Selection

Python for AP CS Principles Lesson 6 2:09 English narration · English + 中文 subtitles burned in

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Selection means the program picks what to do based on a condition. 选择的意思是，程序根据一个条件来决定做什么。

And an if-elif-else is a chain, not three separate questions — watch what happens with the hour set to fourteen. 而 if-elif-else 是一条"链"，不是三个各自独立的问题—— 看看把 hour 设成 14 时会发生什么。

First: is fourteen less than twelve? 先问：14 小于 12 吗？

No, so move on. 不是，往下走。

Then the elif: is it less than eighteen? 再问 elif：小于 18 吗？

That one is true, so Afternoon prints. 这一条是真的，于是打印 Afternoon。

And the else was never even asked, because the chain stopped the moment something matched. 而那个 else 根本没被问到，因为一旦有一条匹配上，这条链就停住了。

A condition is not special syntax — it is just a value. 条件不是什么特殊语法——它就是一个值。

Print a comparison and you see it: score greater-or-equal fifty gives True or False, a real value you could store. 把一个比较打印出来，你就看见了： score >= 50 给出 True 或 False，是一个你甚至可以存起来的真实的值。

And here is the slip that costs marks every year: a double equals TESTS whether two things are the same, while a single equals stores a value into a variable. 而下面这个失误每年都在丢分：双等号是在"测试"两样东西是不是一样，而单个等号是把一个值"存进"变量里。

Boolean operators let one condition ask two things at once. 布尔运算符让一个条件一次问两件事。

and is True only when both sides are True — and on a number line that is a band, the overlap of the two tests. and 只有当两边都为真时才为真—— 而在数轴上，那就是一段区间，是两个测试重叠的部分。

Sixteen falls inside it, so this counts as a teenager. 16 落在里面，所以它算是青少年。

or is easier on you: it needs at least one side to be true. or 就宽松些：它只要求至少有一边为真。

And not simply flips the answer over. 而 not 只是把答案翻过来。

On the exam the same thing is written with braces instead of indentation. 在考卷上，同样的东西是用花括号写的，而不是靠缩进。

IF with the condition in brackets, then a pair of braces around what runs, then ELSE with its own braces. IF 后面把条件放在圆括号里，再用一对花括号把要执行的内容括起来，然后是 ELSE，也有自己的花括号。

And the logic words go in capitals: AND, OR, NOT. 而逻辑词要大写：AND、OR、NOT。

Four things to take with you. 带走四点。

One: selection chooses a branch from a True or False test. 第一：选择根据一个真假测试来挑一条分支。

Two: an elif is only reached when everything above failed. 第二：只有上面全都不成立时，才会问到 elif。

Three: compare with a double equals, not a single one. 第三：比较要用双等号，不是单个等号。

Four: and needs both sides; or needs at least one. 第四：and 要两边都真；or 至少一边真就行。

Now write some conditions in the tasks below. 现在去下面的题里写几个条件。