// This program contains several examples we examined to explore arrays.

import java.util.*;

public class ArraySample {
    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 have to use Arrays.toString to properly format the output
        System.out.println("primes = " + Arrays.toString(primes));

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

        // this example demonstrated that primitive values passed as a
        // parameter are not changed by the method because copies of the values
        // are passed
        int a = 3;
        int b = 5;
        swap(a, b);
        System.out.println("a = " + a + ", b = " + b);

        // 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
        change(list1);
        System.out.println("list = " + 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));

        // this is an example of a good practice problem for arrays
        mystery();
    }
        
    // find the sum of values in an array
    public static int sum(int[] list) {
        int sum = 0;
        for (int i = 0; i < list.length; i++) {
            // do something with list[i]
            // array traversal
            sum = sum + list[i];
        }
        return sum;
    }

    // change some values in an array
    public static void change(int[] list) {
        list[0] = -18;
        list[1] = -2004;
    }
    // this method swaps the local copies of x and y, but not parameters passed
    // to the method
    public static void swap(int x, int y) {
        int temp = x;
        x = y;
        y = temp;
    }
        
    // 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;
        }
    }
        
    // practice problem: what is in the array after the loop?
    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;
            }
        }
    }
}