From 884e56fcf19043d410025e636c2b180fa5c86b3f Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Micha=C5=82=20Por=C4=99ba?= <michalporeba@gmail.com>
Date: Sat, 13 Aug 2022 20:59:51 +0100
Subject: [PATCH 1/2] Basic mentoring notes for the matrix problem

The problem can be solve in a number of ways, and it is good to have the examples ready to guide the students.
The alternative solution with multidimensional tables gets unnecessarily complex quickly so is not included.
---
 tracks/csharp/exercises/matrix/mentoring.md | 71 +++++++++++++++++++++
 1 file changed, 71 insertions(+)
 create mode 100644 tracks/csharp/exercises/matrix/mentoring.md

diff --git a/tracks/csharp/exercises/matrix/mentoring.md b/tracks/csharp/exercises/matrix/mentoring.md
new file mode 100644
index 000000000..c392f3d79
--- /dev/null
+++ b/tracks/csharp/exercises/matrix/mentoring.md
@@ -0,0 +1,71 @@
+### Reasonable solutions: 
+
+Solution without Linq:
+```csharp
+using System.Collections.Generic;
+
+public class Matrix
+{
+  private readonly int[][] matrix;
+  
+  public Matrix(string input)
+  {
+    var output = new List<int[]>();
+    
+    foreach(var rowString in input.Split("\n")) {
+      var row = new List<int>();
+      foreach(var v in rowString.Split(" ")) {
+        row.Add(int.Parse(v));
+      }
+      output.Add(row.ToArray());
+    }
+
+    matrix = output.ToArray();
+  }
+
+  public IEnumerable<int> Row(int row)
+    => matrix[row-1];
+
+  public IEnumerable<int> Column(int col)
+  {
+    foreach(var row in matrix) { 
+      yield return row[col-1]; 
+    }
+  }
+}
+```
+
+Solution with Linq:
+```csharp
+using System.Linq;
+using System.Collections.Generic;
+
+public class Matrix
+{
+    private readonly int[][] matrix;
+    
+    public Matrix(string input)
+    {
+        matrix = input
+          .Split("\n")
+          .Select(r => r.Split(" ").Select(x => int.Parse(x)).ToArray())
+          .ToArray();
+    }
+	
+    public IEnumerable<int> Row(int row)
+      => matrix[row-1];
+	
+    public IEnumerable<int> Column(int col)
+      => matrix.Select(x => x[col-1]);
+}
+```
+
+### Common suggestions: Good suggestions specific to this exercise. Good lessons that emerge from it.
+
+- While it is possible to solve the problem using multidimensional arrays the jagged array is easier to work with. 
+
+### Talking points: Questions to challenge more advance learners with.
+
+- The differences between multidimensional and jagged arrays in general and in C#.
+- There are two problems: paring of input and responding to queries. 
+- The problem can be solved with iteration using loops and using Linq. What's the difference? 

From aca4c29012e04e0eeacbcecf0378130e37cdcfd8 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Micha=C5=82=20Por=C4=99ba?= <michalporeba@gmail.com>
Date: Tue, 13 Sep 2022 13:09:48 +0100
Subject: [PATCH 2/2] Improvements to formatting

It is based on the discussion from PR #2212
---
 tracks/csharp/exercises/matrix/mentoring.md | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/tracks/csharp/exercises/matrix/mentoring.md b/tracks/csharp/exercises/matrix/mentoring.md
index c392f3d79..f662459e6 100644
--- a/tracks/csharp/exercises/matrix/mentoring.md
+++ b/tracks/csharp/exercises/matrix/mentoring.md
@@ -60,12 +60,20 @@ public class Matrix
 }
 ```
 
-### Common suggestions: Good suggestions specific to this exercise. Good lessons that emerge from it.
+### Common suggestions:
+*Good suggestions specific to this exercise. Good lessons that emerge from it.*
 
-- While it is possible to solve the problem using multidimensional arrays the jagged array is easier to work with. 
+- While it is possible to solve the problem using multidimensional arrays the jagged array is easier to work with.
+This is because it is possible to return a single row simply as `matrix[r]`.  
 
-### Talking points: Questions to challenge more advance learners with.
+### Talking points:
+*Questions to challenge more advance learners with.*
 
 - The differences between multidimensional and jagged arrays in general and in C#.
-- There are two problems: paring of input and responding to queries. 
+In this case the data is dense so there are no memory efficiencies to talk about. 
+However, the choice influeces the type of code we can write. 
+- There are two problems: parsing of input and responding to queries. 
+Both are relatively simple and there is no need for input checking. 
 - The problem can be solved with iteration using loops and using Linq. What's the difference? 
+Both the parsing and access to column data change. 
+The change in parsing is more obvious between very imperative use of two loops vs more declarative expression using linq.