sorting arrays of objects

14y1.6kVisual Basic .NET

Following program is all about sorting arrays of objects. It is comparing father's age and my age.

I couldn't understand what is Temp.Age. Anyone knows please explain.

using System;

public class Person : IComparable
{
public Person() { }
// private fields
private string _FirstName;
private string _LastName;
private int _Age;

public Person(string FirstName, string LastName, int Age)
{
this.FirstName = FirstName;
this.LastName = LastName;
this.Age = Age;
}
//Properties
public string FirstName
{
get { return _FirstName; }
set { _FirstName = value; }
}
public string LastName
{
get { return _LastName; }
set { _LastName = value; }
}
public int Age
{
get { return _Age; }
set { _Age = value; }
}
//This was the interface member
public int CompareTo(object obj)
{
Person Temp = (Person)obj;

if (this.Age < Temp.Age) //????????????????????????
{return 1;}

if (this.Age > Temp.Age)
{return -1;}
else
{return 0;}
}
}

//The method int CompareTo (object obj) returns an integer. You can specify your result according to the returned value as follow:

class Program
{
public static void Main(string[] args)
{
Console.WriteLine("Hello World!");

Person me = new Person("Bejaoui", "Bechir", 29);
Person myFather = new Person("Bejaoui", "Massinissa", 65);
Person myGrandFather = new Person("Krayem", "Shishaq", 95);

int state = me.CompareTo(myFather);

if (state == 1) Console.WriteLine("My father is older than me");
if (state == -1) Console.WriteLine("I'm older than my father!!!");
if (state == 0) Console.WriteLine("My Father and I have the same age!");

Console.Write("Press any key to continue . . . ");

Console.ReadKey(true);
}
}
/*
Hello World!
My father is older than me
*/

7 Replies

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

Vulpes14y ago
FWIW, I agree with Sam that templates would have been a better name for generics as the latter term does not immediately convey what they are.

However, templates in C++ are much more powerful (and complex) than generics in .NET and bind to the type parameter(s) at compile time rather than at runtime. This is possibly why the .NET architects chose a different name.
Vulpes14y ago
CompareTo is an instance method which compares the current instance of Employee (i.e. this) with the Employee object passed to it as a parameter.

So this line:

if(this.idNumber

is comparing the idNumber field of the current instance with that of 'emp'.

You can use any basis you like to compare objects as long as it represents some kind of natural ordering which idNumber clearly does.
Sam Hobbs14y ago
The terms generic and non-generic are confusing; they mean the opposite of what I expect them to mean. As Vulpes says, "Generic means that a type is written once to apply generally to all types". That makes sense from the perspective of the developer that writes the class, but as a user of the classes, it is the opposite of what I would think it means. A generic collection object is always for a specific type. When a generic collection object is declared, a type is (or types are) always specified in angle brackets, such as "".

For what it is worth, generic collections are generic classes. Generic classes can exist for many other purposes, not just for collections. Generic classes in C# are essentially the same thing as templates in C++. I think that the term template is more indicative of what they are. So you could say that a generic collection is a template collection in the sense that the class is a template that can be used for a type or types that are specified in the declaration.
14y ago
In this example three objects in the array. Each object contains ID number and salary. I assume that emp.idNumber is commonly representing the ID numbers in the each array object. I wish to know this.idNumber is representing what. In order to compare, two item must be there.

using System;
public class ComparableEmployeeArray_idNumber
{
public static void Main()
{
Console.WriteLine("SORTING idNumber IN ACENDING ORDER");
Console.WriteLine();

Employee[] emp = new Employee[] { new Employee(333, 12.50), new Employee(111, 14.75), new Employee(222, 22.35) };

for (int x = 0; x < emp.Length; x++)
Console.WriteLine("Employee #{0} ID = {1}, Salary = {2}", x + 1, emp[x].GetId(), emp[x].GetSalary().ToString("c2"));

Console.WriteLine("\nSorted employees: ");

Console.WriteLine();

Array.Sort(emp);

for (int x = 0; x < emp.Length; x++)
Console.WriteLine("Employee #{0} ID = {1}, Salary = {2}", x + 1, emp[x].GetId(), emp[x].GetSalary().ToString("c2"));

Console.ReadKey();

}
}
class Employee : IComparable
{
private int idNumber;
private double salary;

public Employee(int id, double pay)
{
idNumber = id;
salary = pay;
}

public int GetId()
{
return idNumber;
}
public double GetSalary()
{
return salary;
}

public int CompareTo(Employee emp)
{
int returnVal;

if (this.idNumber > emp.idNumber)
//tem.idNumber - compared object
returnVal = 1;
//This instance is greater than the compared object.

else
if(this.idNumber//???????????????
returnVal = -1;
//This instance is less than the compared object.
else
returnVal = 0;
//This instance is equal to the compared object.

return returnVal;
}
}
/*
SORTING idNumber IN ACENDING ORDER

Employee #1 ID = 333, Salary = $12.50
Employee #2 ID = 111, Salary = $14.75
Employee #3 ID = 222, Salary = $22.35

Sorted employees:

Employee #1 ID = 111, Salary = $14.75
Employee #2 ID = 222, Salary = $22.35
Employee #3 ID = 333, Salary = $12.50
*/
Vulpes14y ago
The generic equivalent of IComparable is IComparable where T can refer to any type.

The latter also has a CompareTo method but the difference is that the parameter is of type T (not object).

Consequently, if the Person class had implemented the IComparable interface, then you could have coded the CompareTo method as follows:

public int CompareTo(Person person)
{

if (this.Age < person.Age)
{return 1;}

if (this.Age > person.Age)
{return -1;}
else
{return 0;}
}

Notice that you no longer need to declare a local variable Temp and cast the object to the Person type - you can use the parameter directly.

Generic means that a type is written once to apply generally to all types. T is called a type parameter and can be replaced by any specific type (Person here), a process known as 'specialization'.

It is possible for a generic type to have more than one type parameter and to 'constrain' type parameters so that they can only be replaced by certain types, rather than any type.

Non-generic means that a type is not generic i.e. does not have any type parameters and so cannot be specialized to a particular type.
14y ago
What is meant by generic and non-generic in C#.
Vulpes14y ago
As you're implementing the non-generic IComparable interface, the CompareTo method takes a parameter of type object even though in practice it will actually be a Person object.

It therefore first needs to be cast to Person and stored in the local variable Temp.

You can then access its Age property and compare it with the Age property of the current Person object (i.e. this) for sorting purposes.