12 Programming Basics: Statements & Functions

12 编程原理：语句和函数

Hi, I'm Carrie Anne, and welcome to CrashCourse Computer Science!

嗨，我是 Carrie Anne，欢迎收看计算机科学速成课！

Last episode we discussed how writing programs in native machine code,

上集讲到用机器码写程序，

and having to contend with so many low level details, was a huge impediment to writing complex programs.

还要处理那么多底层细节 对写大型程序是个巨大障碍

To abstract away many of these low-level details, Programming Languages were developed that

为了脱离底层细节，开发了编程语言

let programmers concentrate on solving a problem with computation, and less on nitty gritty hardware details.

让程序员专心解决问题，不用管硬件细节

So today, we're going to continue that discussion, and introduce some fundamental building blocks

今天我们讨论

that almost all programming languages provide.

大多数编程语言都有的基本元素

Just like spoken languages, programming languages have statements.

就像口语一样，编程语言有"语句"

These are individual complete thoughts, like "I want tea" or "it is raining".

语句表达单个完整思想，比如"我想要茶"或者"在下雨"

By using different words, we can change the meaning;

用不同词汇可以代表不同含义，

for example, "I want tea" to "I want unicorns".

比如"我想要茶"变成"我想要独角兽"

But we can't change "I want tea" to "I want raining" that doesn't make grammatical sense.

但没法把"我想要茶"改成"我想要雨"语法毫无意义

The set of rules that govern the structure and composition of statements in a language

规定句子结构的一系列规则

is called syntax.

叫语法

The English language has syntax, and so do all programming languages.

英语有语法，所有编程语言也都有语法

a = 5 is a programming language statement.

a=5 是一个编程语言语句

In this case, the statement says a variable named A has the number 5 stored in it.

意思是创建一个叫 a 的变量，把数字 5 放里面.

This is called an assignment statement because we're assigning a value to a variable.

这叫"赋值语句"，把一个值赋给一个变量

To express more complex things, we need a series of statements,

为了表达更复杂的含义，需要更多语句

like "A is 5, B is 10, C equals A plus B"

比如 ，a=5，b=10 ，c=a+b

This program tells the computer to set variable ‘A' equal to 5, variable ‘B' to 10,

意思是，变量 a 设为5，变量 b 设为10

and finally to add ‘A' and ‘B' together, and put that result, which is 15, into -you guessed it -variable C.

把 a 和 b 加起来，把结果 15 放进变量 c

Note that we can call variables whatever we want.

注意，变量名可以随意取

Instead of A, B and C, it could be apples, pears, and fruits.

除了 a b c，也可以叫苹果、梨、水果

The computer doesn't care, as long as variables are uniquely named.

计算机不在乎你取什么名，只要不重名就行

But it's probably best practice to name them things that make sense

当然取名最好还是有点意义，

in case someone else is trying to understand your code.

方便别人读懂

A program, which is a list of instructions, is a bit like a recipe:

程序由一个个指令组成，有点像菜谱：

boil water, add noodles, wait 10 minutes, drain and enjoy.

烧水、加面，等10分钟，捞出来就可以吃了

In the same way, the program starts at the first statement

程序也是这样，从第一条语句开始

and runs down one at a time until it hits the end.

一句一句运行到结尾

So far, we've added two numbers together.

刚才我们只是把两个数字加在一起

Boring.

无聊

Let's make a video game instead!

我们来做一款游戏吧

Of course, it's way too early to think about coding an entire game,

当然，现在这个学习阶段 ，来编写一整个游戏还太早了

so instead, we'll use our example to write little snippets of code

所以我们只写一小段一小段的代码

that cover some programming fundamentals.

来讲解一些基础知识

Imagine we're building an old-school arcade game where Grace Hopper has to capture bugs

假设我们在写一款老派街机游戏：Grace Hopper 拍虫子

before they get into the Harvard Mark 1 and crash the computer!

阻止虫子飞进计算机造成故障

On every level, the number of bugs increases.

关卡越高，虫子越多

Grace has to catch them before they wear out any relays in the machine.

