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c#arrays
JC Grubbs
JC Grubbs

Reputation: 40341

Order an Array like another Array in C#

What is the best algorithm to take array like below:

A {0,1,2,3}

I expected to order it like array below:

B {3,1,0,2}

Any ideas?

Upvotes: 1

Views: 3986

Answers (7)

Yuri Astrakhan
Yuri Astrakhan

Reputation: 10035

Here's my implementation of the comparer (uses LINQ, but can be easily adapted to older .net versions). You can use it for any sorting algorithms such as Array.Sort, Enumerable.OrderBy, List.Sort, etc.

var data = new[] { 1, 2, 3, 4, 5 };
var customOrder = new[] { 2, 1 };
Array.Sort(data, new CustomOrderComparer<int>(customOrder));
foreach (var v in data)
    Console.Write("{0},", v);

The result is 2,1,3,4,5, - any items not listed in the customOrder are placed at the end in the default for the given type (unless a fallback comparator is given)

public class CustomOrderComparer<TValue> : IComparer<TValue>
{
    private readonly IComparer<TValue> _fallbackComparer;
    private const int UseDictionaryWhenBigger = 64; // todo - adjust

    private readonly IList<TValue> _customOrder;
    private readonly Dictionary<TValue, uint> _customOrderDict;

    public CustomOrderComparer(IList<TValue> customOrder, IComparer<TValue> fallbackComparer = null)
    {
        if (customOrder == null) throw new ArgumentNullException("customOrder");

        _fallbackComparer = fallbackComparer ?? Comparer<TValue>.Default;

        if (UseDictionaryWhenBigger < customOrder.Count)
        {
            _customOrderDict = new Dictionary<TValue, uint>(customOrder.Count);
            for (int i = 0; i < customOrder.Count; i++)
                _customOrderDict.Add(customOrder[i], (uint) i);
        }
        else
            _customOrder = customOrder;
    }

    #region IComparer<TValue> Members

    public int Compare(TValue x, TValue y)
    {
        uint indX, indY;
        if (_customOrderDict != null)
        {
            if (!_customOrderDict.TryGetValue(x, out indX)) indX = uint.MaxValue;
            if (!_customOrderDict.TryGetValue(y, out indY)) indY = uint.MaxValue;
        }
        else
        {
            // (uint)-1 == uint.MaxValue
            indX = (uint) _customOrder.IndexOf(x);
            indY = (uint) _customOrder.IndexOf(y);
        }

        if (indX == uint.MaxValue && indY == uint.MaxValue)
            return _fallbackComparer.Compare(x, y);

        return indX.CompareTo(indY);
    }

    #endregion
}

Upvotes: 2

Jeremy
Jeremy

Reputation: 107

Could the issue be resolved using a Dictionary so the elements have a relationship that isn't predicated on sort order at all?

Upvotes: 1

Eric
Eric

Reputation: 11662

What you need to do is determine the ordering of B and then apply that ordering to A. One way to accomplish this is to undo the ordering of B and keep track of what happens along the way. Then you can do the reverse to A.

Here's some sketchy C# (sorry, I haven't actually run this)...

Take a copy of B:

List<int> B2 = new List<int>(B);

Now sort it, using a sort function that records the swaps:

List<KeyValuePair<int,int>> swaps = new List<KeyValuePair<int,int>>();
B2.Sort( delegate( int x, int y ) {
   if( x<y ) return -1;
   if( x==y ) return 0;
   // x and y must be transposed, so assume they will be:
   swaps.Add( new KeyValuePair<int,int>(x,y) );
   return 1;
});

Now apply the swaps, in reverse order, to A:

swaps.Reverse();
foreach( KeyValuePair<int,int> x in swaps )
{
   int t = A[x.key];
   A[x.key] = A[x.value];
   A[x.value] = t;
}

Depending how the built-in sort algorithm works, you might need to roll your own. Something nondestructive like a merge sort should give you the correct results.

Upvotes: 3

EBGreen
EBGreen

Reputation: 37830

If they are nearly the same then here is some pseudo code:

Make an ArrayList
Copy the contents of the smaller array to the arraylist
for each item I in the larger array
    FInd I in the ArrayList
    Append I to a new array
    Remove I from the arraylist

Upvotes: 1

JC Grubbs
JC Grubbs

Reputation: 40341

Both array's contain the same values (or nearly so) but I need to force them to be in the same order. For example, in array A the value "3045" is in index position 4 and in array B it is in index position 1. I want to reorder B so that the index positions of like values are the same as A.

Upvotes: 1

harpo
harpo

Reputation: 43228

In the example you gave (an array of numbers), there would be no point in re-ordering A, since you could just use B.

So, presumably these are arrays of objects which you want ordered by one of their properties.

Then, you will need a way to look up items in A based on the property in question (like a hashtable). Then you can iterate B (which is in the desired sequence), and operate on the corresponding element in A.

Upvotes: 1

EBGreen
EBGreen

Reputation: 37830

So if you have two arrays and they hold the same data just in different order then just do this:

A = B

I suspect that is not your situation so I think we need more info.

Upvotes: 6

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