CODEWITHSUNDEEP
javaarrayssortingelement
Griezy
Griezy

Reputation: 55

Reversing elements in array

I've been looking at a for-loop that reverses the elements in an array, but I do not quite understand what's going on inside it. This is the code:

 int middleIndex = (array.length) / 2;

    for (int i = 0; i < middleIndex; i++) {
        int temporaryVariable = array[i];
        array[i] = array[array.length - 1 - i];
        array[array.length - 1 - i] = temporaryVariable;
    }

What exactly does the two lines below int temporaryVariable = array[i] do? How exactly does it reverse the elements?

Upvotes: 0

Views: 121

Answers (5)

Griezy
Griezy

Reputation: 55

another part was to return the second biggest number, but for some reason it returns the third biggest number, which is really weird.

This is the code:

public static int returnSecondBiggest(int[] array) {
int largestElement = array[0];
for (int i = 1; i < array.length; i++) {
    if (array[i] > largestElement) {
        largestElement = array[i];
    }
}

int secondBiggest = Integer.MIN_VALUE;
for (int i = 0; i < array.length; i++) {
    if (array[i] > secondBiggest && array[i] != largestElement) {
        secondBiggest = array[i];
    }
}

return secondBiggest;

}

How does it return the third when the code should return the second? I'm so lost.

Upvotes: 0

jbx
jbx

Reputation: 22128

First of all remember that array indexes start from 0. So the index of the last element is the array.length - 1.

Those 3 lines are swapping the first item with the last item, then the 2nd item with the 2nd-from-last item, etc. The temporaryVariable is used as a temporary place to store one of the values during swapping, so that it doesn't get lost when it is overwritten by the other value.

  1. Take a copy of the value at i:

    int temporaryVariable = array[i];

  2. Put item i from the end of the array (array.length - 1 - i) instead of it.

    array[i] = array[array.length - 1 - i];

  3. Put the temporarily stored item which was item i at i from the end (array.length - 1 - i).

    array[array.length - 1 - i] = temporaryVariable;

The loop stops when i reaches the middle of the array. (If the array has an odd number of elements the middle one stays where it is.)

Upvotes: 0

dbl
dbl

Reputation: 1109

This algorithm is taking N/2 iterations of swapping values stored in an array. It starts from the beggining of the array (Index 0) and goes until its half(index N/2). It swaps the first element(indexed 0) with the last one (indexed N - 1 - 0), then swaps the second element(indexed 0 + 1) with the one before the last one(indexed N - 1 - (0 + 1)), and so on.

Upvotes: 0

Anirudh Bagri
Anirudh Bagri

Reputation: 2447

The 2 lines after int temporaryVariable = array[i]; simply swap the i'th element with the i'th from last element, and we run this loop half time the number of elements in array.

Upvotes: 1

NiVeR
NiVeR

Reputation: 9786

It effectively reverses the elements of the array by swapping first with last element, second with second_last etc. In this way the number of operations are ayrray_length / 2.

Upvotes: 1

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