infix.c

#include <stdio.h>
#include <stdlib.h>
#define MAX 100
char stk[20];
int top = -1;
int isEmpty()
{
    return top == -1;
}
int isFull()
{
    return top == MAX - 1;
}
char peek()
{
    return stk[top];
}
char pop()
{
    if (isEmpty())
        return -1;
    char ch = stk[top];
    top--;
    return (ch);
}
void push(char oper)
{
    if (isFull())
        printf("Stack Full!!!!");
    else
    {
        top++;
        stk[top] = oper;
    }
}
int checkIfOperand(char ch)
{
    return (ch >= 'a' && ch <= 'z') || (ch >= 'A' && ch <='Z');
    }
int precedence(char ch)
{
    switch (ch)
    {
    case '+':
    case '-':
        return 1;
case '*':
    case '/':
        return 2;
    case '^':
        return 3;
    }
    return -1;
}

int toPostfix(char *exp)
{
    int i, j;
    for (i = 0, j = -1; exp[i]; ++i)
    {
        if (checkIfOperand(exp[i]))
            exp[++j] = exp[i];
        else if (exp[i] == '(')
            push(exp[i]);
        else if (exp[i] == ')')
        {
            while (!isEmpty() && peek() != '(')
                exp[++j] = pop();
            if (!isEmpty() && peek() != '(')
                return -1;
            else
                pop();
        }
        else
        {
            while (!isEmpty() && precedence(exp[i]) <=
                                     precedence(peek()))
                exp[++j] = pop();   
                           push(exp[i]);
        }
    }
    while (!isEmpty())
        exp[++j] = pop();

    exp[++j] = '\0';
    printf("%s", exp);
}
void main()
{
    char exp[MAX];
    char *e;
printf("Enter the expression : ");
    scanf("%s", exp);
    printf("\n");
    e = exp;
    toPostfix(e);
    
}