Question

What is the most efficient way of setting values in C# multi-dimensional arrays using a linear index? For example given an array…

int[,,] arr2 = {   {{0,1,2}, {3,4,5}, {6,7,8}}                 , {{9,10,11}, {12,13,14}, {15,16,17}}                 , {{18,19,20}, {21,22,23}, {24,25,26}}         }; 

How do I set all the elements to 30 using a linear index …

//This code does not work for (int i = 0; i < arr.Length; i++) {     arr.SetValue(30, i); } 

Apparently the SetValue() above does not work with multidimensional arrays.

Here is the best solution that I could come up with…

EDIT: Added some clarifications to the code…

static class Program {     static void Main(string[] args)     {         //Sample input.          int[,,] arr2 = {   {{0,1,2}, {3,4,5}, {6,7,8}}                         , {{9,10,11}, {12,13,14}, {15,16,17}}                         , {{18,19,20}, {21,22,23}, {24,25,26}}                 };          int[] arr1 = { 1, 2, 3, 4 };          setElementsTo30(arr2);         setElementsTo30(arr1);      }      //Must be able to process int arrays of arbitrary dimensions and content     private static void setElementsTo30(Array arr)     {         IList<int> cumulativeLength = getCumulativeLengths(arr);          for (int i = 0; i < arr.Length; i++)         {             SetValue(arr, i, 30, cumulativeLength);         }     }      public static void SetValue(this Array arr, int index, object value, IList<int> cumulativeLength)     {         int[] arrayIndex = new int[arr.Rank];          for (int dim = arr.Rank-1; dim >= 0; dim--)         {             arrayIndex[dim] = index / cumulativeLength[dim] % arr.GetLength(dim);         }          arr.SetValue(value, arrayIndex);     }      private static IList<int> getCumulativeLengths(Array arr)     {         List<int> lengths = new List<int>(arr.Rank);          for (int dim = 0; dim < arr.Rank; dim++)         {             int prod = 1;             for (int i = dim + 1; i < arr.Rank; i++)             {                 prod *= arr.GetLength(i);             }             lengths.Add(prod);         }          return (IList<int>)lengths;     } } 

Is there a way to do the same more efficiently and possibly using something provided by the framework itself (i.e. something which can be used without much hassle.)

Thanks,
SDX2000.

Answer 1

why do you need the IList ?

static void SetValue2(this Array a, object value, int i) {     int[] indices = new int[a.Rank];     for (int d = a.Rank - 1; d >= 0; d--) {         var l = a.GetLength(d);         indices[d] = i % l;         i /= l     }     a.SetValue(value, indices); } 

Test Code:

static void Main(string[] args) {     int[, ,] arr2 = {            {{0,1,2}, {3,4,5}, {6,7,8}},          {{9,10,11}, {12,13,14}, {15,16,17}},          {{18,19,20}, {21,22,23}, {24,25,26}}     };     for (int i = 0; i < arr2.Length; i++) {         arr2.SetValue2(30, i);     } }