// This class contains several small examples of array manipulation.  Each
// is written as a method.

import java.util.*;

public class ArraySample1 {
    public static void main(String[] args) {
        // below are two examples of an array initializer
        int[] primes = {2, 3, 5, 7, 11, 13, 17, 19, 23};
        String[] words = {"four", "score", "and", "seven", "years", "ago"};

        // we wrote a method to sum a list:
        System.out.println("sum of primes = " + sum(primes));

        // and explored this "array mystery"
        mystery();

        // we used Arrays.toString to print array contents:
        System.out.println("primes = " + Arrays.toString(primes));

        // In exploring value semantics vs reference semantics, we saw that
        // this swap method did not change the values of x and y
        int x = 3;
        int y = 7;
        swap(x, y);

        // we saw that we can have two variables that both refer to the same
        // array object:
        int[] list1 = {8, 17, 4, 93};
        int[] list2 = list1;
        list2[0] = -98;
        System.out.println("list1 = " + Arrays.toString(list1));
        // notice that list1 has changed even though we changed list2[0]

        // we saw that the method call below changed the contents of the array
        modify(list1);
        System.out.println("after modify, list1 = " + Arrays.toString(list1));
        
        // and we wrote this method for reversing the contents of the array
        reverse(primes);
        System.out.println("after reverse, primes = " + Arrays.toString(primes));
    }

    // an example of an array mystery problem--there will a problem like this
    // on the final
    public static void mystery() {
        int[] a = {1, 7, 5, 6, 4, 14, 11};
        for (int i = 0; i < a.length - 1; i++) {
            if (a[i] > a[i + 1]) {
                a[i + 1] = a[i + 1] * 2;
            }
        }
        System.out.println("mystery array = " + Arrays.toString(a));
    }

    // we saw that this is correct code for swapping two int values, but it
    // only swaps the local copies of these values and has no effect on any
    // variables that are used in calling the method
    public static void swap(int x, int y) {
        int temp = x;
        x = y;
        y = temp;
    }

    // we saw that this method was able to change the array contents because
    // of reference semantics
    public static void modify(int[] a) {
        a[0] = -924;
        a[1] = -42;
    }

    // this method was written using the standard loop traversal pattern
    public static int sum(int[] data) {
        int sum = 0;
        // traversal pattern
        for (int i = 0; i < data.length; i++) {
            // do something with data[i]
            sum = sum + data[i];
        }
        return sum;
    }

    // this method reverses the sequence of values in an array
    public static void reverse(int[] list) {
        // we found that we only wanted to do the swapping for half of the
        // length of the list (otherwise we unreversed it)
        for (int i = 0; i < list.length / 2; i++) {
            // compute the index for the other value to swap
            int j = list.length - 1 - i;
            // swap two values
            int temp = list[i];
            list[i] = list[j];
            list[j] = temp;
        }
    }
}