Infix to postfix coversion using stack

C Program to convert a string from infix to post fix using stack.                     

#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();  exit(1); } else {  item = stack[top];  top = top-1;  return(item); }}
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;                  /* to pop all higher precendence operators  */    j++;    x = pop();                       /* add them to postfix expresion */   }   push(x);      push(item);                 /* push current oprerator symbol onto stack */  }  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  {printf("\nInvalid infix Expression.\n");        /* the it is illegeal  symbol */   getchar();   exit(1);  }  i++;

  item = infix_exp[i];  }  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'; 
}
/* main function begins */int main(){ char infix[SIZE], postfix[SIZE];         /* declare infix string and postfix string */

 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;}