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In this video, we'll learn how
to define our own functions.

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Function definitions
have four main parts.

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We'll look at the syntax of
functions in the next slide.

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But for now, let's look
at what these parts are.

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The function has to have a name.

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The name of the function
follows the same rules we

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use for naming variables.

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Function names should be
meaningful and provide the reader

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the intent of the function.

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For example, a function called
func1 and another function

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called func2 may be okay when
teaching functions in my slides.

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But what if I read some production
code with function names like that.

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I'd have no idea what these functions
are doing based on those names.

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Functions perform operations.

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So usually, you see function
names as verb or verb phrases.

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This isn't always the case, but
it's a good habit to get into.

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The second part of a function
is the parameter list.

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This is the list of variables
that the function expects

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from you when you call it.

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For example, if you're
calling a function that adds 2 integers,

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then the 2 integers
are in the parameter list.

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Sometimes functions don't
need any parameters at all.

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Suppose I have a function that
simply displays hello or a function

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that returns a system time to me.

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These functions can be called
with an empty parameter list.

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All functions have the ability
to return information to

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the caller of the function.

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In order to return information,
we must specify the type

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of that information.

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So I might return a Boolean or an
integer or a double or a string.

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Also it's possible that a
function returns nothing.

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For example, if I need a
function to just display hello,

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I really don't expect anything
back from that function.

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In this case, we say that the
return type is the keyword void.

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Finally, functions have bodies.

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These are statements to be executed
and they're enclosed in curly braces.

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You've already seen an example
of this in the main function.

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So as you can see, a function
definition is pretty logical, the

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name of the function, what it expects,
what it returns and what it does.

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Let's see how this
looks in c++ syntax.

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Here, we have a simple
function definition.

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Notice the name of the function
is function underscore name.

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And after the function name, we
have open and closed parentheses.

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In this case, this function
expects no parameters.

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So there's nothing
inside the parentheses.

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This function returns an integer.

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We specify that by providing
the type of the return value

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before the function name.

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Finally, we have the body of
the function, which consists of

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statements inside curly braces.

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You can see that this is
the same syntax as we've

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used in our main function.

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Of course, we can have functions
that return nothing expect

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many parameters and so forth.

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The syntax for all of them
is very regular and exactly

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what you would expect.

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Let's see another one.

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In this case, we have the same
function named function name.

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But this time it expects
exactly one parameter.

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That parameter is an integer
type and it's available for us to

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use in the body of the function.

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In this example, we have a function
that returns nothing when called.

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In this case, we provide the keyword
void before the function name.

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This tells the compiler that
there is no return type.

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With void functions, we can still have
simple return statements in the body

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of the function, but they're totally
optional and not often written.

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If there is no return statement,
it's understood that the function

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returns after the last statement
is executed, pretty simple syntax.

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Let's look at one more.

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In this example, we see a function
definition for function name

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that expects multiple parameters.

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In this case, the function
expects two parameters: an

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integer and a standard string.

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When the function is called, these
two parameters must be provided

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as arguments in the same order as
specified in the parameter list.

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Suppose we're going to write
a function that displays hello

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followed by an endline every
time it's called, we could easily

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define that function as follows.

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The function name is say hello.

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It expects no parameters
and returns nothing in.

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The body of the function,
we have an output statement

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that just displays hello.

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Notice that in this case, there is
no return statement required since

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the function doesn't return anything.

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Once the say hello function is
defined, we can call it from

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main or from any other function.

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In this example, you see the
function say hello defined above

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and you also see that it's being
called by the main function below.

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This program will display hello.

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In this example, we're calling the
say hello function 10 times inside

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a for loop in the main function.

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This will display hello
10 times once per line.

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Of course, we can define as
many functions as we need

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in our applications to help
modularize our program.

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In this case, I'm defining
3 functions: say world,

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say hello and main.

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Execution always begins
with the main function.

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So in this case, main calls say hello.

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So we execute say hello.

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And we display hello to the console.

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But then say hello calls say
world and we display world.

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So this program displays
hello world to the console.

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Here's the same example with some
other output statements sprinkled in.

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Again, execution always
begins with the main function.

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So in this case, main calls say hello.

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So we execute say hello.

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Remember that when say hello
is finished, we need to

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complete the code in main.

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So we display hello to the console
then say hello calls say world.

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And when say world is done, we
complete the code and say hello.

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So we execute say world and it
displays world to the console,

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followed by bye from say world then we
go back to where we left off and say

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hello and execute bye from say hello.

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Then we go back to where we left
off in main and execute bye from

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main and our program terminates.

