Navigating Arrays






Navigating Arrays

Problem

You need to iterate through the elements of a single-dimensional, multidimensional, or jagged array using a pointer to that array.

Solution

To enable iteration, create an unsafe pointer that points to an array. The manipulation of the array can then be performed through this pointer.

To create a pointer to a single-dimensional array, declare and initialize the array:

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

and then set a pointer, arrayPtr, to the address of the first element in this array (you must use the fixed keyword to pin the array in the managed heap so that the garbage collector does not move it):

	fixed(int* arrayPtr = &intArray[0])

Note that this line could also be written as:

	fixed(int* arrayPtr = intArray)

without any address of (&) operator or indexer. This is because the array variable always points to the first element, similarly to how C/C++ array pointers operate.

The following code creates and initializes a pointer to a single-dimensional array and then displays the last item in that array:

	unsafe
	{
	    int[] intArray = new int[5] {1, 2, 3, 4, 5};
	    fixed(int* arrayPtr = intArray)
	    {
	        Console.WriteLine(*(arrayPtr + 4)); //Display the last value '5'
	    }
	}

Creating a pointer to an array of enumeration values is very similar:

	unsafe
	{
	    Colors[] intArray = new Colors[2] {Colors.Red, Colors.Blue};
	    fixed(Colors* arrayPtr = intArray)
	    {
	        // Use arrayPtr here.
	    }
	}

where Colors is declared as follows:

	public enum Colors{Red, Green, Blue}

Creating a pointer to a multidimensional array is performed by declaring and initializing a multidimensional array:

	int[,] intMultiArray = new int[2,5] {{1,2,3,4,5},{6,7,8,9,10}};

You then set a pointer to the address of the first element in this array:

	fixed(int* arrayPtr = intMultiArray)

For example, the following code creates and initializes a pointer to a multidimensional array, then displays the last item in that array:

	unsafe
	{
	    int[,] intMultiArray = new int[2,5] {{1,2,3,4,5},{6,7,8,9,10}};
	    fixed(int* arrayPtr = intMultiArray)
	    {
	        Console.WriteLine(*(arrayPtr + 9)); //Display the last value '10'
	    }
	}

See Also

See the "Multidimensional Arrays" and "Jagged Arrays" topics and the "Unsafe at the Limit" article in the MSDN documentation.



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