Break Solution - Intro to Computer ScienceYouTube↗  IA↗(long shot)
Education  2.1K views 18 likes 8 dislikes   02 Mar 2012
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Here's the answer.
答案是
The first one is equivalent.
選項一是相等的
The reason for that is we have our code here that has "if False."
因為這裡的程式碼是 'if False'
"False" will never be true, so we'll never actually run the "break"—
False 永遠不會是 True，所以永遠不會執行 break
we'll never reach that code.
我們永遠不會執行到那些程式碼
We'll just run the statement again and we'll have the same thing that we started with.
我們只會再次執行陳述，和開始的時候一樣
Every time the test is true, we execute the statement.
每次測試是 True，我們就執行陳述
The second one is not equivalent, and the reason for that is the "break" will
選項二是不相等的，原因是
stop the loop.
這個 break 會中斷 loop
If the loop would execute more than once in this version,
在這個版本，如果 loop 在這種情況下執行超過一次
and if we go through the loop twice,
如果我們想執行 loop 兩次
in this case we can only ever go through the statement once,
在這裡，我們只能執行陳述一次
since after we go through the statement once, we reach the "break" and then we're done.
因為執行陳述一次後，我們就到了 break ，然後結束
The third one is not equivalent, so here we're having a "while true" that will run
選項三是不相等的，這裡的 'while true' 會永遠執行下去
forever except for we have the "break" and that would stop it.
除非我們有 break 來將它停止
The problem is the test condition for the break is the same as the
問題在於 break 的測試條件 和 while 的測試條件是一樣的
test condition for the "while," but it should really be the opposite.
但它應該是相反的才對
In the "while" we keep going as long as the test condition is true.
while 的測試條件只要是 True，就會繼續下去
The way I have written in the code here, if the test condition is true, we stop.
但根據這裡的程式碼，如果測試條件是 True ，就會停止
For this to be equivalent, what we should have written instead
想要讓選項三相等，我們應該
would've taken the opposite of the test condition, so if we had "if" not"—
改寫為相反的測試條件
which changes true to false and false to true, then the test condition,
如果是 'if not'，True 變 False，False 變 True， 然後是測試條件
then the break—this would be equivalent to the original loop.
然後是 break，這就和原來的 loop 相等了
What we have is not.
但是選項三並不是這樣的
The final one is the most complicated.
最後一個選項是最複雜的
This one is actually always equivalent to the original loop.
事實上，它總是相等於原來的 loop
The reason for that is if we think through the execution, so in the
我們來思考一下執行的過程
original loop, if the test condition is true, we ask to give the statement "S,"
在原來的 loop 中， 如果測試條件為 True，我們執行陳述 S
and then we keep going.
然後繼續下去
If the test condition is true the second time, we execute "S" again
如果第二次的測試條件是 True，我們就再次執行 S
and we keep going.
然後繼續下去
If we look at what happens here—well, if the test condition is true, we
再來看選項四，如果測試條件是 True
execute "S."
我們執行 S
Then if the test condition is true again, we execute S and we keep going.
如果測試條件再次是 True，我們執行 S，然後繼續下去
That has the same behavior.
它有同樣的作用
We should trace it through the case where the test condition is
我們必須追蹤這個情況
not true.
測試條件不是 True
Here—if the test condition was true, we execute "S."
如果測試條件為 True，我們執行 S
If it's not true the second time, then we're done with the loop
如果第二次不是 True，loop 就結束
and we continue.
然後繼續
In this case if the test condition is true the first time, we'll execute "S."
在這裡，如果第一次測試條件是 True，我們執行 S
If it's not true the second time, then we would go to the "else."
如果第二次不是 True，我們就執行 else
So if it's not true, we go to the "else" and then we go to the "break" and
所以如果不是 True，會執行 else，然後來到 break
the "break" gets out of the loop.
break 跳出了 loop
We have the same exact behavior where we executed "T" once, then
這和原來的 loop 的作用是完全一致的
we executed "S," and then we executed "T."
先執行 T 一次，然後執行 S ，然後執行 T
We didn't execute "S" again, and we broke out of the loop to continue.
我們不再執行 S ，而是中斷，跳出 loop，然後繼續
So these two are equivalent and the other two are different.
所以這兩個是相等的，那兩個不相等
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