infix2postfix.c

/*
3.Write a C program to convert and print a given valid parenthesized infix arithmetic expression to postfix expression.
The expression consists of single character and binary operators + - * /.
*/
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#define STKSZ 100

struct stack { char items[STKSZ]; int top} stk;

void push(char ele)
{
	if (stk.top + 1 == STKSZ) {
		printf("\n[E] Stack Overflow!\n");
		exit(1);
	}
	stk.items[++stk.top] = ele;
}

char pop()
{
	if (stk.top == -1) {
		printf("\n[E] Stack Underflow!\n");
		exit(1);
	}
	return (stk.items[stk.top--]);
}


int priority(char c)
{
	switch (c)
	{
	case '(': return 0;
	case '+': case '-': return 1;
	case '*': case '/': return 2;
	case '$': return 3;
	}
}

int main()
{
	char expr[100], *e, x;
	printf("Enter Infix Expression:");
	scanf("%s", expr);
	printf("Postfix Expression:");
	stk.top = -1;

	for (e = expr; *e != '\0'; e++)
	{
		if (isalnum(*e))
			printf("%c", *e);
		else if (*e == ')')
		{
			while ( (x = pop()) != '(')
				printf("%c", x);
		}
		else if (*e == '(')
			push(*e);
		else
		{
			while ( priority(stk.items[stk.top]) >= priority(*e) )
				printf("%c", pop());
			push(*e);
		}
	}

	while (stk.top != -1)
		printf("%c", pop());
	putchar('\n');
}