Grace 要在虫子损坏继电器之前 抓住虫子

Fortunately, she has a few extra relays for repairs.

好消息是 她有几个备用继电器

To get started, we'll need to keep track of a bunch of values that are important for gameplay

开始编写时，我们需要一些值 来保存游戏数据

like what level the player is on, the score, the number of bugs remaining,

比如当前关卡数、分数、剩余虫子数、

as well as the number of spare relays in Grace's inventory.

Grace 还剩几个备用继电器

So, we must "initialize" our variables, that is, set their initial value:

所以我们要"初始化"变量，"初始化"的意思是设置最开始的值.

level equals 1, score equals 0, bugs equals 5, spare relays equals 4, and player name equals Andre".

关卡=1 分数=0 虫子数=5，备用继电器=4 玩家名=Andre

To create an interactive game, we need to control the flow of the program

为了做成交互式游戏，程序的执行顺序要更灵活

beyond just running from top to bottom.

不只是从上到下执行

To do this, we use Control Flow Statements.

因此用 "控制流语句"

There are several types, but If Statements are the most common.

控制流语句有好几种，最常见的是 if 语句

You can think of them as "If X is true, then do Y".

可以想成是 "如果 X 为真，那么执行 Y"

An English language example is: "If I am tired, then get tea"

用英语举例就是 "如果累了， 就去喝茶"

So if "I am tired" is a true statement, then I will go get tea

如果 "累了" 为真，就去喝茶

If "I am tired" is false, then I will not go get tea.

如果 "累了" 为假，就不喝茶

An IF statement is like a fork in the road.

if 语句就像岔路口

Which path you take is conditional on whether the expression is true or false

走哪条路 取决于 "表达式" 的真假，

so these expressions are called Conditional Statements.

因此这些表达式又叫 "条件语句"

In most programming languages, an if statement looks something like

在大多数编程语言中，if 语句看起来像这样：

If, expression, then, some code, then end the if statement.

if [条件]， then [一些代码]，结束 if 语句.

For example, if "level" is 1, then we set the score to zero, because the player is just starting.

比如，if [第一关]，then [分数设为0]，因为玩家才刚开始游戏

We also set the number of bugs to 1, to keep it easy for now.

同时把虫子数设为 1，让游戏简单些

Notice the lines of code that are conditional on the if-statement are nested between the

注意， 依赖于 if 条件的代码，

IF and END IF.

要放在 IF 和 END IF 之间

Of course, we can change the conditional expression to whatever we want to test, like

当然，条件表达式 可以改成别的，比如：

is score greater than 10 or "is bugs less than 1".

分数 >10 或者 "虫子数 <1"

And If-Statements can be combined with an ELSE statement, which acts as a catch-all if the expression is false.

if 还可以和 else 结合使用，条件为假会执行 else 里的代码

If the level is not 1, the code inside the ELSE block will be executed instead, and the

如果不是第1关，else 里的指令就会被执行

number of bugs that Grace has to battle is set to 3 times the level number.

Grace 要抓的虫子数，是当前关卡数 * 3

So on level 2, it would be six bugs, and on level 3 there's 9, and so on.

所以第 2 关有 6 个虫子，第 3 关有 9 个虫子，以此类推

Score isn't modified in the ELSE block, so Grace gets to keep any points earned.

else 中没有改分数，所以 Grace 的分数不会变

Here are some examples of if-then-else statements from some popular programming languages

这里列了一些热门编程语言 if-then-else 的具体语法

you can see the syntax varies a little, but the underlying structure is roughly the same.

具体语法略有不同，但主体结构一样

If-statements are executed once, a conditional path is chosen, and the program moves on.

if 语句 根据条件执行一次

To repeat some statements many times, we need to create a conditional loop.

如果希望根据条件执行多次，需要"条件循环"

One way is a while statement, also called a while loop.

比如 while 语句，也叫 "while 循环"

As you might have guessed, this loops a piece of code "while" a condition is true.

