How can I sort an arraylist without using collections.sort()? [duplicate]

Question

I have been looking for a while for a way to sort an arraylist without using collections.sort as my own logic is flawed and I have been having a lot of trouble.

I need to sort it in a way that I can use a method I created that basically does what collections.swap does in order to completely sort an arraylist.

Here is my code:

public static void mySort(ArrayList<Double> sort){

    int min = 0;
    int i;
    int j = 0;
        for(i = 0; i < sort.size() - 1; i++) {
            min = i;
            mySwap(sort, j ,min);

            for(j = 0; j < sort.size() -1;j++){
                if(j < min ){
                    min = j;
                }
            }
    }
}

public static void mySwap(ArrayList<Double> a, int x, int y){

    double temp = a.get(x);
    a.set(x,a.get(y));
    a.set(y,temp);
}

I have been having a lot of trouble of with this. Sorry if it is a question is that harming the community.

Homework question I have been working on for a while. This is just a part of it. I can't seem to figure it out though. — Andy Reutzel
– Andy Reutzel, Commented May 7, 2014 at 22:44
@AnubianNoob There is nothing wrong with clarifying - sometimes question askers just don't know about something, and visitors should know that the answers are only relevant under a very specific (artificial) limitation. — Gareth Latty
– Gareth Latty, Commented May 7, 2014 at 22:45
"a way to sort an arraylist without using collections.sort" is pretty clear to me... — Anubian Noob
– Anubian Noob, Commented May 7, 2014 at 22:47
@AndyReutzel I don't mean to come off as abrasive, I understand that new material can be confusing. With that said, Google is your friend. What you are trying to do (based on your snippet above) is what is known as Selection Sort. Maybe this link will be of use. — Whymarrh
– Whymarrh, Commented May 7, 2014 at 23:02

Alexey Malev · Accepted Answer · 2014-05-07 22:52:33Z

I assume you want the following algorithm: find min in the rest of the array, swap it with current element starting with first, reconsider rest to be array starting of increased +1 index.

You should update your code like this:

public static void swap(List<Integer> sort, int i, int j) {
    int tmp = sort.get(i);
    sort.set(i, sort.get(j));
    sort.set(j, tmp);
}

public static void doSort(List<Integer> sort) {
    int min;
    for (int i = 0; i < sort.size(); ++i) {
        //find minimum in the rest of array
        min = i;
        for (int j = i + 1; j < sort.size(); ++j) {
            if (sort.get(j) < sort.get(min)) {
                min = j;
            }
        }

        //do swap
        swap(sort, i, min);
    }
}

You have a bug with finding minimum and then swapping items. Please note that the code can be improved in many ways (I tried to maintain your let's say way of coding as possible), such as swapping integer references in swap(), doing BubbleSort like another answer suggests (same algorithm but simpler implementation), using O(n * log(n)) complexity algorithm, and so on.

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