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And I'm going to create a new file by clicking this little plus,
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and clicking New File.
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>> I'm going to save this next one as, let's say, string.c,
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because I want to play with strings.
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And string in C is just a sequence of characters.
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So now let's go ahead and do the following.
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>> Include standard IO.h-- and it turns out standard IO,
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IO just means input and output.
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So it turns out that this line here is what
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is the neighboring us to use printf.
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Printf, of course, produces output.
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So in order to use printf, it turns out you have to have this line of code
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at the top of your file.
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>> And we'll come back to what that really means before long.
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It turns out that in any C program I write,
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I've got to start it with code that looks like this.
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And you'll notice CS50 IDE, and other integrated development
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environments like it, are going to try as best
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they can to finish your thought.
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In fact, a moment ago if I undo what I just did, I hit Enter.
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>> I then hit open curly brace, hit Enter again.
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And it finished my thought.
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It gave me a new line, indented no less for nice stylistic reasons we'll see.
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And then it automatically gave me that curly brace to finish my thought.
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Now, it doesn't always guess what you want to do.
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But in large part, it does save you some keystrokes.
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So a moment ago, we ran this program-- hello, world, and then compiled it,
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and then ran it.
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But there's no dynamism here.
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What if we wanted to do something different?
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Well, what if I wanted to actually get a string from the user?
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I'm going to use a puzzle piece called exactly that-- get string.
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>> Turns out in C that when you don't want to provide input to a puzzle piece,
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or more properly to a function, you literally just do open parenthesis,
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close parenthesis.
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So it's as though there's no white box to type into.
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The say block before had a little white box.
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We don't have that white box now.
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>> But when I call get string, I want to put the result somewhere.
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So a very common paradigm in C is to call a function, like get string here,
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and then store its return value.
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It's the result of its effort in something.
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>> And what is the construct in programming,
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whether in Scratch or now C, that we can use to actually store something?
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Called it a variable, right?
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And in Scratch, we don't really care what was going in variables.
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>> But in this case, we actually do.
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I'm going to say string.
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And then I could call this anything I want.
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I'm going to call it name, gets get string.
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>> And now even if you're a little new to this,
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notice that I'm lacking some detail.
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I'm forgetting a semi-colon.
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I need to finish this thought.
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So I'm going to move my cursor, and hit semi-colon there.
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And what have I just done?
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In this line of code, number 5 at the moment,
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I'm calling get string with no inputs.
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So there's no little white box like the Save block has.
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>> I'm just saying, hey, computer, get me a string.
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The equal sign is not really an equal sign, per se.
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It's the assignment operator, which means,
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hey, computer, move the value from the right over to the left.
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And in the left, I have the following.
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>> Hey, computer, give me a string-- a sequence of characters.
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And call that string Name.
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And I don't even have to call it Name.
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>> I could call it, conventionally, something like S,
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much like we used i to call the variable i.
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But now I need to do something with it.
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It would be pretty stupid to try compiling this code, running
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this program, even though I'm getting a string,
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because it's still just going to say hello world.
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>> But what if I do want to change this.
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Why don't I do this?
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Percent s, comma s.
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And this is a little cryptic still.
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>> So let me make my variables more clear.
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Let me name this variable Name.
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And let's see if we can't tease apart what's happening here.
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>> So on line five, I'm getting a string.
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And I'm storing that string, whatever the user has typed in
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at his or her keyboard, in a variable called Name.
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And it turns out that printf doesn't just
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take one argument in double quotes, one input in double quotes.
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>> It can take two, or three, or more, such that the second, or third, or fourth,
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are all the names of variables, or specifically values,
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that you want to plug into, dynamically, that string in quotes.
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In other words, what would be wrong with this?
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If I just said hello name, backslash n, saved my file, compiled my code,
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and ran this, what would happen?
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>> It's just going to say, hello name, literally N-A-M-E,
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which is kind of stupid because it's no different from world.
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So anything in quotes is what literally gets printed.
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So if I want to have a placeholder there,
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I actually need to use some special syntax.
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And it turns out if you read the documentation for the printf function,
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it will tell you that if you use percent s,
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you can substitute a value as follows.
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>> After a comma after that double quote, you simply
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