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So notice, line 7, I can call print name.
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Line 10, I can define or implement print name.
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But, unfortunately, that's not enough.
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>> Let me go ahead and recompile this after saving.
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Whoa, now, I've made it worse, it would seem.
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So implicit declaration of function print name is invalid.
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And, again, there's more errors.
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But as I cautioned earlier, even if you get overwhelmed with,
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or a little sad to see so many errors, focus only on the first
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initially, because it might just have had a cascading effect.
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So C, or Clang more specifically, still does not recognize print name.
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>> And that's because Clang, by design, is kind of dumb.
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It only does what you tell it to do.
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And it only does so in the order in which you tell it to do.
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>> So I have defined main on line four, like we've been doing pretty often.
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I've defined print name on line 10.
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But I'm trying to use print name on line seven.
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>> It's too soon, doesn't exist yet.
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So I could be clever, and be like, OK, so let's just play along,
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and move print name up here, and re-compile.
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Oh my God.
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It worked.
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It was as simple as that.
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>> But the logic is exactly that.
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You have to teach Clang what it is by defining the function first.
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Then you can use it.
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But, frankly, this feels like a slippery slope.
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>> So every time I run into a problem, I'm just
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going to highlight and copy the code I wrote, cut it and paste it up here.
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And, surely, we could contrive some scenarios
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where one function might need to call another.
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And you just can't put every function above every other.
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>> So it turns out there's a better solution.
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We can leave this be.
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And, frankly, it's generally nice, and convenient, and good design
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to put main first, because, again, main just like when green flag clicked,
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that is the function that gets executed by default.
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So you might as well put it at the top of the file
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so that when you or any other human looks at the file
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you know what's going on just by reading main first.
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So it turns out, we can tell Clang proactively, hey, Clang, on line four,
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I promise to implement a function called Print
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Name that takes a string called name as input, and returns nothing, void.
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And I'll get around to implementing it later.
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>> Here comes Main.
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Main now on line 9 can use Print Name because Clang
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is trusting that, eventually, it will encounter the definition
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of the implementation of Print Name.
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So after saving my file, let me go ahead and make prototype,
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looks good this time.
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Dot slash, prototype, let me go ahead and type in a name.
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David, hello David, Zamila, hello Zamila, and, indeed, now it works.
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>> So the ingredient here is that we've made a custom function, like a custom
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Scratch block we're calling it.
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But unlike Scratch where you can just create it and start using it,
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now we have to be a little more pedantic,
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and actually train Clang to use, or to expect it.
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Now, as an aside, why all this time have we been just blindly on faith including
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CS50.h, and including standard IO.h?
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>> Well, it turns out, among a few other things,
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all that's in those dot h files, which happen to be files.
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They're header files, so to speak.
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They're still written in C. But they're a different type of file.
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>> For now, you can pretty much assume that all that is inside of CS50.h
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is some one-liners like this, not for functions called Print Name,
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but for Get String, Get Float, and a few others.
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And there are similar prototypes, one liners, inside of standard IO.h
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for printf, which is now in my own Print Name function.
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So in other words, this whole time we've just been blindly copying and pasting
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include this, include that, what's going on?
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Those are just kind of clues to Clang as to what functions
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are, indeed, implemented, just elsewhere in different files
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elsewhere on the system.
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>> So we've implemented print name.
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It does have this side effect of printing something on the screen.
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But it doesn't actually hand me something back.
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How do we go about implementing a program that
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does hand me something back?
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>> Well, let's try this.
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Let me go ahead and implement a file called return.c
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so we can demonstrate how something like Get String, or Get Int,
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is actually returning something back to the user.
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Let's go ahead and define int main void.
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>> And, again, in the future, we'll explain what that int and that void
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is actually doing.
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But for today, we'll take it for granted.
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I'm going to go ahead and printf, for a good user experience, x is.
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And then I'm going to wait for the user to give me x with get int.
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>> And then I'm going to go ahead and print out x to the square.
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So when you only have a keyboard, people commonly
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use the little carrot symbol on the keyboard
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to represent to the power of, or the exponent of.
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So x squared is present i.
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>> And now I'm going to do this.
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I could just do-- what's x squared? x squared is x times x.
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>> And we did this some time ago already today.
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This doesn't feel like all that much progress.
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You know what?
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Let's leverage some of that idea from last time of abstraction.
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>> Wouldn't it be nice if there's a function called
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