Shift elements of array list in java

Question

so I'm trying to shift an array to the right so that x will appear as 4, 5, 6, 7, 1, 2, 3. I think my algorithm is correct but for some reason it won't return x and just skips anything after the "x = rotate(x, 3)" line. Any help or explanation would be appreciated.

    public class Ex1sub3 {
    public static void main(String[] args){

        double[] x = {1, 2, 3, 4, 5, 6, 7};


        System.out.println("Before rotation: ==============================");
            for (int i = 0; i < x.length; i++)
            {
                System.out.println("x[" + i + "]: " + x[i]);
            }

            x = rotate(x, 3);

            System.out.println("After     rotation:==============================");

                for (int i = 0; i < x.length; i++)
                {
                    System.out.println("x[" + i + "]: " + x[i]);
                } 

    }

    private static double[] rotate(double[] x, int n){
        int l = x.length;

        double [] r = x;

        int c = 0;
        int rotation;
        int startRotation = 0;
        for (c = 0; c < l; c++)
        {
            rotation = c+n;

            while (rotation < l-n)
            {
                x[c] = r[rotation];
            }

            if (rotation >= l-n)
            {
            x[c] = r[startRotation];
            startRotation++;
            }

        }

        return x;
    }

}

Peter Lawrey · Accepted Answer · 2015-11-16 23:51:55Z

1

This is where a debugger would help you see but, the one problem is you are overwriting the value you first value to copy to without keeping it. e.g.

Say you have two elements, { 1, 2 } The first thing you do is copy x[1] = x[0] do the array is { 1, 1 } i.e. you over wrote a value you can't get latter. Instead you need to hold the value to be copied. e.g.

double d = x[1]; 
x[1] = x[0]; 
x[0] = d;

This is more complicated when you shift by n but it can be done. A simpler solution is to take a copy of the array before you rotate.

answered Nov 16, 2015 at 23:51

Peter Lawrey

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3 Comments

waterytart Over a year ago

see, I thought I made a copy of the initial array in the r variable, so all of the initial array's elements are intact and can be accessed by x. I'm stumped.

Peter Lawrey Over a year ago

@waterytart In Java there is only primitives and references to an object. You t is a reference to an double[] it is not an array. When you copy r = t you are copying the reference, not the array itself. Instead you can do double[] r = t.clone(); to create a new array which is a copy of t and make r a reference to that. Again this is something you can see in the debugger in your IDE.

Peter Lawrey Over a year ago

@waterytart similarly, when you see String s you have to tell yourself that s is not a String but a reference to a String. Without this knowledge you can't understand what s == "Hi" is really doing. i.e. it is comparing two references to Strings, not their contents.

kandid · Accepted Answer · 2015-11-17 00:01:09Z

0

Your program does not skip any lines but will stuck in your rotate(...) function. I can see no way to leave the inner while loop since all variables in the condition are untouched in the body. But as @Peter Lawrey already mentioned: a debugger might prove useful in cases where the program flow is mysterious.

answered Nov 17, 2015 at 0:01

kandid

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