':return op2-op1;
   case '*':return op2*op1;
   case '/':return op2/op1;
   case '^':return pow(op2,op1);
   }
  return nan("?");
}