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You can walk through this code and
you'll see exactly what's happening.

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There is one caveat about
function definitions.

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The compiler must know the function
details that is the function

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name, the parameters and so forth
before the function is called.

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So look at this simple example.

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I'm calling say hello in main.

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But the compiler hasn't seen
the definition of say hello yet.

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In this case, you'll get a compiler
error telling you that say hello

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has not been declared in this scope.

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We'll talk about how to solve
this issue with function

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prototypes in the next video.

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But before we do that, let's head over
to the IDE and write a few functions.

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Okay, so I'm in the IDE, I'm in
the section 11 workspace in the

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function definitions project.

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What I want to do here is I want to
write a program that calculates the

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area of a circle on the volume of a
cylinder, pretty easy calculations.

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But I want to break everything up
into functions that we define, we

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create our own user-defined functions.

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And this is what I want
my main to look like.

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Two simple function calls: area of
the circle, volume of the cylinder.

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That's it.

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Now within those functions, we can
prompt the user to enter the radius,

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the height, whatever else we need.

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Okay, so let's get started.

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Let's write that area
circle function first.

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Now in this case, I don't want to
pass anything into the function or

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return anything from the function.

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This function is simply going
to ask the user to cut to

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enter the radius of the circle.

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It's going to call another function
that will actually calculate the

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area, and it'll print the result.

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Okay, so let's write that function.

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We'll start with void
since it returns nothing.

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And the name of the function
is area of the circle.

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That's it.

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Simple as that, nothing in the
parameter list right now because

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we're not passing anything in,
as you can see here in the call.

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Okay, so what do we need?

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Well, we need to ask the user for
the radius of the circle, right.

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So we need a variable here.

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And we'll just call it
radius, and double is perfect.

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We'll initialize it to 0, and we'll
say, we'll prompt the user and

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put a new line at the beginning.

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We'll just say enter the
radius of the circle.

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That's it.

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And we'll read the input into
the variable we just created.

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You can see this pattern
happening over and over

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again, it's pretty standard.

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Okay, now what do I want to do.

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Well, now I want to call
another function that

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actually calculates the area.

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That function -- again, remember, the
area for a circle is pi r squared.

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That function needs to know the
radius in order to calculate the area.

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Let's write the function call first.

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We'll say something like cout,
and we'll do something like

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again the area of a circle with
radius whatever they typed in,

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which was radius in this case.

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And then we'll say
is, now what do we do.

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Now we need to call a function
that actually calculates that

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area gives me back the double
that I can display here.

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Okay, so let's call that
function calc area circle.

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And that function expects
the radius, right.

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And then we'll print
a new line at the end.

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And really that's it
for this function.

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Let me clean that space
up right here, right.

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So now we need to write the other
function, this function right

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here, calc area of the circle.

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Obviously, we need to return
something from that function, right.

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The function's calculating the area.

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So it needs to return something to
me so that I can display it here.

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So in this case, I want
to return a double.

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And let's call it calc
area of the circle.

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And what does it expect, it expects
the radius, which is a double.

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What does it do now, it just returns
pi r squared, right, simple as that.

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But we haven't defined pi.

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Let's define it up here.

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We'll say const double pi

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and we'll set it to 3.14159,

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that's close enough for this example.

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So there's my pi.

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It's constant, which is what we want.

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We don't want to be changing pi.

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And in the function body here,
all we need to do really is to say

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return pi times the radius squared.

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We could use the pow function or we
could just say radius times radius,

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which is what I'll do right here.

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That's it. That should do it.

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Let's walk through this.

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We call area of the circle

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transfers control over to this function.

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We have a radius variable right
here, we're asking the user to enter

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the radius of the circle, we're
reading it and then we're passing

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that value into this function,
calculate the area of the circle.

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And it's going to calculate the
area of the circle and return

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that value back to me, which I'm
going to print right in here.

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Now I could store that into
a variable, do anything

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I want with it, right.

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So let's run this and
test it out, oops.

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Let me comment .volume of the cylinder
because we haven't written that one

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yet, that's the next one, right.

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So the radius of the circle 12.5.

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It says here the area of a circle
with radius 12.5 is 490.873.

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Okay, obviously, we'd want to test
that to make sure that that's correct.

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And that's pretty much it, right.

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Pretty straightforward.

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Now let's write this one right here.

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The volume of a cylinder.

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Well, how do you calculate
the volume of a cylinder.

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Well, the volume of a cylinder
is pi r squared times the

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height of the cylinder, right.

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And I'm not using any units here.

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You can think of this as inches
or centimeters or meters or

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whatever you like, it doesn't
really matter too much.

