3.4. List

To write a List version of the program, we will have to introduce several new C# concepts, including the class List. We will also review different kinds of for loops used in C#.

Before going any further, we suggest you run the ListGradebook program in Visual Studio. This program is in csharp-web-dev-lsn2controlflowandcollections. Once you’ve forked and cloned the repository following the directions on how to clone a C# project, let’s look at what is happening in the source code.

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
using System;
using System.Collections.Generic;

namespace ListGradebook
{
   class Program
   {
      static void Main(string[] args)
      {
         List<string> students = new List<string>();
         List<double> grades = new List<double>();
         string newStudent;
         string input;

         Console.WriteLine("Enter your students (or ENTER to finish):");

         // Get student names
         do
         {
            input = Console.ReadLine();
            newStudent = input;

            if (!Equals(newStudent, "")) {
               students.Add(newStudent);
            }

         } while(!Equals(newStudent, ""));

         // Get student grades
         foreach (string student in students) {
            Console.WriteLine("Grade for " + student + ": ");
            input = Console.ReadLine();
            double grade = double.Parse(input);
            grades.Add(grade);
         }

         // Print class roster
         Console.WriteLine("\nClass roster:");
         double sum = 0.0;

         for (int i = 0; i < students.Count; i++) {
            Console.WriteLine(students[i] + " (" + grades[i] + ")");
            sum += grades[i];
         }

         double avg = sum / students.Count;
         Console.WriteLine("Average grade: " + avg);
      }
   }
}

Here we declare and initialize two objects, students and grades, which appear to be of type List<string> and List<double>, respectively. A list in C# is very similar to an array. Like an array, we must let the compiler know what kind of objects our list is going to contain. In the case of students, the list will contain values of type string (representing the names of the students), so we use the List<string> syntax to inform the compiler that we intend to fill our list with strings. Similarly, grades will hold exclusively values of type double and is declared to be of type List<double>.

In lines 10 and 11, we also initialize each List by creating a new, empty List. We could declare and initialize lists in one line like so:

List<string> newList = new List<string> {"Apples", "Oranges", "Avocados"};

Note

You will sometimes see the List class written as List<T>, where T represents a placeholder for the type that a programmer will declare a given List to hold. This is especially true in documentation. You can think of T as representing an arbitrary type.

Classes like List<T> that take another type or class as a parameter are referred to as generic classes or generic types.

3.4.1. List Iteration

3.4.1.1. do-while

We then use a do-while loop to collect the names of each of the students in the class.

17
18
19
20
21
22
23
24
25
26
// Get student names
do
{
   newStudent = Console.ReadLine();

   if (!Equals(newStudent, "")) {
      students.Add(newStudent);
   }

} while(!Equals(newStudent, ""));

Recall that a do-while loop is very similar to a while loop, but the execution condition is checked at the end of the loop block. This has the net effect that the code block will always run at least once. In this example, we prompt the user for a name, which C# processes via Console.ReadLine() when the user hits the enter key. To finish entering names, the user enters a blank line.

Note

On lines 22 and 26, we use a method to compare the value of newStudent and "". The Equals(a,b) compares two strings, a and b, and returns true if the strings are the same. If the strings are not the same, the method returns false.

For each student that is entered (that is, each non-empty line), we add the new string to the end of our List with students.Add(newStudent). The .Add() method is provided by the List Class. There are lots of other List methods to get familiar with, some of which we will discuss in more detail below.

3.4.1.2. foreach

Below the do-while loop are two different loops that demonstrate two ways you can loop through a List in C#. Here’s the first, which collects the numeric grade for each student:

31
32
33
34
35
36
37
// Get student grades
foreach (string student in students) {
   Console.WriteLine("Grade for " + student + ": ");
   string input = Console.ReadLine();
   double grade = double.Parse(input);
   grades.add(grade);
}

This, you may recall, is C#’s foreach loop syntax. You may read this in your head, or even aloud, as: for each student in students. As you might expect at this point, we must declare the iterator variable student with its data type.

