infix_to_postfix.c

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

#define SIZE 100

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

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("\nStack Underflow!");
		exit(1);
	}
	else
	{
		item = stack[top];
		top = top-1;
		return(item);
	}
}

int is_operator(char op)
{
	if(op == '^' || op == '*' ||  op == '/' || op == '+' || op == '-')
	{
		return 1;
	}
	else
	{
		return 0;
	}
}

int check_precedence(char op)
{
	if(op == '^')
	{
		return 3;
	}
	else if(op == '*' || op == '/')
	{
		return 2;
	}
	else if(op == '+' || op == '-')
	{
		return 1;
	}
	else
	{
		return 0;
	}
}

void infixToPostfix(char infix[], char postfix[])
{
	char el, item;
	int i = 0, j = 0;
	push('(');
	strcat(infix, ")");
	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)
		{
			el = pop();
			while(is_operator(el) == 1 && check_precedence(el) >= check_precedence(item))
			{
				postfix[j] = el;
				j++;
				el = pop();
			}
			push(el);
			push(item);
		}
		else if(item == ')')
		{
			el = pop();
			while(el != '(')
			{
				postfix[j] = el;
				j++;
				el = pop();
			}
		}
		else
		{
			printf("\nInvalid expression!");
			exit(1);
		}
		i++;
		item = infix[i];
	}
	postfix[j] = '\0';
}

int main()
{
	char infix[SIZE], postfix[SIZE];
	printf("Enter a valid infix expression: ");
	scanf("%[^\n]", infix);
	infixToPostfix(infix, postfix);
	printf("\nEquivalent postfix expression is %s", postfix);
	return 0;
}