Practical2-1.c

#include<stdio.h>
#include<stdlib.h>     
#include<ctype.h>     
#include<string.h>

#define SIZE 100



char stack[SIZE];
int top = -1;



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

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[] , char postfix[])
{
	int i, j;
	char item;
	char x;

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

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


		item = infix[i];
	} 
	if(top>0)
	{
		printf("\nInvalid infix Expression.\n");       
		getchar();
		exit(1);
	}
	if(top>0)
	{
		printf("\nInvalid infix Expression.\n");        
		getchar();
		exit(1);
	}


	postfix[j] = '\0'; 

}

int main()
{
	char infix[SIZE], postfix[SIZE];         
	printf("\nEnter Infix expression : ");
	gets(infix);

	InfixToPostfix(infix,postfix);                  
	printf("Postfix Expression: ");
	puts(postfix);                     

	return 0;
}