Rotating all elements of the array to left by “k” position but my code moves it to the right

Question

Can someone tell how to fix this code so instead of moving the arrays to the right how do i make it move to the left? Can someone please explain it to me too

public class Test {
  public static void main(String[] args) {
    int[] b = {10, 20, 30, 40, 50, 60};
    System.out.println("\n///// Rotate Left k cell example \n////");
    b = copyArray(a, a.length); 
    b = rotateLeft(b,4); //assuming k = 4
    printArray(b); // This Should Print: { 50, 60, 10, 20, 30, 40 };
  }

  // Rotates all the elements of the source array to the right by 'k' positions
  // Returns the updated array
  public static int[] rotateLeft(int[] source, int k) {
    int[] arr = copyArray(source,source.length); //copies all source to a new array
    int[] temp = new int[arr.length];

    for (int i = 0; i < arr.length; i++){
      temp[(i + k) % arr.length] = arr[i];
    }

    for (int i = 0; i < arr.length; i++){
      arr[i] = temp[i];
    }

    return arr; 
  }
}

So this code rotates the elements to the right side. How should i modify it so that it rotates it to the left side?
Output i am getting:

{ 30, 40, 50, 60, 10, 20 }

but my desired output is:

{ 50, 60, 10, 20, 30, 40 }

Answer 1

I think you can just change your assignment logic for populating the temp array. Change this line:

temp[(i + k) % arr.length] = arr[i];

to this:

temp[i] = arr[(i + k) % arr.length];

The logic here is that the ith entry in temp[] will equal the entry k steps ahead of it in arr[] . In other words, temp[] will contain the original elements shifted to the left by k . Note that you still need the mod operation to make sure that indexing wraps around the size of the array.

Full code:

public static int [] rotateLeft(int [] source, int k){
    int arr[] = copyArray(source, source.length);
    int[] temp = new int[arr.length];
    for (int i=0; i<arr.length; i++) {
        temp[i] = arr[(i + k) % arr.length];
    }
    for (int i=0; i<arr.length; i++) {
        arr[i] = temp[i];
    }

    return arr; 
}

Answer 2

Basically do (i - k) instead of (i + k) . On top of that, remove the copyArray call and the second loop. And because the % operator in java behaves the it does, you have to add source.length to (i - k) before actually % ing it:

public static int [] rotateLeft(int [] source, int k) {
    int arr[] = new int[source.length];

    for (int i = 0; i < arr.length; i++){
        arr[(i - k + source.length) % source.length] = source[i];
    }

    return arr; 
}

Outputting

50
60
10
20
30
40