Data Structure C program Postfix Evaluation Dev c++

 #include <stdio.h>  
 #include <conio.h>  
 #define MAX 20  
 struct stack{  
 int top;  
 float str[MAX];  
 }s;//stack  
 char postfix[MAX];//postfix  
 void push(float);  
 float pop();  
 int isoperand(char);  
 float operate(float,float,char);  
 int main()  
 {  
   int i=0;  
   printf("Enter Expression:");  
   scanf("%s",postfix);  
   float ans,op1,op2;  
   while(postfix[i]!='\0')  
   {  
    if(isoperand(postfix[i]))  
    push(postfix[i]-48);  
    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]);  
    getch();  
 }  
 int isoperand(char x)  
 {  
   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");  
          getch();  
    }  
    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);  
    }  
 }