Random Permutation Interface

Skippums

I would like to create an extension method that randomly permutes the contents of an ordered list (preferably in-place). This is what I have so far:

public static class PermuteExtensions {

public void Permute<T, U>(this T list) where T : IEnumerable<U> {
    // Create a copy of the input array
    U\[\] tempArray = list.ToArray();
    long arraySize = tempArray.LongLength;

    // Create a permutation of indexes, so list\[j\] = tempArray\[indexes\[j\]\]
    long\[\] indexes = new long\[arraySize\];
    // ...

    // Assign the new values to the existing ordered collection
    long j = 0;
    IEnumerator<U> enumer = list.GetEnumerator();
    while (enumer.GetNext()) {
        // Uh-oh, cannot assign to enumer.Current!
        enumer.Current = tempArray\[indexes\[j++\]\];
    }
}

}

There are two problems with this approach: 1. I cannot figure out how to modify the contents of an IEnumerable object. 2. How to specify the constraint on T such that I allow this method to be called on the ordered collections (IList, Stack, Queue, List, LinkedList), but not on the unordered collections (IDictionary, Dictionary, ISet, Set). It is NOT a firm requirement that this be an in-place algorithm. However, I don't know how to construct a new T object that offers a consistent way to add objects for all of the ordered collections listed above. Thanks,

Sounds like somebody's got a case of the Mondays -Jeff

Dave Kreskowiak

You're trying to modify a collection through the IEnumerable interface. This is something you are expressly forbidden to do as modifying the target of an enumerator invalidates the state of the enumerator. Since each of these ordered collections is a different implementation, and only has the IEnumerable interface in common, I don't think there is an extension solution. I think you would have to create new versions of these ordered collection types, inheriting from them so you can manipulate the underlying structures and still maintain the business rules of the parent class. For instance, a List collection, internally, works a bit differently from a Stack and works very differently from LinkedList. Each is going to have to have it's own wrapper implementation. But, why would you even want to randomly rearrange the objects in a Stack or a LinkedList anyway? I see no productive reason to do so.

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Dave Kreskowiak

BillWoodruff

I find this an interesting programming problem, even though I wonder, like Dave K., why you'd want to do this (gambling program, card game shuffle and deal). But, if someone with the intelligence, and skill, of Dave K. says this cannot be done as an extension, and that you will need to have wrappers for each of your Types you wish to "permutate:" I'd accept his opinion, and pursue his suggestions, if you must do this. However, that doesn't stop me, out of curiousity, just starting to see how this might be done (but, not as an extension). Here's some code I started, and will not have time to work-on further for a while, but perhaps it will be useful to you in some way:

// Visual Studio 2012 RC
// Framework 4.5
// WinForms: one Form with one Button it named 'Permute
//
// build a List of Types to permute
private static List<Type> OkayCollectionTypes = new List<Type>()
{
typeof(List<>),
typeof(Stack),
typeof(Queue),
typeof(LinkedList<>)
};

// create some samples to play with
private static List<int> IntList = new List<int>() { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

private static Stack AStack = new Stack(IntList as IList);

// test stub for Permute function
private static dynamic Permute(dynamic permuteTarget)
{
Type currentType = permuteTarget.GetType();

// Note: we cannot use Type.GetType() here
// an error will be thrown
// see comments in the BtnPermuteClick EventHandler

if(OkayCollectionTypes.Contains(currentType))
{
    MessageBox.Show("okay : Type = " + permuteTarget.ToString());
}
else
{
    return null;
}

// at this point we create a new instance
// of 'permuteTarget that we permutate ?
// to be explored ...
return (permuteTarget);

}

private void BtnPermuteClick(object sender, EventArgs e)
{
// this works
// although it has no effect on AStack's contents
AStack = Permute(AStack);

// this will not work
// because the type of IntList is:
// System.Collections
//  .Generic.List\`1
    // \[
        //\[System.Int32,
        // ... snip ...
        //PublicKeyToken=b77a5c561934e089\]
    // \]

// if we tried to use Type.GetType() in the Permute method
// an error would be thrown like this:
//'System.Collections.Generic.List<int>' does not contain a definition for 'Type'

// var mutatedList = Permute(IntList);

}

Pete OHanlon

Skippums wrote:

1. I cannot figure out how to modify the contents of an IEnumerable object.

As you've discovered, you can't change the object you're enumerating over because you'd invalidate the enumerator. What you could do, though, is not use the enumerator - instead, you can iterate over the collection by index using a for loop, and update the underlying array using that instead. So, your loop becomes something like:

for (int i = 0; i < list.Count; i++)
{
list[i] = tempArray[indexes[j++]];
}

Note that this isn't possible for something like a LinkedList. As for the constraint - I'm afraid that you can't do this at the constraint level. You'd have to test this in the actual method body and raise an exception if it was the wrong type.