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So we need to write this
volume cylinder function.

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This is going to be very similar
to the area circle function, right.

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It's just going to prompt the
user for some information.

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In this case, it's going to prompt the
user for the radius of the cylinder

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and the height of the cylinder.

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So let's write it.

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We'll say void volume of a cylinder.

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And the reason that that's popping
up in the completion is because I had

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that in there before and I guess it
remembered, which is kind of handy.

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All right, so we need the
radius of the cylinder.

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And we'll initialize that to 0.

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00:12:05,410 --> 00:12:07,780
And we also need the
height of the cylinder.

241
00:12:09,200 --> 00:12:10,829
We'll initialize that to 0 as well.

242
00:12:11,490 --> 00:12:13,960
All right, so let's ask the user.

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00:12:13,960 --> 00:12:17,660
Let me just copy and paste this
to save a little typing time.

244
00:12:19,410 --> 00:12:22,150
So what is the radius of
the cylinder in this case.

245
00:12:24,140 --> 00:12:26,020
And we'll read that into radius.

246
00:12:29,000 --> 00:12:33,200
Perfect. And then what we'll do here is we'll
copy these guys as well,

247
00:12:36,500 --> 00:12:39,040
and we'll ask them for the height of the cylinder,

248
00:12:41,740 --> 00:12:43,040
enter the height of the cylinder.

249
00:12:43,040 --> 00:12:44,760
We'll read that into
the height variable.

250
00:12:44,760 --> 00:12:46,920
All right, so now we've
got everything we need.

251
00:12:47,210 --> 00:12:48,170
So what do we do now.

252
00:12:48,700 --> 00:12:50,910
Well, let's do the
output statement, right.

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00:12:50,910 --> 00:13:03,260
We'll say cout, and we'll say the
volume of a cylinder with radius, and

254
00:13:03,260 --> 00:13:05,770
we'll provide the radius value here.

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00:13:06,929 --> 00:13:12,460
And then we'll say something
like and height and we'll

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00:13:12,600 --> 00:13:13,740
output the height here.

257
00:13:15,070 --> 00:13:19,790
And finally, we'll just say
just like before is, now we

258
00:13:19,790 --> 00:13:22,330
need to call a function which
we haven't written yet, right.

259
00:13:22,349 --> 00:13:27,709
So let's call it calc
volume of the cylinder.

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00:13:28,619 --> 00:13:29,840
We'll write that function in a minute.

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00:13:29,879 --> 00:13:31,030
Now what do we pass into it.

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00:13:31,050 --> 00:13:34,240
Well, we need to pass in
the radius and the height.

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00:13:34,880 --> 00:13:37,000
It doesn't matter what order you
put it in, but that's the way you

264
00:13:37,000 --> 00:13:38,240
have to provide the variables.

265
00:13:38,250 --> 00:13:41,410
So I could have said height radius
and just sent in the correct ones.

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00:13:41,610 --> 00:13:44,079
So in this case, I'm going to
write that so it expects a radius

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00:13:44,080 --> 00:13:46,819
first and then the height, and
then we'll print a new line

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00:13:46,820 --> 00:13:49,000
at the end, and that'll be it.

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00:13:50,770 --> 00:13:53,089
Okay, so that's where we're
at with this function.

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00:13:53,730 --> 00:13:56,580
It's finished now, so let me clean
up some of this space right here.

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00:13:58,530 --> 00:14:01,640
Now what we need to do is write
that function, calc volume cylinder.

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00:14:02,080 --> 00:14:02,950
So let's do that.

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00:14:02,969 --> 00:14:04,350
And we'll do it right here.

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00:14:05,619 --> 00:14:09,980
We'll say -- actually we'll do it
a couple lines up right under the

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00:14:09,980 --> 00:14:11,580
other calculation, right here.

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00:14:11,850 --> 00:14:16,390
So we'll say double because
it returns a double, calc

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00:14:16,390 --> 00:14:17,570
volume over the cylinder.

278
00:14:18,020 --> 00:14:21,029
It expects the radius,
which is a double.

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00:14:22,719 --> 00:14:27,260
And it expects the height,
which is also a double, right.

280
00:14:27,300 --> 00:14:28,490
So in this case, what do we do?

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00:14:28,500 --> 00:14:38,240
We return pi radius
squared times the height.

282
00:14:39,960 --> 00:14:41,910
And that gives us the
volume of the cylinder.

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00:14:42,629 --> 00:14:43,990
That should do it.

