Versión en español de esta publicación.

Some time ago I was presented with some algorithms common problems related to matrices, today I just found some code in my Windows virtual machine and I through that maybe it was a good idea to publish it here just to keep it online.

I remember I was looking for an algorithm to print a square matrix in spiral, and I found some code in the internet as usual which used a nice approach, I had to modify it because it was in another language and I needed it to be in C#,

Well here is the class I made with the internet approach, the class supports square matrix multiplication and also supports printing a square matrix in spiral using an indirect recursive approach.

**Matrices.cs**

using System; using System.Collections.Generic; using System.Diagnostics; using System.Linq; using System.Text; using System.Threading.Tasks; namespace Samples { public static class Matrices { public static double[,] MtxMult(double[,] mtxA, double[,] mtxB) { if (mtxA == null || mtxA.Length == 0) throw new ArgumentException("Array parameter must be valid"); if (mtxB == null || mtxB.Length == 0) throw new ArgumentException("Array parameter must be valid"); if (mtxA.Length != mtxB.Length) throw new ArgumentException("Arrays must have the same length"); int size = mtxA.GetLength(0); // get first dimmension lenght double[,] mtxC = new double[size, size]; for (int i = 0; i < size; i++) for (int j = 0; j < size; j++) { // Compute dot product of row i and column j. for (int k = 0; k < size; k++) mtxC[i, j] += mtxA[i, k] * mtxB[k, j]; } return mtxC; } private static IList<int> _spiralMtx = null; public static IList<int> MtxProcessSpiral(int[,] mtx) { _spiralMtx = null; _spiralMtx = new List<int>(); int x1 = 0; int y1 = 0; int x2 = mtx.GetLength(0) - 1; int y2 = mtx.GetLength(1) - 1; MtxProcessTopRight(mtx, x1, y1, x2, y2); return _spiralMtx; } // // prints the top and right shells of the matrix // private static void MtxProcessTopRight(int[,] mtx, int x1, int y1, int x2, int y2) { int i = 0; int j = 0; // print the row for (i = x1; i <= x2; i++) _spiralMtx.Add(mtx[y1, i]); //print the column for (j = y1 + 1; j <= y2; j++) _spiralMtx.Add(mtx[j, x2]); // see if we have more cells left if (x2 - x1 > 0 && y2 - y1 > 0) { // 'recursively' call the function to print the bottom-left layer MtxProcessBottomLeft(mtx, x1, y1 + 1, x2 - 1, y2); } } // // prints the bottom and left shells of the matrix // private static void MtxProcessBottomLeft(int[,] mtx, int x1, int y1, int x2, int y2) { int i = 0; int j = 0; // print the row of the matrix in reverse for (i = x2; i >= x1; i--) _spiralMtx.Add(mtx[y2, i]); // print the last column of the matrix in reverse for (j = y2 - 1; j >= y1; j--) _spiralMtx.Add(mtx[j, x1]); if (x2 - x1 > 0 && y2 - y1 > 0) { // 'recursively' call the function to print the top-right layer MtxProcessTopRight(mtx, x1 + 1, y1, x2, y2 - 1); } } } }

Enjoy! 🙂

-Yohan