*pre-emptive celebratory nipple tassle jiggle* - Sean Ewington

"Mind bleach! Send me mind bleach!" - Nagy Vilmos

CodeStash - Online Snippet Management | My blog | MoXAML PowerToys | Mole 2010 - debugging made easier

BobJanova

You can't modify an IEnumerable. You can't modify a Queue or a Stack via array indexing, either. So I don't think it's reasonable to try to do this in place except with an IList. List<T>, Stack<T> and Queue<T> all have a constructor that takes an argument of type IEnumerable<T>, so you could potentially build a new instance of the same type that you started with. However, considering you are already making a copy of the complete contents of the collection, and storing them in a temporary array, why not just return a List<T> with those items?

Pete OHanlon

BobJanova wrote:

You can't modify a Queue or a Stack via array indexing

Damn, I should have mentioned that point. My 5 for that.

*pre-emptive celebratory nipple tassle jiggle* - Sean Ewington

"Mind bleach! Send me mind bleach!" - Nagy Vilmos

CodeStash - Online Snippet Management | My blog | MoXAML PowerToys | Mole 2010 - debugging made easier

Dave Kreskowiak

Pete O'Hanlon wrote:

Note that this isn't possible for something like a LinkedList.

This is exactly why I said he'd have to wrap each collection he wanted to do this to.

A guide to posting questions on CodeProject[^]
Dave Kreskowiak

PIEBALDconsult

Have a look at this Non-recursive Permutations and Combinations[^]. And this LimitedQueue[^] can give insight into indexing a Queue.

Skippums

As to the, "why not just return a list", the answer is that I feel like I should return the same type that is passed in. No particular reason, in fact I really only need this to work on a list, but prefer to write code that is reusable whenever possible.

Sounds like somebody's got a case of the Mondays -Jeff

Skippums

Thanks everyone for the feedback! The solution I went with is the following:

public static class Permutation {

// Random number generator
private static Random rand = new Random();

// Permute a collection of objects, returning an array
public static T\[\] Permute<T>(IEnumerable<T> list) {
    // Create a copy of the input array
    T\[\] permArray = list.ToArray();
    long arraySize = permArray.LongLength;

    // Create a permutation of indexes
    long\[\] perm = Permutation.Random(arraySize);

    // Assign the new values to the existing ordered collection
    long i = 0;
    foreach (T item in list) {
        permArray\[perm\[i++\]\] = item;
    }

    // Return the permuted array
    return permArray;
}

// Generates a random permutation of the integers in the range \[0, size)
public static long\[\] Random(long size) {
    long\[\] perm = new long\[size\];
    lock(rand) {
        for (long i = size - 2; i >= 0; --i) {
            // I implemented the extension method Random.NextInt64
            perm\[i\] = rand.NextInt64(size - i);
            for (long j = i + 1; j < size; ++j) {
                if (perm\[i\] <= perm\[j\])
                    ++perm\[j\];
            }
        }
    }
    return perm;
}

}

This will allow a client a way to get a permutation from a set or dictionary as well. Thanks again for the ideas!

Sounds like somebody's got a case of the Mondays -Jeff

BillWoodruff

Nice work ! The question of what you'd need to do randomly re-order a Dictionary<T,V> remains of interest to me. best, Bill

"When it comes to atoms, language can be used only as in poetry. The poet, too, is not nearly so concerned with describing facts as with creating images." Niels Bohr

PIEBALDconsult

Dictionaries don't have order anyway.

PIEBALDconsult

Skippums wrote:

make the method that returns the array public

That sounds like a better idea. And/or something like a RandomSequence(int n) that returns an array of 0..n in random order.

Skippums

Actually, I did end up making my method that returns a T[] public, and deleted all the wrapper methods. I figured that if clients want the result in a stack, queue, list, etc., they can build it themselves with a simple constructor call. Plus, having an extension method on IEnumerable allows a client to get a permutation from a set, which seems like it could be a useful operation.

PIEBALDconsult wrote:

something like a RandomSequence(int n) that returns an array of 0..n in random order

Actually, in the code I posted, a client can get this with a call to long[] Permutation.Random(long size). I suppose I could add the same code for int, but don't really see the point. I also thought about allowing a client to get a specific permutation, but that only works for permutations up to length 20 (since 21! > 264), unless I include a large integer library.

Sounds like somebody's got a case of the Mondays -Jeff