Finished Pattern Matching with switch

gradebook/src/GradeBook/Book.cs

@@ -52,16 +52,27 @@ namespace GradeBook
                 result.Average += grade;
             }
 
-            // var index = 0;
-            // do
-            // {
-            //     result.lowGrade = Math.Min(grades[index], result.lowGrade);
-            //     result.highGrade = Math.Max(grades[index], result.highGrade);
-            //     result.Average += grades[index];
-            //     index += 1;
-            // } while (index < grades.Count);
-
             result.Average /= grades.Count;
+
+            switch (result.Average)
+            {
+                case var d when d >= 90.0:
+                    result.Letter = 'A';
+                    break;
+                case var d when d >= 80.0:
+                    result.Letter = 'B';
+                    break;
+                case var d when d >= 70.0:
+                    result.Letter = 'C';
+                    break;
+                case var d when d >= 60.0:
+                    result.Letter = 'D';
+                    break;
+                default:
+                    result.Letter = 'F';
+                    break;
+            }
+
             return result;
         }
     }

gradebook/src/GradeBook/Statistics.cs

@@ -5,5 +5,6 @@ namespace GradeBook
         public double Average;
         public double highGrade;
         public double lowGrade;
+        public char Letter;
     }
 }

gradebook/test/GradeBook.Tests/BookTests.cs

@@ -21,6 +21,7 @@ namespace GradeBook.Tests
             Assert.Equal(85.6, result.Average, 1);
             Assert.Equal(90.5, result.highGrade);
             Assert.Equal(77.3, result.lowGrade);
+            Assert.Equal('B', result.Letter);
         }
     }
 }