fixed bug in exercise 1.3 quicksort

Author: la-yumba

Examples/Examples.csproj

@@ -31,7 +31,7 @@
 
     <packagereference Include="Microsoft.NET.Test.Sdk" Version="15.0.0"></packagereference>
     <packagereference Include="NUnit" Version="3.6.1"></packagereference>
-    <packagereference Include="NUnit3TestAdapter" Version="3.8.0-alpha1"></packagereference>
+    <packagereference Include="NUnit3TestAdapter" Version="3.8.0"></packagereference>
   </ItemGroup>
 
   <ItemGroup>

Exercises/Chapter01/Solutions.cs

@@ -1,6 +1,9 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
+using NUnit.Framework;
+using LaYumba.Functional;
+using static LaYumba.Functional.F;
 
 namespace Exercises.Chapter1
 {
@@ -14,22 +17,59 @@ static Func<T, bool> Negate<T>(this Func<T, bool> pred)
       static List<int> QuickSort(this List<int> list)
       {
          if (list.Count == 0) return new List<int>();
+
          var pivot = list[0];
-         var small = from item in list where item <= pivot select item;
-         var large = from item in list where pivot < item select item;
+         var rest = list.Skip(1);
+
+         var small = from item in rest where item <= pivot select item;
+         var large = from item in rest where pivot < item select item;
+
          return small.ToList().QuickSort()
+            .Append(pivot)
             .Concat(large.ToList().QuickSort())
             .ToList();
       }
 
+      // a more terse solution, using helper methods that will be discussed later in the book
+      static List<int> QSort(this List<int> list)
+         => list.Match(
+               () => List<int>(),
+               (pivot, rest) => rest.Where(i => i <= pivot).ToList().QSort()
+                  .Append(pivot)
+                  .Concat(rest.Where(i => pivot < i).ToList().QSort())
+            ).ToList();
+
+      [Test]
+      public static void TestQuickSort()
+      {
+         var list = new List<int> {-100, 63, 30, 45, 1, 1000, -23, -67, 1, 2, 56, 75, 975, 432, -600, 193, 85, 12};
+         var expected = new List<int> {-600, -100, -67, -23, 1, 1, 2, 12, 30, 45, 56, 63, 75, 85, 193, 432, 975, 1000};
+         var actual = list.QuickSort();
+         Assert.AreEqual(expected, actual);
+      }
+
+      [Test]
+      public static void TestQSort()
+      {
+         var list = new List<int> {-100, 63, 30, 45, 1, 1000, -23, -67, 1, 2, 56, 75, 975, 432, -600, 193, 85, 12};
+         var expected = new List<int> {-600, -100, -67, -23, 1, 1, 2, 12, 30, 45, 56, 63, 75, 85, 193, 432, 975, 1000};
+         var actual = list.QSort();
+         Assert.AreEqual(expected, actual);
+      }
+
       // 3.
       static List<T> QuickSort<T>(this List<T> list, Comparison<T> compare)
       {
          if (list.Count == 0) return new List<T>();
+
          var pivot = list[0];
-         var small = from item in list where compare(item, pivot) <= 0 select item;
-         var large = from item in list where 0 < compare(item, pivot) select item;
+         var rest = list.Skip(1);
+
+         var small = from item in rest where compare(item, pivot) <= 0 select item;
+         var large = from item in rest where 0 < compare(item, pivot) select item;
+
          return small.ToList().QuickSort(compare)
+            .Concat(new List<T> { pivot })
             .Concat(large.ToList().QuickSort(compare))
             .ToList();
       }

Exercises/Chapter05/Exercises.cs

@@ -4,14 +4,15 @@
 using System.Collections.Generic;
 using System.Linq;
 using NUnit.Framework;
+using Examples.Chapter3;
 
 namespace Exercises.Chapter5
 {
    public static class Exercises
    {
       // 1. Without looking at any code or documentation (or intllisense), write the function signatures of
       // `OrderByDescending`, `Take` and `Average`, which we used to implement `AverageEarningsOfRichestQuartile`:
-      decimal AverageEarningsOfRichestQuartile(List<Person> population)
+      static decimal AverageEarningsOfRichestQuartile(List<Person> population)
          => population
             .OrderByDescending(p => p.Earnings)
             .Take(population.Count/4)

Exercises/Chapter05/Solutions.cs

@@ -4,14 +4,15 @@
 using System.Collections.Generic;
 using System.Linq;
 using NUnit.Framework;
+using Examples.Chapter3;
 
 namespace Exercises.Chapter5.Solutions
 {
    static class Exercises
    {
       // 1. Without looking at any code or documentation (or intllisense), write the function signatures of
       // `OrderByDescending`, `Take` and `Average`, which we used to implement `AverageEarningsOfRichestQuartile`:
-      decimal AverageEarningsOfRichestQuartile(List<Person> population)
+      static decimal AverageEarningsOfRichestQuartile(List<Person> population)
          => population
             .OrderByDescending(p => p.Earnings)
             .Take(population.Count/4)

Exercises/Exercises.csproj

@@ -11,7 +11,7 @@
 
     <packagereference Include="Microsoft.NET.Test.Sdk" Version="15.0.0"></packagereference>
     <packagereference Include="NUnit" Version="3.6.1"></packagereference>
-    <packagereference Include="NUnit3TestAdapter" Version="3.8.0-alpha1"></packagereference>
+    <packagereference Include="NUnit3TestAdapter" Version="3.8.0"></packagereference>
 
     <ProjectReference Include="..\Examples\Examples.csproj" />
     <ProjectReference Include="..\LaYumba.Functional.Data\LaYumba.Functional.Data.csproj" />