10.5. List Gradebook

Now that we have learned about the List class, let’s apply that to the gradebook example.

Before going any further, we suggest you run the List Gradebook program in your replit IDE.

using System;
using System.Collections.Generic;


class MainClass
{
   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.

In Lines 10 and 11, we also initialize each List by creating a new, empty List. So how do we fill these Lists?

10.5.1. List Iteration

10.5.1.1. do-while

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

// 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).

10.5.1.2. 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:

// 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.

10.5.1.3. for

The next loop on display prints out each student’s name and 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];
}

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:

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

And now we have a gradebook that uses two lists. Let’s compare that to an array gradebook next.