ISC Practicals and Theory - Palindrome Number (String)

A Palindrome Number reads the same from left to right and from right to left. In this version the number is converted into a String and its characters are compared in pairs from the two ends, so no digit arithmetic is needed.

Definition

A number is a Palindrome Number if its digits, written in order, are the same as when written in reverse order.

• $1331$: first and last digits are $1$ and $1$, second and third are $3$ and $3$, so it is a palindrome.
• $1231$: first and last digits are $1$ and $1$, but second and third are $2$ and $3$, so it is not a palindrome.

Algorithm

1. main creates the object ob, calls accept() to read num and then check().
2. check() prints Invalid input if num is negative; otherwise it converts the number to a String with Integer.toString(num) and passes it to isPalindrome().
3. isPalindrome(String s):
   • Set i = 0 (first character) and j = s.length() - 1 (last character).
   • While i < j: if s.charAt(i) != s.charAt(j), return false; otherwise do i++ and j--.
   • If the loop ends without a mismatch, return true.
4. Print whether the number is a Palindrome Number.

Program

isPalindrome() compares characters using length() and charAt(); check() converts the number to a String and prints the result.

import java.util.Scanner;
public class PalindromeStr
{
    int num;
    PalindromeStr()
    {
        num = 0;
    }
    void accept()
    {
        Scanner sc = new Scanner(System.in);
        System.out.print("Enter a number: ");
        num = sc.nextInt();
    }
    boolean isPalindrome(String s)
    {
        int i = 0, j = s.length() - 1;
        while (i < j)
        {
            if (s.charAt(i) != s.charAt(j))
                return false;
            i++;
            j--;
        }
        return true;
    }
    void check()
    {
        if (num < 0)
            System.out.println("Invalid input");
        else
        {
            String s = Integer.toString(num);
            if (isPalindrome(s))
                System.out.println(num + " is a Palindrome Number");
            else
                System.out.println(num + " is not a Palindrome Number");
        }
    }
    public static void main(String args[])
    {
        PalindromeStr ob = new PalindromeStr();
        ob.accept();
        ob.check();
    }
}

Variable description table

Sample input and output

Enter a number: 1331
1331 is a Palindrome Number

Enter a number: 1231
1231 is not a Palindrome Number

Enter a number: -5
Invalid input

Dry run

Input: num = 1231, so s = "1231" and s.length() = 4. Start with i = 0, j = 3.

isPalindrome() returns false, so the output is 1231 is not a Palindrome Number, the same as the real run above.

Variations

1. Palindrome Word "Accept a word and check whether it is a palindrome."
2. Palindrome Sentence "Check whether a sentence is a palindrome, ignoring the spaces."