Question

Improve the code below by creating an addBefore method. Instead of adding an element to the end, addBefore adds it to the beginning, at position 0, causing all existing elements to move forward (their position increased by 1). However, like add, your addBefore method must also guarantee that only O(?) time is needed to perform n addBefores and adds. To accomplish this, as with add, most calls to addBefore must execute very quickly, in O(1) constant time, meaning that you can’t just shift all the elements forward by 1. The changes you make to IntArrayList should preserve the property that the get and set methods take a constant amount of time (not proportional to the array size) and the above property of add that only a linear number of elements is copied. (JAVA)

public class funtion {

private int[] a; // Underlying array

private int length; // Number of added elements in a

public IAList() {

length = 0; // Start with no added elements in a a = new int[4]; // A little room to grow

}

public int get(int i) { // Retrieve an added element, O(1)

if (i < 0 || i >= length) {

throw new ArrayIndexOutOfBoundsException(i);

}

return a[i]; // Retrieve the element at position i

}

public int size() { // Number of added elements, O(1)

return length; // The number of added elements

}

public void set(int i, int x) { // Modify an existing element, O(1)

if (i < 0 || i >= length) {

throw new ArrayIndexOutOfBoundsException(i);

}

a[i] = x; // Change the existing element at position i to x

}

public void add(int x) { // Add an element to the end, O(n) for n

if (length >= a.length) {

// Create new array of double the length

int[] b = new int[a.length * 2];

// Copy the elements of a to the corresponding indexes of b

for (int i = 0; i < a.length; i++) { b[i] = a[i];

}

// Reassign a reference to b a = b;

}

// Place x at the end of the IAList a[length] = x;

// Increase length by 1 length = length + 1;

}

public void addBefore(int x) { /* FILL THIS IN!! */

}

}

Answer 1

thanks for the question, here is the implemented method addBefore() and also corrected the comments incorrectly added

I added a new method printArray() and main () method to test its working fine. You can remove these two methods if you want

===========================================================================================

import java.util.Arrays;

public class IAList {

    private int[] a; // Underlying array

    private int length; // Number of added elements in a

    public IAList() {
        length = 0; // Start with no added elements in a
        a = new int[4]; // A little room to grow
    }

    public int get(int i) { // Retrieve an added element, O(1)
        if (i < 0 || i >= length) {
            throw new ArrayIndexOutOfBoundsException(i);
        }
        return a[i]; // Retrieve the element at position i
    }

    public int size() { // Number of added elements, O(1)
        return length; // The number of added elements
    }

    public void set(int i, int x) { // Modify an existing element, O(1)
        if (i < 0 || i >= length) {
            throw new ArrayIndexOutOfBoundsException(i);
        }
        a[i] = x; // Change the existing element at position i to x
    }

    public void add(int x) { // Add an element to the end, O(n) for n
        if (length >= a.length) {
            // Create new array of double the length
            int[] b = new int[a.length * 2];
            // Copy the elements of a to the corresponding indexes of b

            for (int i = 0; i < a.length; i++) {
                b[i] = a[i];
            }
            // Reassign a reference to b
            a = b;
        }

       // Place x at the end of the IAList
        a[length] = x;
        // Increase length by 1
        length = length + 1;
    }

    public void addBefore(int x) { /* FILL THIS IN!! */

        if (length >= a.length) {
            // Create new array of double the length
            int[] b = new int[a.length * 2];
            // Copy the elements of a to the corresponding indexes of b
            // starting at index 1 and leave index 0 for assigning the value of x at 0
            for (int i = 0; i < a.length; i++) {
                b[i+1] = a[i];
            }
            // Reassign a reference to b
            a = b;
        }else {
            System.out.println(length);
            for (int i = length-1;i>=0;i--) {
                // starting at index 1 and leave index 0 for assigning the value of x at 0
                a[i + 1] = a[i];
            }
        }
        a[0]=x;
        length = length + 1;
    }

    public void printArray(){
        System.out.println(Arrays.toString(a));
    }

    public static void main(String[] args) {
        IAList i = new IAList();
        i.add(1);i.printArray();
        i.add(2);i.printArray();
        i.add(3);i.printArray();
        i.addBefore(4);
        i.printArray();
        i.addBefore(5);
        i.printArray();
        i.addBefore(6);
        i.printArray();
        i.addBefore(7);
        i.printArray();
        i.addBefore(8);
        i.printArray();
        i.addBefore(9);
        i.printArray();

    }

}

======================================================================================

52 53 54 public void addBefore (int x) { /1 FILL THIS IN!! if (length >=a.length) { // Create new array of double the length intll b- new int[a.length 2]: ICopy the elements of a to the corresponding indexes of b /starting at index 1 and leave index 0 for assigning the value of x at for (int i = 0; i < a.length; 1++) 5E 57 5 8 59 60 61 62 63 64 65 // Reassign a reference to b a -b; else AListaddBefore0 AList "C:\Program Files\Java\jdk-11\bin\java.exe" "-javaagent:C:\Program Files\JetBrains\Intell un: [5, 4, 1, 2, 3, o, o, o] [6, 5, 4, 1, 2, 3, 0, 0] [7, 6, 5, 4, 1, 2, 3, 0] [8, 7, 6, 5, 4, 1, 2, 3] [9, 8, 7, 6, 5, 4, 1, 2, 3, 0, 0, 0, 0, 0, 0, 0] Process finished with exit code 0

important lines are commented for your to understand. let me know in case you have any question.

thanks a lot !