Perhaps, the core concept in programming and in C# is the variable. Variables often correspond to the letters used in algebra and stand in for numerical quantities, such as X, Y, and Z and a, b, and c. If you need to keep track of information, such as the player name, score, position, orientation, ammo, health, and a multitude of other types of quantifiable data (expressed by nouns), then a variable will be your friend. A variable represents a single unit of information. This means that multiple variables are needed to hold multiple units, one variable for each. Further, each unit will be of a specific type or kind. For example, the player's name represents a sequence of letters, such as "John", "Tom", and "David". In contrast, the player's health refers to numerical data, such as 100 percent (1) or 50 percent (0.5), depending on whether the player has sustained damage. So, each variable necessarily has a data type. In C#, variables are created using a specific kind of syntax or grammar. Consider the following code sample 1-1 that defines a new script file and class called MyNewScript, which declares three different variables with class scope, each of a unique type. The word "declare" means that we, as programmers, are telling the C# compiler about the variables required:

01 using UnityEngine;
02 using System.Collections;
03 
04 public class MyNewScript : MonoBehaviour 
05 {
06     public string PlayerName = "";
07     public int PlayerHealth = 100;
08     public Vector3 Position = Vector3.zero;
09 
10     // Use this for initialization
11     void Start () {
12 
13     }
14 
15     // Update is called once per frame
16     void Update () {
17 
18     }
19 }

Notice from lines 06-08 of code sample 1-1 that each variable is assigned a starting value, and its data type is explicitly stated as int (integer), string, and Vector3, which represent the points in a 3D space (as well as directions, as we'll see). There's no full list of possible data types, as this will vary extensively, depending on your project (and you'll also create your own!). Throughout this book, we'll work with the most common types, so you'll see plenty of examples. Finally, each variable declaration line begins with the keyword public. Usually, variables can be either public or private (and there is another one called protected, which is not covered here).The public variables will be accessible and editable in Unity's Object Inspector (as we'll see soon, you can also refer to the preceding screenshot), and they can also be accessed by other classes.

Variables are so named because their values might vary (or change) over time. Of course, they don't change in arbitrary and unpredictable ways. Rather, they change whenever we explicitly change them, either through direct assignment in code, from the Object Inspector, or through methods and function calls. They can be changed both directly and indirectly. Variables can be assigned values directly, such as the following one:

PlayerName = "NewName";

They can also be assigned indirectly using expressions, that is, statements whose final value must be evaluated before the assignment can be finally made to the variable as follows:

//Variable will result to 50, because: 100 x 0.5 = 50
PlayerHealth = 100 * 0.5;