#137 – Sorting an Array Using an Independent Comparer Method

Instead of extending a type to implement IComparable to allow sorting, you can create a separate class that knows how to compare objects of that type and use the new class to do the sorting.

Here’s an example of sorting objects of type Person using a custom Compare method.  To start with, we define a new class that implements IComparer.

        public class PersonSorter : IComparer
        {
            public int Compare(object o1, object o2)
            {
                Person p1 = o1 as Person;
                Person p2 = o2 as Person;

                // Sort by LastName, then by FirstName (ignore case)
                int compare = p1.LastName.ToLower().CompareTo(p2.LastName.ToLower());
                if (compare == 0)
                    compare = p1.FirstName.ToLower().CompareTo(p2.FirstName.ToLower());

                return compare;
            }
        }

Now we can sort using this compare function by passing an instance of the IComparer class into the Sort method.

            Person[] folks = new Person[4];
            folks[0] = new Person("Bronte", "Emily");
            folks[1] = new Person("Bronte", "Charlotte");
            folks[2] = new Person("Tennyson", "Alfred");
            folks[3] = new Person("Mailer", "Norman");

            Array.Sort(folks, new PersonSorter());

#136 – Sorting an Array of Values Based on an Array of Keys

You can use the Array.Sort method to sort two arrays at the same time.  The values of one array are used as the keys to determine the sort order and the values of the second array are just changed to match the sequence of the first array.

Array.Sort takes two arrays.  The first is an array of keys, dictating how the elements will be sorted.  The second will just be reordered to match the sequence of the first array.

Here’s an example:

            string[] titles = { "Pride and Prejudice", "Moby-Dick", "A Tale of Two Cities",
                                "Anna Karenina", "Fahrenheit 451" };
            string[] firstWords = {
                "It is a truth universally acknowledged",
                "Call me Ishmael",
                "It was the best of times",
                "All happy families are alike",
                "It was a pleasure to burn" };

            Array.Sort(titles, firstWords);   // Sort by title

Here’s what both arrays look like after sorting:

#134 – Sorting One-Dimensional Arrays

You can sort a one-dimensional array of elements using the static Array.Sort method.  This requires that the underlying type of the array elements implements the IComparable interface.  Types that implement IComparable include: all built-in numeric types (e.g. int, float, double), char, string and DateTime types.

Here’s an example that sorts an array of integers:

 int[] nums = new int[5];
 nums[0] = 3; nums[1] = 2; nums[2] = 4;
 nums[3] = 1; nums[4] = 0;

 Array.Sort(nums);   // Elements now: 0, 1, 2, 3, 4

Here’s an example of sorting an array of strings:

 string[] emps = new string[4];
 emps[0] = "Augustus";
 emps[1] = "Tiberius";
 emps[2] = "Caligula";
 emps[3] = "Claudius";
 Array.Sort(emps);   // Augustus, Caligula, Claudius, Tiberius

You can’t sort an array of objects, if the element type does not implement IComparable:

 Cat[] cats = new Cat[2];
 cats[0] = new Cat("Garfield", "Sleep all day");
 cats[1] = new Cat("Buster", "Scratch everything");
 Array.Sort(cats);    // throws InvalidOperationException