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Welcome back.

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If you write an expression
in a computed value,

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then you probably want to
store that value somewhere.

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Variables are useful because one,
they allow us to store

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the values of expressions to be
used later on in our programs.

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And two, because they allow us to
give us human readable names for

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values that we can then reuse later on.

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So for example, in math class,
if you ever have done geometry,

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then you might be familiar with pi.

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And when you use pi,
you usually don't write out 3.14159, etc.

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Instead, you just use the symbol pi.

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So, pi is almost like a variable.

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It has a name and a value.

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And Python has an equivalent to
variables in its programming language.

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So every variable has a name and a value.

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So in this program,
we create three variables.

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The first variable is called message.

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And it's created on line one.

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So message has the value What's up,
Doc, which is a string.

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And then we create a second variable
whose name is n and whose value is 17.

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And then we create a third
variable whose name is pi and

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whose value is 3.14159.

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Now, the first three lines
are setting variables.

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And then we reference or use those
variables in the last three lines.

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So if I run my code and then you'll see
that I get three things printed out from

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lines five, six and seven.

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So when we print out message,
then we print out What's up, Doc.

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When we print out n, then we print out 17.

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When we print out pi,
then we print out the value 3.14159.

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Now, it's useful to be able to kind
of the execution of programs like

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this in your head.

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And in order to do that, we have what's
called a variables values diagram or

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a reference diagram.

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Now, to draw a reference diagram,

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what we do is we go line
by line from top to bottom.

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So we start at the top, line one,
and then go down and for

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every line, we're going to say what
variable is set to what value.

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A reference diagram has two columns.

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It has variables, And values.

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So what we do is we go line by line from
the top of the program to the bottom and

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we're going to say what
variable is set to what value.

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So in this program on line one,

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then we set the variable message to
the value the string What's up, Doc.

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So in our variables values diagram,
we say the variable message,

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Is set to the value What's up, Doc.

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On line two,
we set the variable n to the value 17.

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On line three, we set the variable

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pi to the value 3.14159.

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And now by the time we get to line five,
when we say print out message.

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Then this expression is
an expression whose value

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evaluates to whatever
message it's pointing to.

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So when Python says print message,
it looks at this and

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it says this is an expression,

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specifically it's an expression that
references the variable message.

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And so the Python interpreter looks
in this variables values diagram and

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it asks what's the value of message.

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We see a message here, and we see that
messages value is What's up, Doc.

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And so what that means is that
when we print out message,

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then we print out What's up, Doc.

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And same thing for
when we actually look up the value of n.

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So when we say print out the value of n,
then Python is going to first look

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in this variables values diagram and
it sees that value of n is 17

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So what that means is that when we print
out the value of n, we print out 17.

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And same thing for
when we print out the value of pi, so

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Phyton looks in this reference diagram,
sees that pi is 3.14159.

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And when we print out pi,
then we print out 3.14159.

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Let me make this green just
to be slightly clearer.

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Okay, So

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you might ask what happens if we actually
overwrite the value of a variable.

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So what happens if we actually
have a variable that I'll

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just call x and we set it to 5 and
then I'll say print x and

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then I'm going to later
on reset x to be 10.

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And I'll say print x, okay.

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So in order to determine
what's going to happen here,

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then I'm going to write out
my variables values diagram.

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So line 1 sets the value of
the variable x to the value 5,

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and then on line 2 when we
print out the value of x,

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then we're going to print out 5.

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On line 4 we set the value of x to 10,

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and so what that does is it
says x no longer points to 5,

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so I'm going to erase this line.

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And I'm going to cross out 5.

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X now instead points to 10.

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And so X now points to 10.

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And here when we print out X,

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then we actually are going to
print out 10 rather than 5.

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And you can see this if
we were in our program.

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So again here, the first line or
the first print statement prints out 5,

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the second print statement prints out 10.

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So every variable has a name and
a value, and

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even though the value of a variable can be
anything, the name has some restrictions.

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So every variable name has
to start out with a letter.

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And variable names can only
contain letters and numbers.

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So for example, x is a valid variable name
because it starts out with the letter and

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only contains letters and numbers.

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Same thing with my variable.

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I can also have capital letters.

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So I can say XYZ, capital XYZ =100.

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And these are all valid variable names.

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So if I print out the value of XYZ,

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then it prints out 100.

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One thing to note is that Python
is what's called case sensitive.

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And what that means is that the variable
name XYZ in all capitals is

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different from the variable
name xyz in lowercase.

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So if I print out XYZ in all capitals,
then I get 100.

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If I print out lower case xyz,
then I get this other variable value.

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If I print out xYz, then there is
no variable that's called xYz and

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so I get an error saying that
this variable is not defined.

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So variable names have to
follow these two rules.

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Start with a letter and
it can only contain letters and numbers.

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And what that means is
that a variable name that

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starts out with a number,
so if I say 2x = 50.

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This is not a valid variable name because
it starts with a number rather than

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starting with the letter.

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And if I try to save and run my program,

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then I'm going to get a syntax error
because I have a bad variable name here.

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I can have a variable called x2
because that starts with the letter,

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but I can't have a variable name 2x.

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One thing to not here is that Python also

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treats underscores like characters, so

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what that means is that
I can have a variable

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myvariable and I can set it to 5.5.

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And if I print out the value of
my variable, then I'll get 5.5.

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So because Python treats this underscore
character as a letter, people

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often use underscores as a substitute for
spaces in their variable names.

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So variables are useful
tool that we'll use to one,

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remembering in expressions
value to use later on.

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And two, to give a nice name to
the value of an expression so

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that it's more human readable.

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Until next time.