Java array order reversing

Question

Im learning arrays and Im trying to reverse the order in an array. This is the method I've got this far but it's only working for the first half of the values in the array. What am I doing wrong?

public static void reverse(int[] anArray)
{
    int[] a = anArray ;
    for (int j = 0; j <= (a.length - 1); j++ )
    {
        anArray[j] = a[(anArray.length - j - 1)];
    }
}

Answer 1

You need to make a temporary location in order to do the swap. This code write the end of the array to the front, but by the time you get to writing the end of the array, the first half of the values have already been lost.

public static void reverse(int[] a)
{
    int l = a.length;
    for (int j = 0; j < l / 2; j++)
    {
        int temp = a[j];
        a[j] = a[l - j - 1];
        a[l - j - 1] = temp;
    }
}

Additionally, you are incrementing j twice. The last part of the for loop definition does the increment between each loop. You don't need to do it manually inside the loop block.

Answer 2

This is not a difficult problem, and I'm sorry that you've received several wrong answers already.

Your goal is to reverse the contents of the existing array. Your idea is to assign to each array index from the corresponding one in order to switch them all around. However, this would only work if all the assignments happened simultaneously. The code actually runs one assignment at a time. Thus, when you get to the second half of the array, the indices that you're copying from have already been altered by previous runs of the loop.

To fix this, you need to swap pairs of indices. Loop up to halfway (since otherwise you'll swap each pair twice, and end up where you started); take the two indices and swap them. Now, for the reason we just discussed, you can't just assign one to the other and then vice-versa. Instead, you need to "remember" one of the values in a variable first. Assign into that index (since you have the old value saved) from the other one, and then finally assign the remembered value to the other index.

Answer 3

If you are able to use Integer array, you could do like this:

    Integer[] anArray = {0,2,4,1}; // an array of Integer because List do not accept int as type.

    List<Integer> list = Arrays.asList(anArray);  // converting the array to List

    Collections.reverse(list); // reversing the "list" variable.

You can also use Guava's Lists.reverse(List) to create a view backed by the given list that is in reverse order without copying it or modifying the original list.

Answer 4

This reverses the source array by an in place technique. Basically, you are ONLY allowed to use the source array and cannot create another one.

public static void reverseArrayInPlace(int [] source){
    for(int col=0; col<source.length/2; ++col){         
        int temp=source[col];
        source[col]=source[(source.length-1)-col];
        source[(source.length-1)-col]=temp;
    }
}

//initially; 10 20 30 40 50 60

// col=0; 60 20 30 40 50 10

// col=1; 60 50 30 40 20 10

// col=2; 60 50 40 30 20 10

// col=3; 60 50 30 40 20 10

// col=4; 60 20 30 40 50 10

// col=5; 10 20 30 40 50 60

This is why we give the condition col<(source.length/2), if it went up to col<source.length then it would be arranged in the same manner it was before i.e, it wouldn't reverse. Try it both ways to see it yourself.

Answer 5

public static void swap (int[] a, int pos1, int pos2)
{
      int tmp = pos1;
      a[pos1] = a[pos2];
      a[pos2] = tmp;
}

public static void reverse (int[] a)
{
    int l = a.length;
    for (int j = 0; j < (l / 2); j++)
    {
        swap (a, j, l - j - 1);
    }
}

The variable l is just for brevity - it is not a recommendation.

• The arrays content is mutable in Java, so you don't need a second one, which would only be a new reference to the same array.

Sorry - I missed to swap, of course.

Answer 6

When you write int[] a = anArray it will not be a copy of the array but a reference to the very same array. That's why it only works for the first half of the array since the when the second half of the array the first part has already been overwritten. To fix this you have to write:

int[] a = anArray.clone();

Answer 7

Here is the classic example for reversing using stack!

public static void main(String[] args) {
    int[] myIntArray = { 10, 20, 30, 40, 50, 60 };
    int [] reversedArray= new int[myIntArray.length];
    int reversedPosition=0;
    Stack<Integer> s = new Stack<>();
    for (int integer : myIntArray)
        s.push(integer);
    while (!s.empty()) {
        reversedArray[reversedPosition]=s.pop();
        reversedPosition++;
    }
}

Answer 8

public static void reverse(int[] anArray)
{
 int[] newArray = new int[anArray.length];
 int index = 0;
 for (int b = anArray.length - 1; b >= 0; b++)
 {
  newArray[index] = anArray[b];
  index++;
 }
}

Start an index off at 0, and take the last element from the "anArray" and put it at the index in the newArray;

Answer 9

Your problem is that you're reading the values out of the same array you're modifying.

public static void reverse(int[] originalArray) {
    int[] newArray = new int[originalArray.length];

    for (int i = 0; i < originalArray.length; i++) {
        newArray[i] = originalArray[originalArray.length - i - 1];
    }
}