How to reverse slice of array java

January 13, 2022

I’ve been attempting to reverse a slice of a list in java.

The equivalent in python (though perhaps not the best way – I don’t care about this, just want to get my point across) would be:

myList = [0,1,2,3,4,5,6,7,8,9,10]
reverseSlice = myList[2:6]
reverseSlice.reverse()
myList[2:6] = reverseSlice

When I tried to manually implement this in Java, I tried this:

 public static int[] reverse(int[] x,int a1, int a2) {
    
    a1--;
    
    int[] rArr = new int[a2-a1+1];
    
    for (int ind = a1; ind<a2; ind++) {
        rArr[a2-ind-1] = x[ind];
    }
    
    for (int ind = a1; ind<a2; ind++) {
        x[ind] = rArr[ind];
    }
    
    return x;
    
}

However, when I run this:

int[] cows1 = new int[]{0,1,2,3,4,5,6,7,8,9,10};
        
cows1 = reverse(cows1,2,6);
        
for (int i : cows1) {
    System.out.print(i + " ");
}

I get 0 4 3 2 1 0 6 7 8 9 10 .

I’m really confused how I got this, as in my function, I don’t introduce new values, so the "0" shouldn’t have appeared.

My Question: Why is my code returning values that are not in the list?

>Solution :

The 0 value which is coming might be because of accessing an uninitialized value from rArr array. For the purpose of reversing, an extra array is not required. You can simply swap the values from the starting index to the end index. I have tested the following code and it gives correct output

 public class Solution2 {
    
    public static void main(String args[])
    {
        int[] cows1 = new int[]{0,1,2,3,4,5,6,7,8,9,10};
        
        cows1 = reverse(cows1,2,6); 
        // both indices are inclusive 
                
        for (int i : cows1) {
            System.out.print(i + " ");
        }
    }
     public static int[] reverse(int[] x,int a1, int a2) {
            
            
            for (int i = 0; i < (a2-a1+1)/2; i++) {
               swap(x,a1+i,a2-i);
            }
            
            return x;
            
        }
    private static void swap(int[] x, int i, int j) {
//      System.out.println("values swappeed are "+ x[i] +  "  "  + x[j]);
        int temp = x[i];
        x[i] = x[j];
        x[j] = temp; 
        
        
    }
}

and the output comes as