10.6. Array

We learned about arrays in C# in a previous lesson, so let’s spend a moment comparing them to Lists. Lists are generally easier to use than C#’s Array. Let’s see why this is.

Why does C# have both Arrays and Lists? The answer is historical, at least in part. C# is a C-style language, and arrays are the most basic data structure in C. Using an Array over a List might be preferred in some circumstances, primarily for performance reasons (array operations are generally faster than List operations). Also note that Arrays are of fixed size. You cannot expand or contract an Array after it is created, so you must know exactly how many elements it will need to hold when you create it. This fact is reason enough to use Lists in most scenarios.

To illustrate Array usage, here is a version of the Array Gradebook program using Arrays instead of Lists:

using System;

class MainClass
{
   static void Main(string[] args)
   {

   // Allow for at most 30 students
   int maxStudents = 30;

   string[] students = new string[maxStudents];
   double[] grades = new double[maxStudents];

   string input;
   string newStudent;
   int numStudents = 0;

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

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

      if (!Equals(newStudent, "")) {
         students[numStudents] = newStudent;
         numStudents++;
      }

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

   // Get student grades
   for (int i = 0; i < numStudents; i++) {
      Console.WriteLine("Grade for " + students[i] + ": ");
      input = Console.ReadLine();
      double grade = double.Parse(input);
      grades[i] = grade;
   }

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

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

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

}

Note that we have to decide up front how large our arrays, students and grades, are going to be. Thus, this program sets an arbitrary maximum amount of students, likely larger than any user will enter. It may seem obvious, then, that Array has no equivalent Add() method. The only way to access and alter an element in an Array is with bracket notation, using an explicit index. For example, gradebook defines a counter variable, numStudents. When the first student is entered by the user, the value is stored in newStudent. If the value is not the empty string, then the value in students at position 0 is assigned the newStudent value. The next time the do-while loop executes, the value of students at position 1 will be assigned. This process continues until the user enters an empty string for newStudent. Because we must always access and assign Array elements using an explicit index, our code can seem littered with Array counter variables (like our friends i and j), making it more difficult to read (not to mention more error-prone).

Like Lists, however, we can loop through an Array using a foreach loop as long as we don’t need to use the index of the current item. If we only wanted to print each student’s name, and not their grade, at the end of our program, we could do the following:

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

We’ll use Arrays in C# from time-to-time, but for the most part you should rely on Lists to store collections of values, or ordered data.

10.6.1. Check Your Understanding

Question

Array size and element values cannot be changed once defined.

1. True

2. False

Question

Given the Array below, which of the following options is a valid action?

int[] randomNumbers = new int[5];

1. randomNumbers.Add(3);

2. randomNumbers.Add("one");

3. randomNumbers[0] = "three";

4. randomNumbers[0] = 1;