IComparable.CompareTo

14y1.9kVisual Basic .NET

This program is altered for simplicity. For example data given to ID 100, 200 and 300. Return will be 1, -1 or 0. this.ID can take 100, 200 or 300. What value tem.ID can take. Problem is highlighted.

using System;
namespace ConsoleApplication1
{
class StudetCompare
{
static void Main(string[] args)
{
int x;

Student[] student = new Student[3];

for (x = 0; x < student.Length; ++x)
{
student[x] = new Student();
Console.Write("Student #{0} ID = ", x + 1);
student[x].ID = Convert.ToInt32(Console.ReadLine());
}
Console.WriteLine();

Array.Sort(student);

for (x = 0; x < student.Length; ++x)
{
Console.WriteLine("Student #{0} ID = {1}", x + 1, student[x].ID);
}

Console.ReadKey();
}
}

class Student : IComparable
{
public int ID { get; set; }

public int CompareTo(object student)
{
Student tem = (Student)student; //Note: IComparable is non generic therefore casting

return (this.ID - tem.ID);
}
}
}
/*
Student #1 ID = 300
Student #2 ID = 200
Student #3 ID = 100

Student #1 ID = 100
Student #2 ID = 200
Student #3 ID = 300
*/

7 Replies

Know the answer? Post it — somebody with the same question will find it here.

Vulpes14y ago
Accepted answer
If you insert the highlighted line in the CompareTo method, then you'll be able to see what values are used on each call:

public int CompareTo(object student)
{
Student tem = (Student)student; //Note: IComparable is non generic therefore casting
Console.WriteLine("this.ID is {0}, tem.ID is {1}", this.ID, tem.ID);
return (this.ID - tem.ID);
}

The output on my machine was:

this.ID is 300, tem.ID is 200
this.ID is 200, tem.ID is 100
this.ID is 300, tem.ID is 200
this.ID is 100, tem.ID is 200
this.ID is 200, tem.ID is 300

So the answer to your question is that when this.ID is 100, tem.ID is 200.
14y ago
Thank you for your explanation and given link.
Vulpes14y ago
The Array.Sort method uses a version of the in-place Quicksort algorithm to sort the array:

http://en.wikipedia.org/wiki/Quicksort

This is very efficient for sorting large arrays but inefficient for sorting small arrays (up to about 9 elements)

You can certainly see that here as it's taking 5 comparisons to sort only 3 elements and one of them is repeated.

If you change the program to accept 4 numbers and input 400, 300, 200, 100 then you'll find that 12 comparisons are needed and that there are a lot of repetitions amongst them:

this.ID is 400, tem.ID is 300
this.ID is 300, tem.ID is 100
this.ID is 400, tem.ID is 300
this.ID is 100, tem.ID is 300
this.ID is 300, tem.ID is 400
this.ID is 300, tem.ID is 200
this.ID is 100, tem.ID is 200
this.ID is 100, tem.ID is 200
this.ID is 100, tem.ID is 200
this.ID is 300, tem.ID is 400
this.ID is 300, tem.ID is 400
this.ID is 300, tem.ID is 400

In practice, this inefficiency doesn't really matter as sorting small arrays is fast on today's machines, whatever method is used.
14y ago
Highlighted one is repeating. Can you tell me please why it is repeating.

this.ID is 300, tem.ID is 200
this.ID is 200, tem.ID is 100
this.ID is 300, tem.ID is 200
this.ID is 100, tem.ID is 200
this.ID is 200, tem.ID is 300
14y ago
Thank you very much for your explanation.

If I have problem I will get back to you.
14y ago
For instance we say that this.ID takes 100. What value will tem.ID take.
Vulpes14y ago
tem.ID can take exactly the same values : 100, 200, 300 as this.ID.

The reason is that the Array.Sort routine will call the CompareTo method using successive elements in the array until eventually those elements are in sorted order.