How to get reverse of number with specific base in java?

Question

I am new to java. I was just writing a function to accept a number and its base as argument and then show the palindrome number (if any). The main problem is that I don't exactly get it how to reverse the number of base other than 10? The Code I wrote (according to my poor logic ) is below:

 void   CheckPalidrome(    int num,    int base)
    {
        long     n=num;
        
        boolean palid=false;
        long TNum= num;
        for(int i=1;    i<=10   &&  palid==false;  ++i)    
        {   
            int     rev=0;
            while(n>0 )
            {
                int d= (int) (n%base);
                rev=    rev*base  + d;
            
                n/=base;
            }
            if(TNum==rev)
            {
                palid=true;
            }
            else{
                n=TNum+rev;
                TNum=n;
            }
            
            System.out.println("TNum= "+TNum+"\trev="+rev);
        
        }
        
        if(palid==true){
            System.out.println("Palidrome= "+TNum);
        }
        else{
            System.out.println("None, "+TNum);
        }
    }

But, there is a problem in this. It works fine for any number of base 10. But doesn't work for another base. For example:- 87 with base 10 will be reversed and checked if it is a Palindrome, if not it the reverse and number will be added and checked again Just Like: 87+78= 165+ 561 = 726 + 627= 1353 + 3531 = Palindrome !

But I don't know how to do this for other bases except 10. Note: 1211 with base 3 forms Palindrome 112211 and 3112 with base 4 forms Palindrome 233332 . I used these results to test my code. But the output doesn't match.

---

Please forgive my poor English. Kindly share your knowledge or give me any idea to solve the problem. I shall be glad and thankful to you :)

Answer 1

Your logic seems to be good, but remember to parse and print numbers in the given base.

E.g. the number 1211 (base 3) is the same as 49 (base 10).

To see this, I rewrote your code like this:

private static int findPalindrome(int num, int base) {
    for (int n = num, rev; n > 0; n += rev) {
        if ((rev = reverse(n, base)) == n)
            return n;
        System.out.printf("%-6s + %-6s", Integer.toString(n, base), Integer.toString(rev, base));
        if (base != 10)
            System.out.printf("   = (%-4d + %-4d) ", n, rev);
        System.out.printf("%n  = ");
    }
    return 0;
}
private static int reverse(int num, int base) {
    int rev = 0;
    for (int n = num; n != 0; n /= base)
        rev = rev * base + n % base;
    return rev;
}

Then tested it like this:

public static void main(String[] args) {
    test("87", 10);
    test("1211", 3);
    test("3112", 4);
}
private static void test(String num, int base) {
    System.out.printf("%2d: ", base);
    int p = findPalindrome(Integer.parseInt(num, base), base);
    System.out.printf("%-15s", Integer.toString(p, base));
    if (base != 10)
        System.out.printf("   = (%-11d)", p);
    System.out.printf("%n%n");
}

Which produced this output:

10: 87     + 78    
  = 165    + 561   
  = 726    + 627   
  = 1353   + 3531  
  = 4884           

 3: 1211   + 1121     = (49   + 43  ) 
  = 10102  + 20101    = (92   + 172 ) 
  = 100210 + 12001    = (264  + 136 ) 
  = 112211            = (400        )

 4: 3112   + 2113     = (214  + 151 ) 
  = 11231  + 13211    = (365  + 485 ) 
  = 31102  + 20113    = (850  + 535 ) 
  = 111221 + 122111   = (1385 + 1685) 
  = 233332            = (3070       )

The numbers in () are the base 10 numbers. The other numbers are shown in the base listed before the :.