Not understanding the issue with my solution to a palindrome problem-CodePudding

I'm trying to solve a question where your code is supposed to determine if a given number is a palindrome or not, and I don't understand why it isn't working.

(You can skip this and just read the code if you want) The idea was that I create a string with the value of the integer, and create a for loop using the length of said string that uses % 10 to reverse the integer and store it in a separate string. I would then compare the 2 strings to determine if the number is a palindrome or not

public static boolean isPalindrome(int x) {
        String s = String.valueOf(x);
        int count = s.length();
        String palindrome = "";
        for(int i = 0; i < count; i  ){
            palindrome  = x % 10;
        }
        System.out.print(palindrome);
        if(palindrome == s){
            return true;
        }
        else{
            return false;
        }
    }

The problem is that the code only returns false, and when I added a print statement to check the what the reversed number (String palindrome) is I got a different number. For ex. I used 121 to test it and after the for loop the print statement outputted 111. I'm not really looking for a solution, I just want to understand why it's behaving like this. Thanks in advance.

CodePudding user response：

    for(int i = 0; i < count; i  ){
        palindrome  = x % 10;
    }

Since x does not change in that loop (or indeed anywhere in the code), each execution yields the same thing, the least significant digit of x.

Thus palindrome has some number of copies of the same digit, and will almost never equal s

You need to divide x by 10 each time around the loop.

CodePudding user response：

The problem is your for-loop. Every time the the for-loop iterates you have also to divide by 10 your x

public static boolean isPalindrome(int x) {
        String numStr = String.valueOf(x);
        StringBuilder palindrome = new StringBuilder();

        for(; x>0; x /=10)  {
            palindrome.append(x % 10);
        }

        return palindrome.toString().equals(numStr);
    }

In this for-loop is missing the initialisation because you don't need the index. In fact you can replace the for loop with a while loop:

while (x>0) {
    palindrome.append(x % 10);
    x /= 10;
}

Note: I have also simplify your return. You can use the method equals() that return true if the two strings are equal.

Note: When you want to append strings its a better practice use a StringBuilder than use the = operator.