当 while 条件为真，代码会重复执行

Regardless of the programming language, they look something like this:

不管是哪种编程语言，结构都是这样

In our game, let's say at certain points, a friendly colleague restocks Grace with relays!

假设到达一定分数会冒出一个同事，给 Grace 补充继电器

Hooray!

棒极了！

To animate him replenishing our stock back up to a maximum of 4, we can use a while loop.

把继电器补满到最大数 4 个，我们可以用 while 语句来做

Let's walk through this code.

来过一遍代码

First we'll assume that Grace only has 1 tube left when her colleague enters.

假设同事入场时，Grace 只剩一个继电器

When we enter the while loop, the first thing the computer does is test its conditional…

当执行 while 循环，第一件事是检查条件

is relays less than 4?

继电器数量<4？

Well, relays is currently 1, so yes.

继电器数量现在是1，所以是真

Now we enter the loop!

进入循环！

Then, we hit the line of code: "relays equals relays plus 1".

碰到这一行：继电器数量=继电器数量+1

This is a bit confusing because the variable is using itself in an assignment statement,

看起来有点怪，变量的赋值用到了自己

so let's unpack it.

我们讲下这个

You always start by figuring out the right side of the equals sign first,

总是从等号右边开始，

so what does "relays plus 1" come out to be?

继电器数量+1 是多少？

Well, relays is currently the value 1, so 1 plus 1 equals 2.

当前值是1，所以 1+1=2

Then, this result gets saved back into the variable relays, writing over the old value,

结果存到"继电器数量"，覆盖旧的值

so now relays stores the value 2.

所以现在继电器数量是 2

We've hit the end of the while loop, which jumps the program back up.

现在到了结尾，跳回开始点

Just as before, we test the conditional to see if we're going to enter the loop.

和之前一样，先判断条件，看要不要进入循环

Is relays less than 4?

继电器数量<4？

Well, yes, relays now equals 2, so we enter the loop again!

是，继电器数量是2，所以再次进入循环！

2 plus 1 equals 3.

2+1=3

so 3 is saved into relays.

3 存入"继电器数量"

Loop again.

回到开头

Is 3 less than 4?

3<4？

Yes it is!

是！

Into the loop again.

进入循环

3 plus 1 equals 4.

3+1=4

So we save 4 into relays.

4 存入"继电器数量"

Loop again.

回到开头

Is 4 less than 4?....

4<4？

No!

不！

So the condition is now false, and thus we exit the loop and move on to any remaining code

现在条件为假，退出循环，执行后面的代码

That's how a while loop works!

while 循环就是这样运作的！

There's also the common For Loop.

另一种常见的叫 "for 循环"

Instead of being a condition-controlled loop that can repeat forever until the condition is false

不判断条件，

a FOR loop is count-controlled; it repeats a specific number of times.

判断次数，会循环特定次数

They look something like this:

看起来像上图

Now, let's put in some real values.

现在放些真正的值进去

This example loops 10 times, because we've specified that variable ‘i'

上图例子会循环10次，因为设了变量 i

starts at the value 1 and goes up to 10.

从 1 开始，一直到 10

The unique thing about a FOR loop is that each time it hits NEXT, it adds one to ‘i'.

for 的特点是，每次结束，i 会 +1

When ‘i' equals 10, the computer knows it's been looped 10 times, and the loop exits

当 i 等于10，就知道循环了10次，然后退出.

We can set the number to whatever we want -10, 42, or a billion -it's up to us.

我们可以用任何数字，10， 42， 10 亿

Let's say we want to give the player a bonus at the end of each level

假设每关结束后 给玩家一些奖励分

for the number of vacuum relays they have left over.

奖励分多少取决于 继电器剩余数量

As the game gets harder, it takes more skill to have unused relays,

随着难度增加，剩下继电器会越来越难

so we want the bonus to go up exponentially based on the level.

因此奖励分会根据当前关卡数，指数级增长

We need to write a piece of code that calculates exponents -

我们要写一小段代码来算指数

that is, multiplying a number by itself a specific number of times.

指数是一个数乘自己，乘特定次数

A loop is perfect for this!