3.4.1.3. for

The next loop on display prints out each student’s name and grade:

34
35
36
37
38
39
40
41
// Print class roster
Console.WriteLine("\nClass roster:");
double sum = 0.0;

for (int i = 0; i < students.Count; i++) {
   Console.WriteLine(students[i] + " (" + grades[i] + ")");
   sum += grades[i];
}

Here, we introduce the syntax students.Count which utilizes the Count property of List. This property holds the integer representing the number of items in the List. This is similar to string’s .Length property.

In this for loop, we use a loop index to define the starting point, ending point, and increment for iteration. It may be helpful for you to consider this kind of construction as something like, for integer i in the range of the number of items in students.... The first statement inside the parenthesis declares and initializes a loop index variable i. The second statement is a boolean expression that is our exit condition. In other words, we will keep looping as long as this expression evaluates to true. The third statement is used to increment the value of the loop index variable at the end of iteration through the loop.

Again, the syntax i++ is C# shorthand for i = i + 1. C# also supports the shorthand i-- to decrement the value of i. We can also write i += 2 as shorthand for i = i + 2.

In the final lines of the program, we compute the average grade for all students:

43
44
double avg = sum / students.Count;
Console.WriteLine("Average grade: " + avg);

3.4.2. List Methods

Let’s gather up a few of the List methods that we’ve encountered so far, along with a few new ones. While these will be the most common methods and properties that you use with this class, they by no means represent a complete record. Refer to the official documentation on the List class for such a record, and for more details.

To demonstrate the use of these methods, we’ll create a new List called planets.

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

Ok, we’ve got an empty List. We need to use the class’s .Add() method to populate this collection with items.

Using .Add() to populate planets:

1
2
3
4
5
6
7
8
planets.Add("Mercury");
planets.Add("Venus");
planets.Add("Earth");
planets.Add("Mars");
planets.Add("Jupiter");
planets.Add("Saturn");
planets.Add("Uranus");
planets.Add("Neptune");

Thus, the first item in this table:

List Methods in C#
C# Syntax Description Example
Add() Adds an item to the List planets.Add("Pluto") adds Pluto to planets
Contains() Checks to see if the List contains a given item, returning a Boolean planets.Contains("Earth") returns true
IndexOf() Looks for an item in a List, returns the index of the first occurrence of the item if it exists, returns -1 otherwise planets.IndexOf("Jupiter") returns 4
Sort() Rearranges the elements of an List into ascending order. planets.Sort() produces {"Earth", "Jupiter", "Mars", "Mercury", "Neptune", "Pluto", "Saturn", "Uranus", "Venus"}
ToArray() Returns an Array containing the elements of the List planets.ToArray() returns {"Earth", "Jupiter", "Mars", "Mercury", "Neptune", "Pluto", "Saturn", "Uranus", "Venus"}

Example

In order to use ToArray, we could first declare a planetsArray of the same size as planets or do it in one line of code.

1
2
3
4
5
6
// Option A
string[] planetsArray = new string[planets.Count];
planetsArray = planets.ToArray();

// Option B
string[] planetsArray = planets.ToArray();

In addition to these different methods we can use, the List class has a number of properties that are very helpful. You may find yourself using the Count property quite a bit. This property holds the number of values in the list. In our example, after we add all of the planets in the solar system, planets.Count has a value of 9.

Speaking of arrays, let’s see the array version of Gradebook next.

3.4.3. Check Your Understanding

Question

The number of entries in a List may not be modified.

  1. True
  2. False

Question

Create a List called charStars containing a, b, and c.

  1. 1
    2
    3
    4
    List<string> charStars = new List<string>();
    charStars.add('a');
    charStars.add('b');
    charStars.add('c');
    
  2. 1
    2
    3
    4
    List<char> charStars = new List<string>();
    charStars.add('a');
    charStars.add('b');
    charStars.add('c');
    
  3. List<char> charStars = new List<char>('a', 'b', 'c');
    
  4. 1
    2
    3
    4
    List<string> charStars = new List<string>();
    charStars.add("a");
    charStars.add("b");
    charStars.add("c");