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The programs in this course are going
to make heavy use of variables.

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In fact, almost every program you
ever write will use variables.

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They aren't a complex concept, but
they are fundamental to programming.

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You probably remember working with
variables in your algebra class.

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They were often denoted with a x or
a y, and

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were used to hold a variety
of possible values.

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That is, variables are names for
values that can vary.

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In this equation y = x + 5,
if we set x = 1 then y would equal 6.

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If we set x = 10,
then y would equal 15 and so on.

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Variables in a programming
language are like that but

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they can hold other
things besides numbers.

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They can hold text or a calendar date, or

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any other piece of data that can be
stored in your computer's memory.

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When you assign a value to a variable,

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you're giving that value a name
that you can refer to it by.

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There are couple of different
ways to create variables.

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The first, which we'll show you in
this video, is to use the VAR keyword.

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You type the keyword var,
a variable name an equals sign, and

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then a value you are going
to assign to the variable.

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And as we saw earlier, all # statements
being to end in a semicolon, so

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we type a semicolon at
the end of each line.

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So line 5 here we'll create a variable
named number and assignment the value 4.

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Let me create a second
variable called greeting, and

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I'll assign it the string value hello.

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Once we've declared a variable,

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we can use it anywhere we might
use the original piece of data.

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Remember in our hello world program
we used system.console.writeline to

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print some text to the terminal?

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We can do the same thing with
the contents of variables.

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So here I'll use System.Console.WriteLine
to print the contents of the number

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variable.

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And then I'll use System.Console.WriteLine
again to print the contents

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of the greeting variable.

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Let me save this and
let's try running it, dotnet run.

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And it'll print the contents
of the two variables.

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We can also use a variable that holds
a number in math operations, and

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print the results of
those to the terminal.

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So here I'll use
System.Console.WriteLine to print

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the result of adding 2
to the number variable.

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And here, I'll subtract the variable
number from the value 12 and

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print the result of that.

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Let me save this and run it.

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And you can see that first,
it prints the values of the number and

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greeting variables by themselves.

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And then it prints the result
of the math operations we did.

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The number variable holds the value 4, so

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first it prints the result of 4 + 2,
which is 6.

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And then it prints the result
of 12- 4 which is 8.

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If we change the values to variables hold,

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the program will use that new
value instead of the old one.

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So let me change the number
variable from 4 to 6, and

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let me change the greeting variable
to the string hello to the string hi.

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Save this, and rerun the program, and

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you can see that it prints the new value,
6, for number.

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It prints the new value, hi, for greeting.

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And because number now holds 6 instead
of 4, 6 + 2 is 8, and 12- 6 is 6.

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We can even assign new
values to a variable,

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that is replace the value a variable
holds in the middle of a program.

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For example,
I can reassign the value 10 to number.

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So I type the name of the variable I want
to assign for, a single equal sign and

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the new value I want to assign to it.

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Let me save this and run it.

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Now on the line before
re-assignment number holds 6, so

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number + 2 still prints 8.

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But after the re-assignment number
holds 10, so 12- number will print 2.

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Notice that I didn't use the var keyword
when reassigning a value to the variable.

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That's because the var keyword
is used to declare a variable.

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To declare a variable is to say that
you intend to use a variable with

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a particular name.

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You can only declare one variable at a
time with the same name in the same scope.

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We'll be talking about
scope later in the course.

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If I use the var keyword with
the reassignment is treated as declaring

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another variable with the same name.

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If I save this and try to rerun it,
I'll get an error.

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I need to remove the var keyword so

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that it's treated as an assignment,
not a declaration.

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If I try running this now,
the error goes away.

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So remember,
you can only declare a variable once, but

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you can assign to it as
many times as you need.

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

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

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Variable names have to start with
a letter either lowercase or uppercase or

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an underscore character.

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After that they can contain any
combination of letters numbers or

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underscore characters.

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Other characters like symbols or
spaces are not allowed.

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You can't use C# keywords
as variable names

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because that would confuse the compiler.

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Keywords we've seen so
far include var, class, and static.

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Obviously we haven't had a chance
to learn all the keywords yet.

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So for now just remember that if
you accidentally use a keyword

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as a variable name you'll get a compiler
error and you'll need to change it.

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Beyond the rules that compiler enforces,
there are rules of example or

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conventions that the C#
community follows for names.

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Following these conventions will
make your code easier to read,

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both for you and for other developers.

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The first convention is that variable
names should always begin with

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a lowercase letter.

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I know we just said the compiler will
allow you to begin a variable name with

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an uppercase letter.

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But the convention is to start
variables with lowercase letters.

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It makes them stand out from method and
class names.

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You should also avoid using
underscore characters.

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Again, they're legal, but
their use is frowned upon.

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If there are multiple words
in a variable name words,

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after the first word
should be capitalized.

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This style is often called camel case,
because the capital

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letters stick out of the name,
like the humps on a camel's back.

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If a variable name consists of a single
word, it should be all lowercase.

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Names should clearly describe
what the variable holds.

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Avoid abbreviations unless
they're really common.

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Also avoid single letters.

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We mentioned there are two major
ways to declare variables.

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We should you the var
keyword in this video.

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Up next we'll show you the other way.