Make a Static method in the main class that writes on the screen all even numbers from 1 to 10000 that are palindromes

Question

This is my method:

public static void B()
{
    int vet[]=new int [100];
    int vet2[]=new int [vet.length/2];
    for(int i=0;i<=vet.length;i++)

    {
        vet[i]=i+1;
    }
    for(int i=0;i<vet.length;i++)
    {
        int rest = vet[i]%2;
        if(rest==0)
        {
            vet2[i]=vet[i];
        }
    }
    for (int i = 0; i <= vet2.length; i++) 
    {
        if(vet2[i]==(vet2[i-1])-i)
        {
            System.out.println(vet2[i]+"É PALINDROMO");
        }
    }

And this is the error that appears:

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 100
  at lista1.pkg05.Lista105.B(Lista105.java:59)
  at lista1.pkg05.Lista105.main(Lista105.java:15)
C:\Users\Pichau\AppData\Local\NetBeans\Cache\8.2\executor-snippets\run.xml:53:

Java returned: 1

Answer 1

The algorithm has an error in for because it reads one more number. You cannot buy with <= to size because the count starts at 0, it has to be with <. It would also work to use this comparison by subtracting 1, but this generates a larger computational effort and it makes no sense to use it like this.

The access to check if it is palindromic has a wrong formula and causes problem as well. I improved, but I think it’s still not what you want, because it’s to check if the array is palindromic and does not say about each element, but will know.

One of the arrays has no function in the algorithm other than complicate.

By this algorithm will never be palindromic, but I hope it will generate the array with some other algorithm. Would it be to check if the digits of each number is palindromic? Then the algorithm is another. But you’re already in trouble.

I only kept the first 100 as asked in the question, maybe to make it easier to test, to go up to 10000, just change the size of the array.

I gave one organized in the code too.

class HelloWorld {
    public static void main (String[] args) {
        int vet[] = new int[50];
        for (int i = 0; i < vet.length; i++) if (i % 2 == 0) vet[i] = (i + 1) * 2;
        for (int i = 0; i < vet.length; i++) if (vet[i] == vet[vet.length - i - 1]) System.out.println(vet[i] + " É PALINDROMO");
    }
}

Behold working in the ideone. And in the repl it.. Also put on the Github for future reference. Data is missing, but at least the error disappears.

Answer 2

Well, I’ll approach it in a more legible way.

public class HelloWorld {
   public static void main(String[] args) {
      Palindromas();
   }
   public static void Palindromas()
   {

       StringBuilder sb = new StringBuilder();//Alocar Espaço para um SB

       for(int i = 1; i <= 10000; i++) {

         sb.append(i);//Adicionar o número no sb

         sb.reverse();//Inverter o número

         if(
            i % 2 == 0 // se  é par
            && // E
            Integer.parseInt(sb.toString()) == i //Ele ao contrário é igual a ele normal
            ){
             System.out.println(i);//Printa
         }

         sb.setLength(0);//Limpa o Buffer pra reutiliza-lo no proximo item do loop

      };
   }
}

I only did so because I already had more answers, mine is a workaround ;)

Answer 3

The problem is in these two lines;

for(int i=0;i<=vet.length;i++)

for (int i = 0; i <= vet2.length; i++) 

The length property returns the total size of elements in the array, so when using to iterate an array like the code above, in an array of 10 positions it will run 11 times from 0 to 10 thus giving the Arrayindexoutofboundsexception error in the first iteration, to solve the problem follow the code below.

for(int i=0;i<=vet.length -1;i++)

for (int i = 0; i <= vet2.length -1; i++) 

Done so the code will go through 10 times from 0 to 9.