IComparer
in [CSharp]
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From: Kamal on 14 May 2010 08:55 How Icomparer works ? I want to know the fundamental of its working .. for 3 object in a array it does comparision 9 times so how the comparisation is going on? can any one help me.
From: sloan on 14 May 2010 09:47 You can look at an example I did here: http://sholliday.spaces.live.com/Blog/cns!A68482B9628A842A!141.entry .............. The gist of it is this: internal sealed class EmployeeComparer : IComparer { public int Compare( Employee x , Employee y ) { //////Write your own logic here that takes 2 Employees (x and y) and then compares them ANYWAY YOU CHOOSE. // You write the code which makes 1 Employee come before the other Employee.........whether it would be LastName, HireDate or whatever Property(ies) you pick. } } That example is 1.1'ish, but you can learn the basic concept. "Kamal" <Kamal(a)discussions.microsoft.com> wrote in message news:5EBEBF1C-CDF6-45DD-91F4-C6D8F4F9B258(a)microsoft.com... > How Icomparer works ? > I want to know the fundamental of its working .. > for 3 object in a array it does comparision 9 times so how the > comparisation > is going on? > can any one help me. >
From: Peter Duniho on 14 May 2010 11:35 Kamal wrote: > How Icomparer works ? > I want to know the fundamental of its working .. > for 3 object in a array it does comparision 9 times so how the comparisation > is going on? > can any one help me. If you can be more specific about your question, I'm sure we can. The IComparer interface does not dictate when it's called at all, never mind how many times. And the way it works is the way any interface works: it defines specific members that must be implemented by implementors, and then a reference to implementors can be passed as the interface type itself so that other code can access those members. If you are using IComparer in a particular context — for example, by passing it a Sort() method of a type like Array — then the real question is how the Sort() method works, which is independent of which interface is used for comparisons. In general, .NET sorting methods use quicksort, which will call the comparison function being used (IComparer.Compare, if you're using IComparer) each time the algorithm needs to compare two objects. The quicksort algorithm has a "big-O" complexity of "O(n log n)", where n is the number of elements being sorted. That means that the number of iterations in the algorithm will be roughly proportional to n log n. But it's not exact. It just happens that for the input data you're using, the 3-element array requires comparisons between the elements of the array 9 times. That actually seems high to me; after all, a plain bubble sort could reliably sort a 3-element array with 3 comparisons (it's an O(n^2) algorithm, but for very small values of 'n', the big-O notation is very inaccurate). I would expect a quicksort algorithm to do about the same. Even if the implementation picks the pivot randomly and so has to compare the pivot element with itself, you get something like this: • iteration 1: three comparisons to build two partitions • iteration 2: two comparisons to build two more partitions (one of the partitions is already only length 1) • iteration 3: all partitions are length 1, no more comparisons needed So not as good as bubble sort, but still only five comparisons, not nine. So I couldn't explain why the .NET quicksort implementation does 9 comparisons, without taking the time to actually look at the exact implementation. I'll leave doing that as an exercise for the reader. :) Pete
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