Where did I go wrong in reversing an array in Java?

Question

My Code:

public static int invertieren(int[] werte) {
    int[] werte1 = new int[werte.length];
    for (int i = 0; i < werte.length; i++) {
        for (int j = werte.length - 1; i < j; j--) {
            werte1[j] = werte[i];
        }
    }
    return werte1[0];
}

This is my code, I developed a Method that should reverse my Array. for example: in the main Method:

public static void main(String[] args) {
    // TODO Auto-generated method stub

    int[] a = {1,2,3,4,7,5};
    System.out.println(invertieren(a));
}

and I need the that in position 0 - werte1[0] should give me 5 back.

werte1[0] = 5
werte1[1] = 7
werte1[2] = 4

my new Array should look like that: int[] werte = {5,7,4,3,2,1}

my quastion I have not been answered before, cause I'm asking about my code, to learn from making it right.

possible duplicate of [How do I reverse an int array in Java?](http://stackoverflow.com/questions/2137755/how-do-i-reverse-an-int-array-in-java) — Anderson Vieira, Apr 01 '15 at 20:06
Can you explain idea behind your code a little? How do you think it should work? — Pshemo, Apr 01 '15 at 20:08
Tip: use only a single loop that increments i and decrements j on each iteration. — Mick Mnemonic, Apr 01 '15 at 20:11
it should store the value of the first array(werte) in the secound (werte1) in the upside down order — Samara92, Apr 01 '15 at 20:16

Anderson Vieira · Accepted Answer · 2015-04-02T00:30:03.077

You don't need nested loops to reverse the array. You just need to loop through the arrays like this:

static int[] invertieren(int[] werte) {
    int[] werte1 = new int[werte.length];
    for (int i = 0; i < werte.length; i++) {
        werte1[i] = werte[werte.length - i - 1];
    }
    return werte1;
}

If you want to return the reversed array, you need to change the return type of your original method to return a int[] instead of a int, and then return werte1 instead of werte1[0].

To understand how this works assume that werte.length == 6. So for i = 0 we have:

werte1[i] = werte[werte.length - i - 1]
werte1[0] = werte[6 - 0 - 1] = werte[5] = 5

For i = 1:

werte1[1] = werte[6 - 1 - 1] = werte[4] = 7

And so on. So we go through werte1 from 0 to 5 storing the values from werte from 5 to 0.

If you analise how invertieren() currently works you will see why the result is wrong:

Initial values:
    werte = {1,2,3,4,7,5}
    werte1 = {0,0,0,0,0,0}

for i = 0:
    for j from 5 to 1:
        werte1[j] = werte[0]

intermediate result: werte1 = {0,1,1,1,1,1}

for i = 1:
    for j fom 5 to 2:
        werte1[j] = werte[1]

intermediate result: werte1 = {0,1,2,2,2,2}

for i = 2:
    for j from 5 to 3:
        werte1[j] = werte[2]

intermediate result: werte1 = {0,1,2,3,3,3}

for i = 3:
    for j from 5 to 4:
        werte1[j] = werte[3]

intermediate result: werte1 = {0,1,2,3,4,4}

for i = 4:
    for j from 5 to 5:
        werte1[j] = werte[4]

final result: werte1 = {0,1,2,3,4,7}

for i = 5 it will do nothing, since j starts at 5.

It does not reverse the original array at all, so werte1[0] == 0.

I need actually to change it, that is right, but I set it as int cause I wanted to chick it if it does work...!! — Samara92, Apr 01 '15 at 23:12
What I don't get, why you did this : werte[werte.length - i - 1] ? — Samara92, Apr 01 '15 at 23:14

Sujan Reddy A · Answer 2 · 2015-04-14T21:28:21.643

1

 public static int[] invertieren(int[] werte){
    int[] werte1 = new int[werte.length];
    int length = werte.length-1;
    for(int i= 0; i < werte.length; i++){

            werte1[length-i] = werte[i];

        }
    return werte1;
    }

edited Apr 14 '15 at 21:28

answered Apr 01 '15 at 20:12

Sujan Reddy A

132
2
6

Malik Brahimi · Answer 3 · 2015-04-01T20:24:47.077

1

You're incorrectly using a nested loop, use a single loop instead:

public int[] invertArray(int[] oldInts)
{
    int[] newInts = new int[oldInts.length];

    for (int i = oldInts.length - 1; i >= 0; i--)
    {
        newInts[oldInts.length - i - 1] = oldInts[i];
    }

    return newInts;
}

edited Apr 01 '15 at 20:24

answered Apr 01 '15 at 20:19

Malik Brahimi

16,341
7
39
70

score 0 · Answer 4 · answered Apr 01 '15 at 20:21

0

If you are doing this for a class or for your own knowledge then this is not your answer. Otherwise, I would use Apache Commons ArrayUtils. They have several overloaded reverse methods.

answered Apr 01 '15 at 20:21

Troy

146
4

score 0 · Answer 5 · edited Apr 01 '15 at 21:04

This is working code with a modified main method:

public static int[] invertieren(int[] werte) {
    int[] werte1 = new int[werte.length];
    for (int i = 1; i <= werte.length; i++) {
        werte1[i - 1] = werte[werte.length - i];
    }
    return werte1;
}

Your modified main method:

public static void main(String[] args) {
    int[] a = {1, 2, 3, 4, 7, 5};
    for (int revVal : invertieren(a)) {
        System.out.println(revVal);
    }
}

Kevin Mee · Answer 6 · 2015-04-01T20:17:50.350

-1

It looks as though you are returning the value at index 0.

public static int[] invertieren(int[] werte) {
    int[] werte1 = new int[werte.length];
    int len = werte.length - 1;
    for (int i = 0; i < len; i++) {
        werte1[len-i] = werte[i];
    }
    return werte1;
}

edited Apr 01 '15 at 20:17

answered Apr 01 '15 at 20:10

Kevin Mee

539
3
14

what I did here, was that I started the werte array from positin 0 till werte.length and the werte1 from the end till 0 and saved the value of the positin i in j but in the upside down order. – Samara92 Apr 01 '15 at 20:14
It still doesn't work. You're carrying forward logic errors from the question into your code (in this case the loop boundaries). You need to test before you post. – azurefrog Apr 01 '15 at 20:21

Where did I go wrong in reversing an array in Java?

6 Answers6