Java：未排序的数组到降序的数组

Question

I am a beginner learning java and I am trying to transfer the variables in an array to another array in descending order. This is my code:

    int num[] = new int[6];
    int DecSort[] = new int[6];
    System.out.println("Input 6 numbers: ");
    for (int i = 0; i < 6; i++) {
        num[i] = Integer.parseInt(br.readLine());
    }
    //sorted array is descending
    int max = num[0];
    int maxpos = 0;
    for (int j = 0; j < 6; j++) {
        for (int i = 0; i < 6; i++) {
            if (num[i] > max) {
                max = num[i];
                maxpos = i;
            }
        }
        DecSort[j] = max;
        max = num[0];
        num[maxpos] = Integer.MIN_VALUE;
    }

    System.out.println("sorted array");
    for (int i = 0; i < 6; i++) {
        System.out.println(DecSort[i]);
    }

When I input 213, 45, 1234, 765, 23, 5 as the array, it outputs: sorted array 1234 765 213 213 213 213

I am not sure where my mistake is, please help!

Answer 1

The problem is that you're initializing max to num[0] . This means that any number in the array smaller than the first will never be found, because num[i] > max will always be false . You could fix it by setting your initial value of max to Integer.MIN_VALUE .

However, are you aware that the Java Standard Library already has sort functionality. You could use that to sort you array in ascending order, and then just print it in reverse:

int num[] = new int[6];
System.out.println("Input 6 numbers: ");
for (int i = 0; i < 6; i++) {
    num[i] = Integer.parseInt(br.readLine());
}

Arrays.sort(num);

System.out.println("sorted array");
for (int i = 5; i >= 0; --i) {
    System.out.println(num[i]);
}

I'll leave you to work out how to turn an ascending array into a descending one if that's what you need. ;)

---

Alternatively, using a NavigableSet , you can even get Java to sort it for you on the fly:

NavigableSet<Integer> nums = new TreeSet<>();
System.out.println("Input 6 numbers: ");
for (int i = 0; i < 6; i++) {
    nums.add(Integer.valueOf(br.readLine()));
}

Integer[] sortedNums = nums.descendingSet().toArray(new Integer[6]);

Answer 2

I changed just a few your code:

1 - Removed DecSort array beacause its not needed.

2 - Applied bubble sort descending algorithm.

3 - Change de break condition to the for in order to be used for any size array.

Have a look:

public void orderDescending() {
    int num[] = { 213, 45, 1234, 765, 23, 5 };
    // sorted array is descending
    int maxpos = 0;
    for (int j = 0; j < num.length; j++) {
        for (int i = j; i < num.length - 1; i++) {
            if (num[i] > num[j]) {
                int tempA = num[i];
                num[i] = num[j];
                num[j] = tempA;
            }
        }
    }

    System.out.println("sorted array");
    for (int i = 0; i < 6; i++) {
        System.out.println(num[i]);
    }
}

Just a few changes and works perfect!

Answer 3

Can create Comparator and that can be control your ascending and descending order.

Comparator<Integer> comparator = new Comparator<Integer>() {

        @Override
        public int compare(Integer o1, Integer o2) {
            return o2.compareTo(o1);
        }
    };

    // option 1
    Integer[] array = new Integer[] { 1, 24, 4, 4, 345 };
    Arrays.sort(array, comparator);