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HW4.java

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+
+/**
+ * Write a description of class HW4 here.
+ *
+ * @author (your name)
+ * @version (a version number or a date)
+ */
+public class HW4 extends HW4_AbstractClass
+{
+    // instance variables - replace the example below with your own
+    //int left;
+    //int right;
+    //int k;
+    //int[] A;
+
+    /**
+     * Constructor for objects of class HW4
+     */
+    public HW4()
+    {
+        // initialise instance variables
+
+    }
+
+    protected int selectionNew(int [] A, int left, int right, int k)
+    {
+        // put your code here
+        int pivotIndex = findPivot(A, left, right);
+        swap(A, pivotIndex, right);
+        int i = partition(A, left, right-1, A[right]);
+        swap(A, i, right);
+        if((k-1)==i){
+            System.out.println("A[i] = " + A[i]);
+            return A[i];
+        }
+        else if(k <= i){
+            return selectionNew(A, left, i -1, k);
+        }
+        else if(k> i +1){
+            return selectionNew(A, i+1, right, k);
+        }
+        return 0;
+    }
+
+    //partition method provided by our textbook found at 
+    //I have made a few small changes to the provided method
+    int partition(int[] A, int left, int right, int pivot) {
+        while (left <= right) { //(left <= right) Move bounds inward until they meet
+            while (A[left]< pivot){//(A[left]< pivot)
+                left++;
+            }
+            while ((right >= left) && (A[right] >=pivot)){//((right >= left) && (A[right] >=pivot))
+                right--;
+            }
+            if (right > left){ //right > left
+                swap(A, left, right); // Swap out-of-place values
+            }
+        }
+        return left; // Return first position in right partition
+    }
+
+    public int findPivot(int []A, int left, int right){
+        //return (left+right)/2;
+        // int middle = (int)Math.floor((left + right)/2); //middle value is average of left and right
+        int middle = (left+right/2);
+
+        if(A[right]< A[middle]){ //the following three if statements swap left right and pivot to order them
+            swap(A, right, middle);
+        }
+        if(A[middle]< A[left]){
+            swap(A, middle, left);
+        }
+        if(A[middle]< A[right]){
+            swap(A, middle, right);
+        }
+        //swap(A, middle, right-1); //was right-1
+        return middle;  //was right-1
+    }
+
+    public int selectionNew(int [] A, int k){
+        //System.out.println("A.length-1=: "+ (A.length-1));
+        return selectionNew(A, 0, A.length-1, k); //recursively calls itself
+    }
+    //takes an array and the index of two elements to be swapped
+    public void swap(int [] A, int m, int n){ 
+        //System.out.println("m= "+m);
+        //System.out.println("n= "+n);
+        int temp = A[m];
+        A[m]=A[n];
+        A[n]=temp;
+    }
+
+    public static void main(String args[]){
+        HW4 obj = new HW4();
+        //int []A = {9,8,7,6,5,4,3,2,1};
+        int []A = {1,2,3,4,5,6,7,8,9};
+        //int []A = {4,3,2,1,10,20,30,40};
+        //int []A = {3,2,1};
+        //obj.selectionNew(A,1);
+        obj.selectionNew(A,2);
+        obj.selectionNew(A,3);
+        obj.selectionNew(A,4);
+        obj.selectionNew(A,5);
+        obj.selectionNew(A,6);
+        obj.selectionNew(A,7);
+        obj.selectionNew(A,8);
+        //obj.selectionNew(A,9);
+        System.out.println("----------------");
+    }
+}