Sorting array by item type

Question

I have an array of objects that I sort by type. Within that array is a set of objects of one type that are already in their preferred relative order. After the sort, these objects are grouped together, but no longer in the same order. I am using the sort a follows

Array.Sort(shapes, GetVisualComparer());

I have looked for a sort method that promises to preserve original order, but have found nothing.

I will note that I have a workaround, but it adds unnecessary confusion to the code, and does not address the general problem if it comes up in the future.

" a sort method that promises to preserve original order" Huh? "adds unnecessary confusion" I know what you mean... please show sample data and a desired result. — Tim Schmelter
– Tim Schmelter, Commented Aug 13, 2013 at 22:08
You will have to imbue the comparer with information regarding the existing order of those items so that it can keep them in the same order. A workaround would be to sort everything but these items and insert the pre-sorted group at the desired point afterwards, but that's probably going to be more clunky. — Jon
– Jon, Commented Aug 13, 2013 at 22:09
Why don't you use 'group by' first, and then sort the 'group by' list? — Gerhard Powell
– Gerhard Powell, Commented Aug 13, 2013 at 22:11
GroupBy preserves the order, that is guaranteed. "The IGrouping<TKey, TElement> objects are yielded in an order based on the order of the elements in source that produced the first key of each IGrouping<TKey, TElement>. Elements in a grouping are yielded in the order that the elements that produced them appear in source." — Tim Schmelter
– Tim Schmelter, Commented Aug 13, 2013 at 22:13

Jan Dörrenhaus · Accepted Answer · 2013-08-13 22:20:28Z

2

Make a copy of the original array. Then, make your comparer compare the original indexes of your elements if their type is equal.

I don't know on what you base your type sorting, but sorting by name, it would look somewhat like this:

public class TypeComparer : IComparer<Type>
{
    public int Compare(Type x, Type y)
    {
        int result = StringComparer.InvariantCulture.Compare(x.Name, y.Name);
        if (result == 0)
        {
            result = Array.IndexOf(originalArray, x).CompareTo(Array.IndexOf(originalArray, y));
        }
        return result;
    }
}

edited Aug 13, 2013 at 22:20

answered Aug 13, 2013 at 22:12

Jan Dörrenhaus

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2 Comments

John Gardner Over a year ago

well, it looks good, but then you end up performing 2 more linear searches of the array on every equals comparison. That could completely destroy the performance of the sort itself... with an array full of dups, this is like N^2 plus the cost of the sort, isn't it?

Jan Dörrenhaus Over a year ago

Unless that array has millions of entries, the performance impact is negligible.

alc · Accepted Answer · 2013-08-13 22:11:03Z

2

The problem is that Array.Sort<T>(T[], IComparer<T>) is not using a stable sort.

This implementation performs an unstable sort; that is, if two elements are equal, their order might not be preserved. In contrast, a stable sort preserves the order of elements that are equal.

You'll have to sort your array using some other method.

answered Aug 13, 2013 at 22:11

alc

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2 Comments

spainchaud Over a year ago

Thanks, I did not know that stable sort was the type of sort I was looking for. This should help me in my search.

John Gardner Over a year ago

stable isn't a type of sort, its kindof a property of any/all sorting algorithms. either the algorithm is stable or it isn't. many sacrifice stability for performance, thinking that if the items are equal, their order isn't important.

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