Objects in a List

The type parameter used in creating a list defines the type of the variables that are added to the list. For instance, List<string> includes strings, List<int> integers, and List<double> floating point numbers.

In the example below we first Add strings to a list, after which the strings in the list are printed one by one.

List<string> names = new List<string>();

// string can first be stored in a variable
string betty = "Betty Jennings";
// then Add it to the list
names.Add(betty);

// strings can also be directly added to the list:
names.Add("Betty Snyder");
names.Add("Frances Spence");
names.Add("Kay McNulty");
names.Add("Marlyn Wescoff");
names.Add("Ruth Lichterman");

// several different repeat statements can be
// used to go through the list elements

// 1. while loop
int index = 0;
while (index < names.Count)
{
  Console.WriteLine(names[index]);
  index = index + 1;
}

// 2. for loop with index
for (int i = 0; i < names.Count; i++)
{
  Console.WriteLine(names[i]);
}

Console.WriteLine();
// 3. for each loop (no index)
foreach (string name in names)
{
  Console.WriteLine(name);
}

Adding Object to a List

Strings are objects, so it should come as no surprise that other kinds of objects can also be found in lists. Next, let’s examine the cooperation of lists and objects in more detail.

Let’s assume we have access to the class defined below, describing a person.

public class Person
{
  public string name { get; }
  public int age { get; set; }
  public int weight { get; set; }
  public int height { get; set; }

  public Person(string name)
  {
    this.age = 0;
    this.weight = 0;
    this.height = 0;
    this.name = name;
  }

  public double BodyMassIndex()
  {
    double heigthPerHundred = this.height / 100.0;
    return this.weight / (heigthPerHundred * heigthPerHundred);
  }

  public void GrowOlder()
  {
    if (this.age < 100)
    {
      this.age = this.age + 1;
    }
  }

  public bool IsOfLegalAge()
  {
    return this.age >= 18;
  }


  public override string ToString()
  {
    return this.name + ", age " + this.age + " years";
  }
}

Handling objects in a list is not really different in any way from the previous experience we have with lists. The essential difference is only to define the type for the stored elements when you create the list.

In the example below we first create a list meant for storing Person type object, after which we add persons to it. Finally the person objects are printed one by one.

static void Main(string[] args)
{
  List<Person> persons = new List<Person>();

  // a person object can be created first
  Person john = new Person("John");
  // and then added to the list
  persons.Add(john);

  // person objects can also be created "in the same sentence" that they are added to the list
  persons.Add(new Person("Matthew"));
  persons.Add(new Person("Martin"));

  foreach (Person person in persons)
  {
    Console.WriteLine(person);
  }
}

John, age 0 years
Matthew, age 0 years
Martin, age 0 years

Adding User Inputted Objects to a List

The structure we have learned earlier for reading inputs is still very useful.

static void Main(string[] args)
{
  List<Person> persons = new List<Person>();

  // Read the names of persons from the user
  while (true)
  {
    Console.Write("Enter a name, empty will stop: ");
    String name = Console.ReadLine();
    if (name == "")
    {
      break;
    }


    // Add to the list a new person
    // whose name is the previous user input
    persons.Add(new Person(name));
  }

  // Print the number of the entered persons, and their individual information
  Console.WriteLine();
  Console.WriteLine("Persons in total: " + persons.Count);
  Console.WriteLine("Persons: ");

  foreach (Person person in persons)
  {
    Console.WriteLine(person);
  }
}

Enter a name, empty will stop: Matt
Enter a name, empty will stop: Mike
Enter a name, empty will stop: Bob
Enter a name, empty will stop: 

Persons in total: 3
Persons: 
Matt, age 0 years
Mike, age 0 years

Multiple Constructor Parameters

If the constructor demands more than one parameters, you can query the user for more information. Let’s assume we have the following constructor for the class Person.

public class Person
{
  public string name { get; }
  public int age { get; set; }
  public int weight { get; set; }
  public int height { get; set; }

  public Person(string name, int age)
  {
    this.age = age;
    this.name = name;
    this.weight = 0;
    this.height = 0;
  }

  // Rest of the Person
}

In this case, an object is created by calling the two-parameter constructor.

If we want to query the user for this kind of objects, they must be asked for each parameter separately. In the example below, name and age parameters are asked separately from the user. Entering an empty name will end the reading part.

The persons are printed after they have been read.

static void Main(string[] args)
{
  List<Person> persons = new List<Person>();

  // Read the names of persons from the user
  while (true)
  {
    Console.Write("Enter a name, empty will stop: ");
    String name = Console.ReadLine();
    if (name == "")
    {
      break;
    }

    Console.Write("Enter the age of the person " + name + ": ");

    int age = Convert.ToInt32(Console.ReadLine());

    // Add to the list a new person
    // whose name is the previous user input
    persons.Add(new Person(name, age));
  }

  // Print the number of the entered persons, and their individual information
  Console.WriteLine();
  Console.WriteLine("Persons in total: " + persons.Count);
  Console.WriteLine("Persons: ");

  foreach (Person person in persons)
  {
    Console.WriteLine(person);
  }
}

Enter a name, empty will stop: Mike Modeler 
Enter the age of the person Mike Modeler: 42
Enter a name, empty will stop: Nelson M
Enter the age of the person Nelson M: 1000
Enter a name, empty will stop: Harry Booter
Enter the age of the person Harry Booter: 1
Enter a name, empty will stop: 

Persons in total: 3
Persons: 
Mike Modeler, age 42 years
Nelson M, age 1000 years
Harry Booter, age 1 years

In the example below, the required information was entered line by line. By no means is it impossible to ask for input in a specific format, e.g. separated by a comma.

If the name and age were separated by a comma, the program could work in the following manner.

static void Main(string[] args)
{
  List<Person> persons = new List<Person>();

  // Read the names of persons from the user
  while (true)
  {
    Console.WriteLine("Enter the person details separated by a comma, e.g.: Randall, 2");
    string details = Console.ReadLine();
    if (details == "")
    {
      break;
    }

    string[] parts = details.Split(",");
    string name = parts[0];
    int age = Convert.ToInt32(parts[1]);
    persons.Add(new Person(name, age));
  }

  // Print the number of the entered persons, and their individual information
  Console.WriteLine();
  Console.WriteLine("Persons in total: " + persons.Count);
  Console.WriteLine("Persons: ");

  foreach (Person person in persons)
  {
    Console.WriteLine(person);
  }
}

Enter the person details separated by a comma, e.g.: Randall, 2
Matt, 23
Enter the person details separated by a comma, e.g.: Randall, 2
Mike Pence, 3
Enter the person details separated by a comma, e.g.: Randall, 2


Persons in total: 2
Persons: 
Matt, age 23 years
Mike Pence, age 3 years

Filtered Printing from the List

You can also examine the objects on the list as you go through it. In the example below, we first ask the user for an age restriction, after which we print all the objects whose age is at least the number given by the user.

// Let's make a list with couple of entries
List<Person> persons = new List<Person>();
persons.Add(new Person("Martin", 11));
persons.Add(new Person("Matthew", 12));

// Ask for age limit
Console.Write("What is the age limit? ");
int ageLimit = Convert.ToInt32(Console.ReadLine());

// Print only those who are above the limit
foreach (Person person in persons) {
  if (person.age >= ageLimit)
  { 
    Console.WriteLine(person);
  }
}

What is the age limit? 12
Matthew, age 12 years

You can now do the exercises for Objects in a List.