284
00:14:44,990 --> 00:14:48,180
Calling volume cylinder now, so we'll
do it 2 times now, we'll get the

285
00:14:48,180 --> 00:14:51,620
area of the circle, which let's say
it's -- again, let's just say 10.

286
00:14:52,670 --> 00:14:53,490
And that's correct.

287
00:14:53,520 --> 00:14:54,610
That's what we expect, right.

288
00:14:54,660 --> 00:14:55,800
Pi times 100.

289
00:14:56,219 --> 00:14:59,370
So what's the radius of
the cylinder this time?

290
00:14:59,370 --> 00:15:00,269
Well, let's say it's 12.1.

291
00:15:00,660 --> 00:15:01,420
And what's the height?

292
00:15:01,420 --> 00:15:04,329
Let's say it's a really
tall cylinder, 25.2.

293
00:15:05,360 --> 00:15:09,759
So the volume of the cylinder with
radius 12.1 and height 25.2 is

294
00:15:09,990 --> 00:15:13,749
11.591, and we just need to put a
little space after the is there.

295
00:15:15,049 --> 00:15:17,500
Okay, now there's one thing that we
could do to make this a little better.

296
00:15:17,779 --> 00:15:22,260
You'll notice that this calculation
right here pi r squared, we're

297
00:15:22,260 --> 00:15:23,670
doing it again right here.

298
00:15:24,000 --> 00:15:26,500
So what we could do is we
could refactor this code

299
00:15:26,500 --> 00:15:27,640
just so it's a little better.

300
00:15:28,099 --> 00:15:33,310
And we could say return and we could
do we can calculate the area of the

301
00:15:33,310 --> 00:15:37,900
circle and pass in the radius to it
and then multiply that by the height.

302
00:15:38,670 --> 00:15:40,530
That way we're reusing that function.

303
00:15:41,410 --> 00:15:43,150
Okay, so hopefully, you can
see what's going on here.

304
00:15:43,160 --> 00:15:48,439
The volume of a cylinder is
the area's -- the area of

305
00:15:48,440 --> 00:15:50,010
the circle times its height.

306
00:15:50,290 --> 00:15:53,390
So we may as well just call
that function to do it for us.

307
00:15:53,390 --> 00:15:54,700
You can see the repeated code.

308
00:15:55,850 --> 00:15:56,770
Okay, so there you go.

309
00:15:56,770 --> 00:15:57,250
That's it.

310
00:15:57,280 --> 00:16:00,170
We've created -- and let me run that
again just to be sure that we're

311
00:16:00,170 --> 00:16:04,920
working correctly, see 12.1, so what's
the radius of the cylinder, 10.2.

312
00:16:05,460 --> 00:16:09,200
And let's say it's
30.1., and there we go.

313
00:16:11,539 --> 00:16:12,319
Okay, so that's it.

314
00:16:12,340 --> 00:16:14,460
You can see how really
straightforward it is.

315
00:16:14,460 --> 00:16:16,390
And the nice thing about
this is when you read your

316
00:16:16,390 --> 00:16:17,689
code, you can see your main.

317
00:16:17,870 --> 00:16:19,490
It's really easy to follow, right.

318
00:16:19,720 --> 00:16:22,130
Before in our main, we had
all of this stuff in there.

319
00:16:22,420 --> 00:16:24,690
Now we're just seeing area
of the circle, oh okay.

320
00:16:24,860 --> 00:16:27,230
And remember, I can call
these as many times as I want.

321
00:16:27,589 --> 00:16:29,869
In this case, it's going
to ask me two times.

322
00:16:29,869 --> 00:16:30,729
I could put them in loops.

323
00:16:30,730 --> 00:16:31,599
I can do anything I want.

324
00:16:31,850 --> 00:16:33,170
So what's the radius of a circle?

325
00:16:33,490 --> 00:16:34,780
10.1.

326
00:16:34,780 --> 00:16:39,079
Okay, it's asking me again as I
called it again, 10.2 this time.

327
00:16:39,790 --> 00:16:41,660
Then it goes to the
radius of the cylinder.

328
00:16:44,290 --> 00:16:45,199
Okay, so that's it.

329
00:16:45,210 --> 00:16:46,600
That finishes off this video.

330
00:16:46,830 --> 00:16:50,079
You can see how easy it is to
create these user-defined functions.

331
00:16:50,450 --> 00:16:51,800
And this is really what you want.

332
00:16:51,820 --> 00:16:54,830
You want your program to be
modularized and have these small,

333
00:16:55,030 --> 00:16:58,460
well-defined units that do what
you'd want it to do and just do

334
00:16:58,460 --> 00:16:59,750
one thing really, really well.