How to create an array through another?

Question

I have a problem that deals with providing an array to the program and this should return only the odd numbers of this array, and for this I create a new array that receives the values:

public void maiorValorImpar(int[] a) {
    int b= a.length-1;
    int[] arr2= {};
    int i=0;
    while(i<b) {
        if(a[i]%2 != 0) {
            arr2[i]= a[i];
            System.out.println(arr2);
        }
        i++;
    }
} 

However with my current code, I get the following error:

Exception in thread "main" java.lang.Arrayindexoutofboundsexception: Index 0 out of Bounds for length 0

Answer 1

This error is due to the attempt to access the first element (Index 0) of arr2, whereas arr2 has no element (length 0) because you declared her with int[] arr2= {}, without any element between the keys.

In the loop you are declaring b as a.length-1, which makes the loop never check the last element of a, because he’s on the index a-length-1, and the condition is i < b, for example:

a.length: 5;
b: a.length-1 = 4;
while (i < b); // o while só passará pelas posições 0, 1, 2 e 3, pois são menores que b (4)

Another thing I noticed was that you assign arr2[i] = a[i], leaving empty spaces if there are even numbers before an odd, because arr2 will be assigned in the position i, for example:

// Usando o seu código, porém assumindo que arr2 possui um tamanho suficiente
a:    [0, 4, 2, 87, 6, 9, 3]
arr2: [ ,  ,  , 87,  , 9, 3]

To correct this, use a variable that stores the current index of arr2:

public static void maiorValorImpar(int[] a) {
    int b = a.length; // Correção da não verificação do último elemento
    int[] arr2 = new int[b];
    int i = 0, j = 0; // j guarda onde será colocado o próximo número em arr2
    while(i < b) {
        if(a[i]%2 != 0) {
            arr2[j]= a[i];
            System.out.println(arr2[j]);
            j++;
        }
        i++;
    }
}

When working with arrays you don’t initially know the size, I recommend using Arraylist (documentation) (w3schools), so you don’t need to allocate spaces in the array that you might not use, like on line 3 of my example (int[] arr2 = new int[b];).

Answer 2

Java arrays cannot grow dynamically after you set their length that can no longer be changed. When you do int[] arr2= {}; you are telling the compiler to create the array arr2 with length 0.

To solve you can use a list List<int> and the method ArrayList.toArray( T[] a ) to get the array.

public static void main (String[] args) throws java.lang.Exception
{
    int[] a = { 1, 3, 5 };
    int b = a.length;
    // cria uma lista de int
    List<Integer> arr2 = new ArrayList<Integer>();
    int i = 0; 
    while (i < b) {
        if (a[i] % 2 != 0) {
            // Adiciona a[i] a lista arr2
            arr2.add(a[i]);
            // Imprime o item i da lsta arr2
            System.out.println( arr2.get(arr2.size() - 1) ); // Leia os comentários               
        }
        i++;
    }
    System.out.println(arr2);
}