infix postfix notation.

#include<stdio.h> 
#include<stdlib.h> /* for exit() */ 
#include<ctype.h> /* for isdigit(char ) */ 
#include<string.h> 
#define SIZE 100 
char stack[SIZE]; 
int top = -1; 
/* define push operation */ 
void push(char item) 
{ 
 if(top >= SIZE-1) 
 { 
 printf("\nStack Overflow."); 
 } 
 else 
 { 
 top = top+1; 
 stack[top] = item; 
 } 
} 
/* define pop operation */ 
char pop() 
{ 
 char item ; 
 if(top <0) 
 { 
 printf("stack under flow: invalid infix expression"); 
 getchar(); 
 /* underflow may occur for invalid expression */ 
 /* where ( and ) are not matched */ 
 exit(1); 
 } 
 else 
 { 
 item = stack[top]; 
 top = top-1; 
 return(item); 
 } 
} 
/* define function that is used to determine whether any symbol is operator or not 
(that is symbol is operand) 
* this fucntion returns 1 if symbol is opreator else return 0 */ 
int is_operator(char symbol) 
{ 
 if(symbol == '^' || symbol == '*' || symbol == '/' || symbol == '+' || symbol =='-') 
 { 
 return 1; 
 } 
 else 
 { 
 return 0; 
 } 
} 
int precedence(char symbol) 
{ 
 if(symbol == '^')/* exponent operator, highest precedence*/ 
 { 
 return(3); 
 } 
 else if(symbol == '*' || symbol == '/') 
 { 
 return(2); 
 } 
 else if(symbol == '+' || symbol == '-') /* lowest precedence */ 
 { 
 return(1); 
 } 
 else 
 { 
 return(0); 
 } 
} 
void InfixToPostfix(char infix_exp[], char postfix_exp[]) 
{ 
 int i, j; 
 char item 
 char x; 
 push('('); /* push '(' onto stack */ 
 strcat(infix_exp,")"); /* add ')' to infix expression */ 
 i=0; 
 j=0; 
 item=infix_exp[i]; /* initialize before loop*/ 
 while(item != '\0') /* run loop till end of infix expression */ 
 { 
 if(item == '(') 
 { 
 push(item); 
 } 
 else if( isdigit(item) || isalpha(item)) 
 { 
 postfix_exp[j] = item; /* add operand symbol to postfix expr */ 
 j++; 
 } 
 else if(is_operator(item) == 1) /* means symbol is operator */ 
 { 
 x=pop(); 
 while(is_operator(x) == 1 && precedence(x)>= precedence(item)) 
 { 
 postfix_exp[j] = x; /* so pop all higher precendence operator and */ 
 j++; 
 x = pop(); /* add them to postfix expresion */ 
 } 
 push(x); 
 /* because just above while loop will terminate we have 
 oppped one extra item 
 for which condition fails and loop terminates, so that one*/ 
 push(item); 
 } 
 else if(item == ')') /* if current symbol is ')' then */ 
 { 
 
 x = pop(); /* pop and keep popping until */ 
 while(x != '(') /* '(' encounterd */ 
 { 
 postfix_exp[j] = x; 
 j++; 
 x = pop(); 
 } 
 } 
 else 
 { /* if current symbol is neither operand not '(' nor ')' and nor 
 operator */ 
 printf("\nInvalid infix Expression.\n"); /* the it is illegeal symbol */ 
 getchar(); 
 exit(1); 
 } 
 i++; 
 item = infix_exp[i]; /* go to next symbol of infix expression */ 
 } /* while loop ends here */ 
 if(top>0) 
 { 
 printf("\nInvalid infix Expression.\n"); /* the it is illegeal symbol */ 
 getchar(); 
 exit(1); 
 } 
 if(top>0) 
 { 
 printf("\nInvalid infix Expression.\n"); /* the it is illegeal symbol */ 
 getchar(); 
 exit(1); 
 
} 
 postfix_exp[j] = '\0'; /* add sentinel else puts() fucntion */ 
 /* will print entire postfix[] array upto SIZE */ 
} 
/* main function begins */ 
int main() 
{ 
 char infix[SIZE], postfix[SIZE]; /* declare infix string and postfix string */ 
 /* why we asked the user to enter infix expression 
 * in parentheses ( ) 
 * What changes are required in porgram to 
 * get rid of this restriction since it is not 
 * in algorithm 
 * */ 
 printf("ASSUMPTION: The infix expression contains single letter variables and single digit constants 
only.\n"); 
 printf("\nEnter Infix expression : "); 
 gets(infix); 
 InfixToPostfix(infix,postfix); /* call to convert */ 
 printf("Postfix Expression: "); 
 puts(postfix); /* print postfix expression */ 
 return 0; 
}