Name: 2016年CS50--第一週--C (CS50 2016 - Week 1 - C)
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>> DAVID J. MALAN: All right, this is CS50.

So recall that last time in week zero, we focused on computational thinking.

And we transitioned from that to Scratch, a graphical programming

language from our friends at MIT's Media Lab.

>> And with Scratch, did we explore ideas like functions, and conditions,

and loops, and variables, and even events, and threads, and more.

And today, we're going to continue using those ideas,

and really taking them for granted, but translate them

to another language known as C. Now, C is a more traditional language.

It's a lower level language, if you will.

And so at first glance, it's all going to look rather cryptic

We're going to have semi-colons, and parentheses,

But realize that even though the syntax is

about to look a little unfamiliar to most of you, see past that.

And try to see the ideas that are, indeed, familiar,

because here in week one what we'll begin to do is to compare,

>> So, for instance, recall that when we implemented the first of our programs

last time, we had a block that looked a little something like this-- when

green flag clicked, and then we had one or more puzzle pieces below it,

So, indeed, in Scratch, when I click that green flag

to run my program, so to speak, these are

And, specifically, Scratch said, hello, world.

>> Now, I could have specified different words here.

But we'll see that, indeed, many of these blocks-- and indeed,

in C many functions-- can be parametrized or customized

In fact, in C if we want to convert, now,

this Scratch program to this other language,

we're going to write a little something like this.

>> Granted, there is some unfamiliar syntax there most likely, int,

But printf-- even though you would think it would just be print.

But print means print formatted, as we'll soon see.

This literally will print to the screen whatever

is inside of those parentheses, which of course in this case is, hello world.

>> But you'll notice some other syntax, some double quotes,

that the parentheses at the end, the semi-colon and the like.

So there's a bit of overhead, so to speak, both cognitively

and syntactically, that we're going to have to remember before long.

But realize that with practice, this will start to jump out at you.

>> In fact, let's focus on that one function specifically-- in this case,

Hello world is its parameter, or argument, its customisation.

>> And the equivalence in C is just going to be this one line here,

where printf is equivalent to, say, the double quoted string, hello

world is equivalent, of course, to what's in the white box there.

And the backslash n, though a little strange and absent from Scratch,

simply is going to have the effect we'll see in a computer, like my Mac or a PC,

of just moving the cursor to the next line.

It's like hitting Enter on your keyboard.

But first, let's take a look at this other example in the case of loops.

We had this forever loop last time, which was a series of puzzle pieces

that did something literally forever-- in this case,

say, hello world, hello world, hello world, hello world.

>> In C, if we want to implement this same idea, we might simply do this.

While true, printf hello world-- now while, just semantically, kind of

conjures up the idea of doing something again, and again, and again,

Well, true-- recall that true is just on or one.

So it's kind of a meaningless statement just to say true.

But indeed, this is deliberate, because if true is just always true,

than while true just implies, if a little indirectly,

that the following lines of code in between those curly braces

should just execute again, and again, and again, and never actually stop.

>> But if you do want your loop to stop, as we

did last time with something like this, repeat the following 50 times,

in C we can do the same with what's called a for loop-- the keyword

And then we have some new syntax here, with int i equals 0, i less than 50,

But this is simply how we would translate the set of Scratch blocks

And, in fact, we just saw one a moment ago.

And in the case of Scratch, if we wanted to declare a variable called i

for i being integer, just a number, and we want to set it to some value,

we would use this orange block here-- set i to 0.

>> And we'll see today and beyond, just like last week,

programmers do almost always start counting from zero, really

But also because recall from our discussion of binary,

the smallest number you can represent with any number of bits

And so we'll generally start initializing even our variables to 0.

>> And in C to do the same, we're going to say int

I could have called this variable anything I want, just like in Scratch.