Practical-3 || Data Structures || infix to postfix.c

#include<stdio.h>
#include<stdlib.h>     
#include<ctype.h>    
#include<string.h>
#define SIZE 100
char stack[SIZE];
int top = -1;

//push function for stacks
void push(char item)
{
	if(top >= SIZE-1)
	{
		printf("\nStack Overflow.");
	}
	else
	{
		top = top+1;
		stack[top] = item;
	}
}

//pop function
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 == '^')
	{
		return(3);
	}
	else if(symbol == '*' || symbol == '/')
	{
		return(2);
	}
	else if(symbol == '+' || symbol == '-')          
	{
		return(1);
	}
	else
	{
		return(0);
	}
}

void InfixToPostfix(char infix_exp[], char postfix_exp[])
{ 
	int i, j;
	char item;
	char x;

	push('(');                               
	strcat(infix_exp,")");                 

	i=0;
	j=0;
	item=infix_exp[i];        

	while(item != '\0')      
	{
		if(item == '(')
		{
			push(item);
		}
		else if( isdigit(item) || isalpha(item))
		{
			postfix_exp[j] = item;             
			j++;
		}
		else if(is_operator(item) == 1)       
		{
			x=pop();
			while(is_operator(x) == 1 && precedence(x)>= precedence(item))
			{
				postfix_exp[j] = x;                 
				j++;
				x = pop();                      
			}
			push(x);
		
			push(item);                 
		}
		else if(item == ')')       
		{
			x = pop();                  
			while(x != '(')               
			{
				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]; 
	}
	if(top>0)
	{
		printf("\nInvalid infix Expression.\n");      
		getchar();
		exit(1);
	}
	if(top>0)
	{
		printf("\nInvalid infix Expression.\n");       
		getchar();
		exit(1);
	}


	postfix_exp[j] = '\0';

}

//main function
int main()
{
	char infix[SIZE], postfix[SIZE];
	printf("\nEnter Infix expression : ");
	gets(infix);
	InfixToPostfix(infix,postfix);                  
	printf("Postfix Expression: ");
	puts(postfix);  
	return 0;
}