Question

I use a custom Matrix class in my application, and I frequently add multiple matrices:

Matrix result = a + b + c + d; // a, b, c and d are also Matrices 

However, this creates an intermediate matrix for each addition operation. Since this is simple addition, it is possible to avoid the intermediate objects and create the result by adding the elements of all 4 matrices at once. How can I accomplish this?

NOTE: I know I can define multiple functions like Add3Matrices(a, b, c), Add4Matrices(a, b, c, d), etc. but I want to keep the elegancy of result = a + b + c + d.

Answer 1

You could limit yourself to a single small intermediate by using lazy evaluation. Something like

public class LazyMatrix {     public static implicit operator Matrix(LazyMatrix l)     {         Matrix m = new Matrix();         foreach (Matrix x in l.Pending)         {             for (int i = 0; i < 2; ++i)                 for (int j = 0; j < 2; ++j)                     m.Contents[i, j] += x.Contents[i, j];         }          return m;     }      public List<Matrix> Pending = new List<Matrix>(); }  public class Matrix {     public int[,] Contents = { { 0, 0 }, { 0, 0 } };      public static LazyMatrix operator+(Matrix a, Matrix b)     {         LazyMatrix l = new LazyMatrix();         l.Pending.Add(a);         l.Pending.Add(b);         return l;     }      public static LazyMatrix operator+(Matrix a, LazyMatrix b)     {         b.Pending.Add(a);         return b;     } }  class Program {     static void Main(string[] args)     {         Matrix a = new Matrix();         Matrix b = new Matrix();         Matrix c = new Matrix();         Matrix d = new Matrix();          a.Contents[0, 0] = 1;         b.Contents[1, 0] = 4;         c.Contents[0, 1] = 9;         d.Contents[1, 1] = 16;          Matrix m = a + b + c + d;          for (int i = 0; i < 2; ++i)         {             for (int j = 0; j < 2; ++j)             {                 System.Console.Write(m.Contents[i, j]);                 System.Console.Write('  ');             }             System.Console.WriteLine();         }          System.Console.ReadLine();     } }