10.8. Dictionary Gradebook

We suggest you open the following Dictionary Gradebook in your replit IDE.

using System;
using System.Collections.Generic;

class MainClass
{
   static void Main(string[] args)
   {
      Dictionary<string, double> students = new Dictionary<string, double>();
      string newStudent;

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

      // Get student names and grades
      do
      {
            Console.WriteLine("Student: ");
            string input = Console.ReadLine();
            newStudent = input;

            if (!Equals(newStudent, ""))
            {
               Console.WriteLine("Grade: ");
               input = Console.ReadLine();
               double newGrade = double.Parse(input);
               students.Add(newStudent, newGrade);

               // Read in the newline before looping back
               Console.ReadLine();
            }

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

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

      foreach (KeyValuePair<string, double> student in students)
      {
            Console.WriteLine(student.Key + " (" + student.Value + ")");
            sum += student.Value;
      }

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

Notice how a Dictionary called students is declared on Line 8:

Dictionary<string, double> students = new Dictionary<string, double>();

Here, <string, double> defines the data types for this dictionary’s <key, value> pairs.

We can add a new item with a .Add() method, specifying both key and value:

students.Add(newStudent, newGrade);

And while we don’t do so in this example, we may also access Dictionary elements using bracket notation. If we had a key/value pair of "jesse"/4.0 in the students dictionary, we could access the grade with:

double jesseGrade = students["jesse"];

Variables may be used to access elements:

string name = "jesse";
double jesseGrade = students[name];

Looping through a dictionary is slightly more complex than it is for ordered lists. Let’s look at the foreach loop from this example:

foreach(KeyValuePair<string, double> student in students) {
   Console.WriteLine(student.Key + " (" + student.Value + ")");
   sum += student.Value;
}

The iterator variable, student, is of type KeyValuePair<string, double>. The class KeyValuePair<T,T> is specifically constructed to be used in this fashion, to represent key/value pairs within dictionaries. Each KeyValuePair object has a Key property and a Value property.

If you only need to access the key of each item, you can construct a simpler loop and use the Keys property of the Dictionary class:

foreach (string studentName in students.Keys) {
   Console.WriteLine(studentName);
}

A similar structure applies if you only need the values, using students.Values:

foreach (double grade in students.Values) {
   Console.WriteLine(grade);
}

We have only brushed the surface of how arrays, Lists, and dictionaries work. We leave it to you to refer to the official documentation linked througout the chapter for more details. You’ll certainly be using Lists and dictionaries in more ways than those covered in this lesson, but with the knowledge you have now, you should be able to use C# collections and learn new uses as you go.