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>> SPEAKER 1: So I feel like I tend to
write printf hello world or printf

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hello percent s quite often in contexts
where I want to print out a

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string, sometimes asking the
user part of that string.

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Well, wouldn't it be nice if we created
a function of our own called,

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say, print name, whose purpose in life
is simply to print hello comma

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so-and-so by accepting the so-and-so
as an argument to that function?

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We can declare a function of our own
just as we can in Scratch as follows.

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>> In other words, suppose I'd like to
printf your name and then ask the user

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for his or her name with string
s equals GetString.

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And then I want to call a function
called print name

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passing an s as its input.

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>> Now unfortunately, there is no function
called print name in C or in

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the cs50 library.

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But no matter, because we
can make it ourselves.

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To declare a function of our own and
then implement it, we can simply make

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some room in this file and declare
this function as follows, void--

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signifying that the function's not
actually going to return something,

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even though it will have
a side effect--

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print name, and then let's specify in
parentheses that print name expects an

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argument of type string.

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And in general, we'll think
of that as a name.

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>> And let's open our brace
and close our brace.

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And then printf inside hello comma
percent s new line comma name.

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In other words, even though this
function's only one line of code, it

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does now have some higher
level purpose to print

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someone's name with a greeting.

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>> Now inside of main, notice that I'm
calling print name by passing an s.

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And so I seem now to have
a complete program.

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The only difference being from others
we've written is that not only does

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this program have main, it also has a
declaration and implementation of

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print name, a function of my own.

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>> Let's now compile my program with make
function zero, and run it with dot

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slash function zero.

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When prompted for my name
I'll type in David.

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And I indeed am greeted
with "Hello, David."

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>> Now, it turns out I very deliberately
declared print name above main,

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thereby telling C in advance that there
exists a function called print

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name so that I could use
it later in main.

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Now, had I instead put print name at
the bottom of my file, as frankly I

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probably should do so that main remains
the first thing that I or

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another user sees in my code,
it turns out that my code is

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not going to compile.

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>> Specifically, if I go back to my
terminal window and run make function

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zero now, notice that I'm yelled at.

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If I scroll up in my terminal window,
I'll see this first error in red,

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implicit declaration of
function print name.

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>> Now, what does that mean?

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Well, even though I've implemented
print name in this file, I've not

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implemented it before I've
used it in main.

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Now, we could go back to the old
version and put print name's

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implementation up top.

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>> But I can instead just give C a hint
that there's a function to come.

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Specifically, above main I can declare
print name just as I did earlier, but

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end the line with a semicolon, thereby
promising to implement but not yet

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actually implementing the function
called print name.

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I can then have main at the top of my
file as I hoped, and I can actually

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implement print name at the bottom.

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>> If I now return to my terminal window
and recompile function zero with make,

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this time it compiles.

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And if I run it with dot slash function
zero providing my name, I'm

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greeted as "Hello, David."

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