Reversing an sorting algorithm

Question

I got this algorithm that sort int[] a from low to high.

public static void sortering(int[] a){
    int temp;               

    for(int i = 0; i < a.length; i++){
    for(int j = i + 1; j < a.length; j++){
        if(a[i] > a[j]){
            temp = a[j];
            a[j] = a[i];
            a[i] = temp;
        }
    }
}
}

What i want to do is to reverse it, make it sort from high to low. I thought this would be a walk in the park doing something like this:

public static void sorteringU(int[] a){
    int temp;

    for(int i = a.length; i < a.length; i--){
        for(int j = i - 1; j < a.length; j--){
            if(a[i] > a[j]){
                temp = a[j];
                a[j] = a[i];
                a[i] = temp;
            }
        }
    }
}

I was wrong, this apparently does nothing. Anyone willing to help?

Edit: Thx Jesper and Satya, it worked.

The only thing you need to do is change in the original code is if(a[i] > a[j]) to if(a[i] < a[j]). — Jesper
– Jesper, Commented Oct 28, 2013 at 9:04
change this if(a[i] > a[j]){ to if(a[i] < a[j]){ and se if it is working — Satya
– Satya, Commented Oct 28, 2013 at 9:04
if(a[i] > a[j]) this is where logic is hidden not in loop taversing — Subhrajyoti Majumder
– Subhrajyoti Majumder, Commented Oct 28, 2013 at 9:07

Tony Dinh · Accepted Answer · 2013-10-28 09:05:33Z

3

This is enough:

public static void sorteringU(int[] a){
    int temp;               

    for(int i = 0; i < a.length; i++){
    for(int j = i + 1; j < a.length; j++){
        if(a[i] < a[j]){ // Change ">" to "<"
            temp = a[j];
            a[j] = a[i];
            a[i] = temp;
        }
    }
}
}

answered Oct 28, 2013 at 9:05

Tony Dinh

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Comments

X-Pippes · Accepted Answer · 2013-10-28 09:05:34Z

1

public static void sortering(int[] a){
    int temp;               

    for(int i = 0; i < a.length; i++){
for(int j = i + 1; j < a.length; j++){
    if(a[i] < a[j]){
        temp = a[j];
        a[j] = a[i];
        a[i] = temp;
    }
    }
}
}

try this, instead of if(a[i] > a[j]), make it if(a[i] < a[j])

answered Oct 28, 2013 at 9:05

X-Pippes

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Comments

Bernhard Barker · Accepted Answer · 2013-10-28 09:09:04Z

0

It should be:

                 note -1   note condition change
                    V       V
for(int i = a.length-1; i >= 0; i--){
  for(int j = i - 1;    j >= 0; j--){

It needs to be -1, as arrays go from 0 to length-1, you previously started off at 0, thus you need to start off at the other side, i.e. length-1.

You need to check >= 0. If you check < length, it will go on forever, since it only gets smaller, so it will never be larger than length. If you compare against 0, it will stop as soon as it gets to the beginning of the array.

But just changing if(a[i] > a[j]) to if(a[i] < a[j]) would be a simpler fix.

answered Oct 28, 2013 at 9:09

Bernhard Barker

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Comments

Subhrajyoti Majumder · Accepted Answer · 2013-10-28 09:11:36Z

0

Believe me you do not need to change the travering loop :). Sorting logic is hidden into if(a[i] > a[j]). Simply change change if(a[i] > a[j]) to if(a[i] < a[j]). It will work.

for(int i = 0; i < a.length; i++){
    for(int j = i + 1; j < a.length; j++){
        if(a[i] < a[j]){
            temp = a[j];
            a[j] = a[i];
            a[i] = temp;
        }
    }
}

answered Oct 28, 2013 at 9:11

Subhrajyoti Majumder

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Comments

nickcen · Accepted Answer · 2013-10-28 09:19:22Z

0

this is a bubble sort algorithm , just change the comparator method will be ok

    change it from if(a[i] > a[j]) to if(a[i] < a[j])

let other code untouched.

answered Oct 28, 2013 at 9:19

nickcen

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2 Comments

Abhishek Bansal Over a year ago

IMO it is not bubble sort.

Bernhard Barker Over a year ago

It's still not bubble sort. Bubble sort only swaps adjacent elements. It's more like some variant of selection sort.

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