Iteration
Python for IGCSE CS Lesson 4 2:18 English narration · English + 中文 subtitles burned in
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Iteration means repeating steps, and the everyday loop is the one that repeats a fixed number of times.
迭代就是重复执行步骤，而最常见的循环，是重复固定次数的那一种。
In Python that is for i in range.
在 Python 里就是 for i in range。
Range on its own counts from nought.
只给一个数字，range 从 0 开始数。
Give it two numbers and it will start at one, or wherever you say.
给两个数字，它就可以从 1 开始，或者从你说的任何地方开始。
Give it three and it counts in twos.
给三个数字，它就会隔一个数一次。
But here is the catch, and it differs from the exam: range stops BEFORE the number you give it, so you write six to get five.
但这里有个要注意的地方，而且和考卷不同： range 在你给的那个数字之前就停了， 所以你要写 6 才能数到 5。
The syllabus names three kinds of loop and asks you to tell them apart.
考纲列出三种循环，并且会让你区分它们。
A FOR loop repeats a number of times you already know.
FOR 循环重复的次数是你事先就知道的。
A WHILE loop tests before each pass.
WHILE 循环在每一轮之前判断。
A REPEAT loop tests after each pass.
REPEAT 循环在每一轮之后判断。
And that placement decides the one fact the exam actually wants: zero times versus at least once.
而判断放在哪里，决定了考试真正想考的那个事实： 可能一次都不跑，还是至少跑一次。
If the test is false at the very start, a while loop never runs its body, and a repeat loop has already run it once.
如果判断一开始就是假的， while 循环的循环体一次都不会执行， 而 repeat 循环已经执行过一次了。
A while loop has a hazard that a for loop does not.
while 循环有一个 for 循环没有的隐患。
Something inside the body has to move the test toward false — here, the counter going up.
循环体里必须有东西让那个判断朝"假"的方向走—— 这里就是那个不断增加的计数器。
Forget that line and the test is true forever, and the program hangs.
漏掉那一行，判断就永远为真，程序就卡住了。
A for loop counts for you; a while loop does not, so you have to move it toward its end yourself.
for 循环替你计数；while 循环不会， 所以你得自己让它走向终点。
Now the lesson's second task, in both notations: add up one to ten.
现在看这一课的第二道题，两种写法并排：把 1 到 10 加起来。
Look at where the total is set to nought.
注意 total 被设为 0 的那一行在哪里。
It is outside the loop, before it starts.
它在循环外面，在循环开始之前。
Put that line inside and the total resets on every single pass, so the answer comes out as ten — the last number, not the sum.
把那一行放进循环里，total 每一轮都会被重置， 于是答案变成 10——最后那个数字，而不是总和。
That is next lesson's whole topic, and it starts here.
这正是下一课的全部主题，而它从这里开始。
Four things to take with you.
带走四点。
One: a FOR loop repeats a known number of times.
第一：FOR 循环重复的次数是已知的。
Two: a WHILE tests first, so it may run zero times.
第二：WHILE 先判断，所以可能一次都不跑。
Three: a REPEAT tests last, so it always runs once.
第三：REPEAT 后判断，所以至少跑一次。
Four: range stops before its end — write six to reach five.
第四：range 在终点之前停下——要数到 5，就写 6。
Now do the three tasks.
现在去做那三道题。