用循环来实现简直完美！

First lets initialize a new variable called "bonus" and set it to 1.

首先，创建一个叫"奖励分"的新变量，设为 1

Then, we create a FOR loop starting at 1, and looping up to the level number.

然后 for 循环，从 1 到 [当前关卡数]

Inside that loop, we multiply bonus times the number of relays,

[奖励分] x [继电器剩余数]，结果存入 [奖励分]

and save that new value back into bonus.

[奖励分] x [继电器剩余数]，结果存入 [奖励分]

For example, let's say relays equals 2, and level equals 3.

比如继电器数是2，关卡数是3

So the FOR loop will loop three times, which means bonus is going to get multiplied by

for 会循环3次，奖励分会乘

relays... by relays... by relays.

继电器数量 x 继电器数量 x 继电器数量

Or in this case, times 2, times 2, times 2, which is a bonus of 8!

也就是1×2×2×2，奖励分是8，2的3次方

That's 2 to the 3rd power!

也就是1×2×2×2，奖励分是8，2的3次方

This exponent code is useful, and we might want to use it in other parts of our code.

这个指数代码很实用，其他地方可能会用到

It'd be annoying to copy and paste this everywhere, and have to update the variable names each time.

如果每次想用就复制粘贴，会很麻烦，每次都要改变量名

Also, if we found a bug, we'd have to hunt around and update every place we used it.

如果代码发现问题，要补漏洞时，要把每一个复制黏贴过的地方都找出来改

It also makes code more confusing to look at.

而且会让代码更难懂

Less is more!

少即是多！

What we want is a way to package up our exponent code so we can use it, get the result, and

我们想要某种方法，把代码"打包"，可以直接使用，得出结果，

not have to see all the internal complexity.

不用管内部复杂度.

We're once again moving up a new level of abstraction!

这又提升了一层抽象！

To compartmentalize and hide complexity,

为了隐藏复杂度

programming languages can package pieces of code into named functions,

可以把代码打包成 "函数"

also called methods or subroutines in different programming languages.

也叫 "方法" 或 "子程序"，（有些编程语言这么叫）

These functions can then be used by any other part of that program just by calling its name.

其他地方想用这个函数，直接写函数名就可以了

Let's turn our exponent code into a function! First, we should name it.

现在我们把指数代码变成函数. 第一步，取名.

We can call it anything we want, like HappyUnicorn,

叫什么都行，比如"快乐独角兽"

but since our code calculates exponents, let's call it exponent.

但因为是算指数， 直接叫"指数"合适一些

Also, instead of using specific variable names, like "relays" and "levels",

还有，与其用特定变量名，比如 "继电器" 和 "关卡数"

we specify generic variable names, like Base and Exp,

用更通用的名字，比如 底数(Base) 和 指数(Exp)

whose initial values are going to be "passed" into our function from some other part of the program.

Base 和 Exp 的初始值需要外部传入

The rest of our code is the same as before

剩余代码和之前一样

Now tucked into our function and with new variable names.

现在完成了，有函数名和新变量名.

Finally, we need to send the result of our exponent code back to the part of the program that requested it.

最后， 我们还需要把结果 交给使用这个函数的代码

For this, we use a RETURN statement, and specify that the value in ‘result' be returned.

所以用 RETURN 语句，指明返回什么.

So our full function code looks like this:

完整版代码是这样

Now we can use this function anywhere in our program,

现在可以随意用这个函数

simply by calling its name and passing in two numbers.

只需要写出名字 然后传入2个数字 就可以了

For example, if we want to calculate 2 to the 44th power, we can just call "exponent 2 comma 44."

如果要算 2 的 44 次方，写 exponent(2，44)

and like 18 trillion comes back.

结果是 18 万亿左右

Behind the scenes, 2 and 44 get saved into variables Base and Exp inside the function,

幕后原理是，2 和 44 存进 Base 和 Exp

it does all its loops as necessary, and then the function returns with the result.

跑循环，然后返回结果

Let's use our newly minted function to calculate a score bonus.

我们来用这个新函数 算奖励分

First, we initialize bonus to 0.

