C#

toBeH_S wrote:

What is the following code doing:

// 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);
}

> ? currentType is IList
    false
    > ? currentType is List<int>
    false

    * the following edited for brevity
    > ? currentType.GetType()
    {Name = "RuntimeType" FullName = "System.RuntimeType"}
        [System.RuntimeType]: {Name = "RuntimeType" FullName = "System.RuntimeType"}
        base {System.Reflection.MemberInfo}: {Name = "RuntimeType" FullName = "System.RuntimeType"}
        AssemblyQualifiedName: "System.RuntimeType, ... snip ... PublicKeyToken=b77a5c561934e089"
        BaseType: {Name = "TypeInfo" FullName = 
        GenericParameterPosition: 'currentType.GetType().GenericParameterPosition' threw an exception of type 'System.InvalidOperationException'
        GenericTypeArguments: {System.Type[0]}
        GUID: {97e69551-7329-39c5-ba61-2ca4b573e0e5}
        IsClass: true
        IsGenericType: false
    UnderlyingSystemType: {Name = "RuntimeType" FullName = "System.RuntimeType"}

Skippums wrote:

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

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

Pete O'Hanlon wrote:

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

BobJanova wrote:

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

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;
    }
}

Skippums wrote:

make the method that returns the array public

PIEBALDconsult wrote:

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

rachel_m wrote:

I tried what you suggested and that did not work.

rachel_m wrote:

do you have any other suggestions?

Bastard Programmer from Hell
if you can't read my code, try converting it here[^]