Lab4.c

#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#define SIZE 20
char stack[SIZE];
int top = -1;
void push(char elem)
{
    stack[++top] = elem;
}
char pop()
{
    return (stack[top--]);
}
int precedence(char elem) /* Decides the precedence */
{
    switch (elem)
    {
    case '#':
        return 0;
    case '(':
        return 1;
    case '+':
    case '-':
        return 2;
    case '*':
    case '/':
    case '%':
        return 3;
    case '^':
        return 4;
    default:
        printf("Not a Valid Expression\n");
        exit(0);
    }
}
main()
{
    char infix[20], postfix[20], ch, elem;
    int i = 0, k = 0, pr;
    printf("Enter the Infix Expression: ");
    scanf("%s", infix);

    push('#'); /* Initial element of stack. It is a handler */
    while ((ch = infix[i++]) != '\0')
    {
        if (ch == '(') /* Verifying left parenthesis */
        {
            push(ch);
        }
        else if (isalnum(ch)) /* Verifying operand */
        {
            postfix[k++] = ch;
        }
        else if (ch == ')') /* Verifying right parenthesis */
        {
            while (stack[top] != '(')
            {
                postfix[k++] = pop();
                if (stack[top] == '#')
                {
                    printf("Not a Valid Expression\n");
                    exit(0);
                }
            }
            elem = pop(); /* Removing left parenthesis */
        }
        else /* Verifying operators */
        {
            pr = precedence(ch);
            if (ch == '^')
            {
                pr++; /* If ^ operator appears more than once evaluation takes place from

                right to left */
            }
            while (precedence(stack[top]) >= pr)
            {
                postfix[k++] = pop();
            }
            push(ch); /* Push the operator to stack */
        }
    }

    while (stack[top] != '#') /* Pop from stack till empty */
    {
        postfix[k++] = pop();
    }
    postfix[k] = '\0'; /* Make postfix as valid string */
    printf("Given Infix Expn: %s\nPostfix Expn: %s\n", infix, postfix);
}