Explanation:

When a vacancy occurs in the office of the President due to his  resignation, removal, death or otherwise, the Vice-President acts as the President until a new President is elected. Further, when the sitting President is unable to discharge his functions due to absence, illness or any other cause, the Vice-President discharges his functions until the President resumes his office. In case the office of Vice-President is vacant, the Chief Justice of India (or if his office is also vacant, the senior-most judge of the Supreme Court available) acts as the President or discharges the functions of the President.