2

Is it possible, in C#, to convert a multi-dimensional array into a 1D array without having to copy all the elements from one to the other, something like:

int[,] x = new int[3,4];
int[] y = (int[])x;

This would allow the use of x as if it were a 12-element 1D array (and to return it from a function as such), but the compiler does not allow this conversion.

As far as I'm aware, a 2D array (or higher number of dimensions) is laid out in contiguous memory, so it doesn't seem impossible that it could work somehow. Using unsafe and fixed can allow access through a pointer, but this doesn't help with returning the array as 1D.

While I believe I can just use a 1D array throughout in the case I'm working on at present, it would be useful if this function was part of an adapter between something which returns a multidimensional array and something else which requires a 1D one.

Philip C
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  • Is this simply for the convenience of indexing as if it were 1D? What about just wrapping it in a type with an appropriate indexer? And more generally, use an abstraction with the interface you want rather than naked arrays? – anton.burger Mar 12 '12 at 09:03
  • @Anton It's partly for the convenience of a single index, but it also got me wondering whether you could convert one to the other without having to copy it all (in the case that you have a pair of libraries, one which gives you a multi-dimensional array and one which requires it in a 1D format). – Philip C Mar 12 '12 at 10:11

6 Answers6

2

You can't, it's not possible in C# to convert array's this way. You maybe could do it by using a external dll ( C/C++ ), but then you need to keep your array at a fixed location.

Speed

Generally i would advice to avoid using a 2D array because theese are slow in C#, better use jagged-array or even better single dimensionals with a little bit of math.

Int32[] myArray = new Int32[xSize * ySize];

// Access
myArray[x + (y * xSize)] = 5;
Community
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Felix K.
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  • A 2D (`[,]`) array is a little slower than a jagged (`[][]`) array. But don't exaggerate that and use what's most appropriate. – H H Mar 12 '12 at 09:13
  • @HenkHolterman http://www.dotnetperls.com/jagged-array Even then single dimensional arrays are faster, because they don't need to access the reference in the reference of the jagged array. – Felix K. Mar 12 '12 at 09:17
2

In C#, arrays cannot be resized dynamically. One approach is to use System.Collections.ArrayList instead of a native array. Another (faster) solution is to re-allocate the array with a different size and to copy the contents of the old array to the new array. The generic function resizeArray (below) can be used to do that.

One example here : 

// Reallocates an array with a new size, and copies the contents
// of the old array to the new array.
// Arguments:
//   oldArray  the old array, to be reallocated.
//   newSize   the new array size.
// Returns     A new array with the same contents.
public static System.Array ResizeArray (System.Array oldArray, int newSize) {
   int oldSize = oldArray.Length;
   System.Type elementType = oldArray.GetType().GetElementType();
   System.Array newArray = System.Array.CreateInstance(elementType,newSize);
   int preserveLength = System.Math.Min(oldSize,newSize);
   if (preserveLength > 0)
      System.Array.Copy (oldArray,newArray,preserveLength);
   return newArray; }
Biard Kévin
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2

You can already iterate over a multidim as if it were a 1 dimensional array: 

  int[,] data = { { 1, 2, 3 }, { 3, 4, 5 } };

  foreach (int i in data)
     ...  // i := 1 .. 5

And you could wrap a 1-dim array in a class and provide an indexer property this[int x1, int x2].

But everything else will require unsafe code or copying. Both will be inefficient.

H H
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  • You wouldn't be able to modify the array through a foreach variable, though - it'd be read only (and you lose the random access nature of arrays). – Philip C Mar 12 '12 at 10:13
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Riding on the back of Felix K.'s answer and quoting a fellow developer:

You can't convert a square to a line without losing information

Jamie Taylor
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0

try

int[,] x = {{1, 2}, {2, 2}};
int[] y = new int[4];
System.Buffer.BlockCopy(x, 0, y, 0, 4);
PraveenVenu
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    It's a copy, not a conversion of the given type. – Felix K. Mar 12 '12 at 08:58
  • @FelixK. Please read this.. The BlockCopy method accesses the bytes in the src parameter array using offsets into memory, not programming constructs such as indexes or upper and lower array bounds. http://msdn.microsoft.com/en-us/library/system.buffer.blockcopy.aspx – PraveenVenu Mar 12 '12 at 09:01
  • That's right, but he doesn't want to copy the bytes. Even then your code is incorrect, because you need to copy 4 * 4 bytes not 4 bytes. – Felix K. Mar 12 '12 at 09:06
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You cannot cast, you'll have to copy the elements:

int[] y = (from int i in y select i).ToArray();
ionden
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