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square that does exactly that?
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It still, at the end of the day, does the same math.
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But let's abstract away the idea of taking
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one number multiplied by another, and just give it a name,
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like square this value.
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>> And, in other words, in C, let's create a function
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called square that does exactly that.
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It's going to be called square.
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It's going to take an int.
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And we'll will just call it n, by default.
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>> But we could call it anything we want.
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And all that it's going to do, literally, is return
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the result of n times n.
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But because it is returning something, which
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is the keyword in purple we've never seen before, I, on line 11,
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can't just say void this time.
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>> Void, in the example we just saw rather of print name, just means,
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do something.
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But don't hand me something back.
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In this case, I do want to return n times n,
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or whatever that is, that number.
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>> So I can't say, hey, computer, I return nothing, void.
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It's going to return, by nature, an int.
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And so that's all that's going on here.
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>> The input to square is going to be an int.
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And so that we can use it, it has to have a name, N. It's going to output
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an int that doesn't need a name.
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We can leave it to main, or whoever is using me to remember this value if we
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want with its own variable.
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>> And, again, the only new keyword here is Return.
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And I'm just doing some math.
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If I really wanted to be unnecessary, I could say int product gets n times n.
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>> And then I could say, return product.
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But, again, to my point earlier of this just not being good design--
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like, why introduce a name, a symbol, like product,
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just to immediately return it?
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It's a little cleaner, a little tighter, so
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to speak, just to say return n times n, get rid of this line altogether.
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>> And it's just less code to read, less opportunity for mistakes.
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And let's see if this actually now works.
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Now, I'm going to go ahead and make return.
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>> Uh-oh, implicit declaration of function.
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I made this mistake before, no big deal.
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Let me just type, or highlight and copy, the exact same function prototype,
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or signature, of the function up here.
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Or I could move the whole function.
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>> But that's a little lazy.
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So we won't do that.
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Now, let me make return again, dot slash return.
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>> x is 2. x squared is 4. x is 3. x squared is 9.
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And the function seems now to be working.
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So what's the difference here?
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I have a function that's called square, in this case, which I put in an input.
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And I get back an output.
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And yet, previously, if I open the other example
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from earlier, which was called prototype.c,
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I had print name, which returned void, so to speak,
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Or it returned nothing, and simply had a side effect.
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>> So what's going on here?
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Well, consider the function get string for just a moment.
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We've been using the function get string in the following way.
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>> We've had a function get string, like include CS50.h,
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include standard IO.h, int, main, void.
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And then every time I've called get string thus far,
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I've said something like, string s gets get string, because get string--
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let's call this get.c-- get string itself returns a string that I can then
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use, and say, hello, comma, percent s, backslash n, s.
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>> So this is the same example, really, that we had earlier.
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So get string returns a value.
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But a moment ago, print string does not return a value.
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It simply has a side effect.
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So this is a fundamental difference.
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We've seen different types of functions now,
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some of which have returned values, some of which don't.
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So maybe it's string, or int, or float.
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Or maybe it's just void.
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>> And the difference is that these functions that
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get data and return a value are actually bringing something back to the table,
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so to speak.
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So let's go ahead and look at one final set
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of examples that gives a sense, now, of how we might, indeed, abstract better,
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and better, and better, or more, and more, and more, in order
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to write, ultimately, better code.
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Let's go ahead, and in the spirit of Scratch, do the following.
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>> Let me go ahead and include CS50.h and standard IO.h.
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Let me go ahead and give myself an int, main, void.
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And let me go ahead, call this cough.c.
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>> And let me go ahead and just like Scratch, print out cough/n.
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And I want to do this three times.
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So I'm, of course, just going to copy and paste three times.
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I'm now going to make cough dot slash cough.
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Let's give myself a little more room here, Enter, cough, cough, cough.
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>> There's, obviously, already an opportunity for improvement.
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I've copied and pasted a few times today.
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But that was only so I didn't have to type as many characters.
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I still changed what those lines of code are.
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>> These three lines are identical, which feels lazy and indeed is,
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and is probably not the right approach.
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So with what ingredient could we improve this code?
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We don't have to copy and paste code.
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