Variables

The programs in this course are going to make heavy use of variables. In fact, almost every program you ever write will use variables. They aren't a complex concept, but they are fundamental to programming.

You probably remember working with variables in your algebra class. They were often denoted with an x, or a y, and were used to hold a variety of possible values. That is, variables are names for values that can vary. In this equation, "Y = X + 5", if we set X equal to 1, then Y would equal 6. If we set X equal to 10, then Y would equal 15. And so on.

• Variables in a programming language are like that, but they can hold other things besides numbers. They can hold text, or a calendar date, or any other piece of data that can be stored in your computer's memory.
• When you assign a value to a variable, you're giving that value a name that you can refer to it by.
• There are a couple different ways to create variables. The first, which we'll show you in this video, is to use the var keyword.
• You type the keyword var, a variable name, an equals sign, and then a value you're going to assign to the variable.
• And as we saw earlier, all C# statements need to end in a semicolon (;), so we type a semicolon at the end of each line.

var number = 4;
var greeting = "hello";

• We can then use that variable anywhere we might use the original piece of data.
  • Remember in our "Hello, World!" program, we used System.Console.WriteLine to print some text to the terminal?
  • We can do the same thing with the contents of variables.
  • We can also use a variable that holds a number in math operations, and print the results of those to the terminal.

System.Console.WriteLine(number);
System.Console.WriteLine(greeting);
System.Console.WriteLine(number + 2);
System.Console.WriteLine(12 - number);

• If we change the values the variables hold, the program will use that new value instead of the old one.

var number = 6;
var greeting = "hi";
System.Console.WriteLine(number);
System.Console.WriteLine(greeting);
System.Console.WriteLine(number + 2);
System.Console.WriteLine(12 - number);

• We can even assign new values to a variable, that is, replace the value a variable holds in the middle of a program.
  • For example, I can re-assign the value 10 to number: number = 10;
  • Now, on the line before the re-assignment, number holds 6, so number + 2 still prints 8.
  • After the re-assignment, number holds 10, so 12 - number will print 2.

var number = 6;
var greeting = "hi";
System.Console.WriteLine(number);
System.Console.WriteLine(greeting);
System.Console.WriteLine(number + 2);
number = 10; // Update the variable value.
System.Console.WriteLine(12 - number);

• Notice that I didn't use the var keyword when re-assigning a value to the variable.
  • That's because the var keyword is used to declare a variable.
  • To declare a variable is to say that you intend to use a variable with a particular name.
  • You can only declare one variable at a time with the same name in the same scope. (We'll be talking about scope later in the course.)
  • If I use the var keyword with the re-assignment, it's treated as declaring another variable with the same name: var number = 10;
  • If I save this and run the program, I'll get an error.
  • I need to remove the var keyword so that it's treated as an assignment, not a declaration.
  • Now the program will run again.
• So remember, you can only declare a variable once, but you can assign to it as many times as you need.

You can name variables almost whatever you want, but there are a few restrictions.

First up are the rules enforced by the C# language.

• Variable names have to start with a letter, either lower case or upper case, or an underscore character.
• After that, they can contain any combination of letters, numbers, or underscore characters. Other characters, like symbols or spaces, are not allowed.
• You can't use C# keywords as variable names, because that would confuse the compiler. Keywords we've seen so far include var, class, and static. Obviously we haven't had a chance to learn all the keywords yet, so for now just remember that if you accidentally use a keyword as a variable name, you'll get a compile error and you'll need to change it.

OK:

• grade
• angle2
• taxRate

Not OK:

• 2ndAngle
• class
• tax rate

Beyond the rules the compiler enforces, there are some rules of thumb, or conventions, that the C# community follows for names. Following these conventions will make your code easier to read, both for you and for other developers.

• Variable names should always begin with a lower-case letter. I know we just said the compiler will allow you to begin a variable name with an upper-case letter, but the convention is to start variables with lower-case letters. It makes them stand out from method and class names.
• You should also avoid using underscore characters. Again, they're legal, but their use is frowned upon.
• If there are multiple words in a variable name, words after the first word should be capitalized. This style is often called "camel case" because the capital letters stick out of the name like the humps on a camel's back. If a variable name consists of a single word, it should be all lower-case.
• Names should clearly describe what the variable holds. Avoid abbreviations unless they're really common. Also avoid single letters.

OK:

• studentFirstName
• name

Not OK:

• StudentFirstName
• student_first_name
• studentfirstname
• stdfstnm
• x

We mentioned there are two major ways to declare variables. We showed you the var keyword in this video. Up next, we'll show you the other way.