首先，奖励分初始化为 0

Then we check if the player has any remaining relays with an if-statement.

然后用 if 语句，看剩不剩继电器（看上图的 > 0）

If they do, we call our exponent function, passing in relays and level,

如果还剩，用指数函数，传入 [继电器数] 和 [关卡数]

which calculates relays to the power of level, and returns the result, which we save into bonus.

它会算 [继电器数]的[关卡数]次方， 存入奖励分

This bonus calculating code might be useful later, so let's wrap it up as a function too!

这段算奖励分的代码，之后可能还会用，也打包成一个函数

Yes, a function that calls a function!

没错，这个函数 (CalcBonus)，会调用另一个函数 (Exponent)

And then, wait for it…. we can use this function in an even more complex function.

还有！这个 CalcBonus 函数，可以用在其他更复杂的函数

Let's write one that gets called everytime the player finishes a level.

我们来写一个函数， 每一关结束后都会调用

We'll call it "LevelFinished"

叫 LevelFinished (关卡结束)

it needs to know the number of relays left, what level it was, and the current score;

需要传入 [剩余继电器数] [关卡数] [当前分]

those values have to get passed in.

这些数据必须传入.

Inside our function, we'll calculate the bonus, using our CalcBonus function,

里面用 CalcBonus 算奖励分，

and add that to the running score.

并加进总分

Also, if the current score is higher than the game's high score,

还有，如果当前分 > 游戏最高分

we save the new high score and the players name.

把新高分和玩家名 存起来

Now we're getting pretty fancy.

现在代码变得蛮"花哨"了

Functions are calling functions are calling functions!

函数调函数调函数

When we call a single line of code, like this the complexity is hidden.

我们写这样一行代码时，复杂度都隐藏起来了

We don't see all the internal loops and variables,

不需要知道内部的循环和变量

we just see the result come back as if by magic…. a total score of 53.

只知道结果会像魔术一样返回，总分 53

But it's not magic, it's the power of abstraction!

但是这不是魔术，是抽象的力量

If you understand this example, then you understand the power of functions,

如果你理解了这个例子，就明白了函数的强大之处

and the entire essence of modern programming.

和现代编程的核心

It's not feasible to write, for example, a web browser as one gigantically long list of statements.

比如浏览器这样的复杂程序，用一长串语句来写是不可能的

It would be millions of lines long and impossible to comprehend!

会有几百万行代码，没人能理解

So instead, software consists of thousands of smaller functions,

所以现代软件由上千个函数组成

each responsible for different features.

每个负责不同的事

In modern programming, it's uncommon to see functions longer than around 100 lines of code

如今超过100行代码的函数很少见

because by then, there's probably something that

如果多于 100 行，

should be pulled out and made into its own function.

应该有东西可以拆出来做成一个函数

Modularizing programs into functions not only allows a single programmer to write an entire app

模块化编程 不仅可以让单个程序员独立制作 App

but also allows teams of people to work efficiently on even bigger programs.

也让团队协作可以写更大型的程序

Different programmers can work on different functions,

不同程序员写不同函数

and if everyone makes sure their code works correctly,

只需要确保自己的代码工作正常

then when everything is put together, the whole program should work too!

把所有人的拼起来，整个程序也应该能正常运作！

And in the real world, programmers aren't wasting time writing things like exponents.

现实中，程序员不会浪费时间写指数函数这种东西

Modern programming languages come with huge bundles of pre-written functions, called Libraries.

现代编程语言 有很多预先写好的函数集合，叫 "库"

These are written by expert coders, made efficient and rigorously tested, and then given to everyone.

由专业人员编写，不仅效率高，而且经过了仔细检查

There are libraries for almost everything, including networking, graphics, and sound

几乎做所有事情都有库，网络、图像、声音

topics we'll discuss in future episodes.

我们之后会讲这些主题.

But before we get to those, we need to talk about Algorithms.

但在此之前，我们先讲算法

Intrigued?

好奇吗？

You should be.

你应该才是！

I'll see you next week.

下周见