Array required but int found

Question

What's wrong with my progam?

public class Square{
  public int x;
  public Square() {
    int x[] = new int[10];
    int y;
    x[0] = 7;
  }

  public void root() {
    for (int i = 0; i < 10; i++) {
        x[i+1] = x[i] - (Math.pow(x[i] - 2.5, 2)) / (2 * (x[i] - 2.5));
        System.out.println(x[i + 1]);
    }
  }
}

I don't get what's wrong, my for loop does not seem to be working and it keeps on displaying the error for some reason. Could someone help me figure this out?

Okay, I wrote this program now:

    public class Square
    {
       public double x[];
       public void root()
       {
    double x[] = new double[10];
    x[0]=7;
    for(int i=0; i<8; i++)
    {
        x[i+1]=x[i]-(Math.pow(x[i]-2.5,2))/(2*(x[i]-2.5));
        System.out.println(x[i+1]);
    }
}
}

And it is showing this output:

3.625
3.0625
2.78125
2.640625
2.5703125
2.53515625
2.517578125
   java.lang.NullPointerException
    at Square.root(Square.java:14)
java.lang.ArrayIndexOutOfBoundsException: Index 10 out of bounds for length 10 at Square.root(Square.java:11)

I don't know why I'm getting these errors. Also, the answer should be 6.25 at some point. But, it doesn't show that output.

Answer 1

This is because you have defined x previously as just an int , instead of an array of ints.

Try this:

public class Square {
    public int x[];

    public Square() {
        this.x = new int[10];
        int y;
        x[0] = 7;
    }

    public void root() {
        for(int i = 0; i < 10; i++) {
            x[i + 1] = x[i] - (Math.pow(x[i] - 2.5, 2)) / (2 * (x[i]  -2.5));
            System.out.println(x[i + 1]);
        }
    }
}

Answer 2

Your constructor has a local variable int[] x which is disarded at the end of the constructor.

Try this:

public class Square{
  // initialize to array of ten ints
  public int x[] = new int[10];

  public Square() {
    x[0] = 7;
  }

  public void root() {
    for (int i = 0; i < 10; i++) {
        x[i+1] = x[i] - (Math.pow(x[i] - 2.5, 2)) / (2 * (x[i] - 2.5));
        System.out.println(x[i + 1]);
    }
  }
}

Edit: The int y is local to the constructor, it is discarded at the end of the constructor scope, too.

Answer 3

Here is the commented code explaining the problem :

public class Square
{
  // Here, x is defined as an attribute of class Square, of type int
  public int x;
  public Square()
  {
    // Here, x is locally defined as a local variable, of type int[]
    // It shadows the attribute x. This is considered as a bad practice.
    int x[] = new int[10];
    int y;
    // Here, x is the local variable, of type int[]. It IS an array so
    // this line is valid.
    x[0]=7;
  }// From this point, the local variable x is not defined anymore
   // (that is the point of a local variable)

Now here :

  public void root()
  {
    for(int i=0; i<10; i++)
    {
      // Here, x is referencing the attribute of class Square, which is an int
      // But you try to access it as if it was and int[]
      x[i+1]

Answer 4

First you must declare x as an array:

public int[] x;

notice that the java style is not int x[];
Then inside Square() you have to initialize x like this:

x = new int[10];

Finally, this:

(Math.pow(x[i] - 2.5, 2)) / (2 * (x[i] - 2.5))

returns a double so you have to cast it to int :

x[i+1] = x[i] - (int) ((Math.pow(x[i] - 2.5, 2)) / (2 * (x[i] - 2.5)));

So your code should be:

public int[] x;
public void Square() {
    x = new int[10];
    x[0] = 7;
}
public void root() {
    if (x == null)
        Square();
    for(int i = 0; i < x.length - 1; i++) {
        x[i+1] = x[i] - (int) ((Math.pow(x[i] - 2.5, 2)) / (2 * (x[i] - 2.5)));
        System.out.println(x[i+1]);
    }
}

Inside the loop you are accessing the i + 1 item so the counter of the loop must take values up to x.length - 2 , this is why I have in the code: i < x.length - 1 .
I have removed the declaration of y from Square() as it is not used.

Answer 5

public class Square
{
   public double x[];
   public Square()
  {
    this.x = new double[10];
    x[0]=7;
  }
  public void root()
  {
    System.out.println(x[0]);
    for(int i=1; i<10; i++)
    {
        x[i]=x[i-1]-(Math.pow(x[i-1]-2.5,2))/(2*(x[i-1]-2.5));
        System.out.println(x[i]);
    }
  }
}

Answer 6

Try use this code

public class Square
{
   public double x[];
   public void root()
   {
        x = new double[10];
        x[0]=7;
        for(int i=1; i<10; i++)
        {
            x[i]=x[i-1]-(Math.pow(x[i-1]-2.5,2))/(2*(x[i-1]-2.5));
            System.out.println(x[i]);
        }
    }
}