Question

I tried shuffling a list of byte (List) using either code:

myList.Sort((a, b) => this._Rnd.Next(-1, 1));

or

myList.Sort(delegate(byte b1, byte b2)
{
    return this._Rnd.Next(-1, 1);
});

and they threw the following error:

Unable to sort because the IComparer.Compare() method returns inconsistent results. Either a value does not compare equal to itself, or one value repeatedly compared to another value yields different results. x: ‘{0}’, x’s type: ‘{1}’, IComparer: ‘{2}’.

What is wrong with using a random rather than the compare function of byte?

I tried using LINQ function instead and it works.

var myNewList = myList.OrderBy(s => Guid.NewGuid());
var myNewList = myList.OrderBy(s => this._Rnd.NextDouble());

I did read that these methods are slower than Fisher–Yates shuffle giving O(n) runtime only. But was just wondering on using the Sort function and random.

Answer 1

Because as the error says, Random is not consistent. You must have a comparer that always returns the same result when given the same parameters. otherwise the sort will not be consistent.

Knuth has a random sort algorithm which worked like an insertion sort, but you swapped the value with a randomly chosen location in hhe existing array.