November 2017
Beginner to intermediate
344 pages
7h 2m
Chinese
Content preview from 深入理解運算原理｜從簡單的機器到無所不能的程式
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不可能的程式
|
295
至此，為什麼程式的行為難以預測，原因有兩個：
1. 任何具有足夠能力而能自我參照的系統，都無法正確回答和它自己相關的每個問
題
20
。我們始終能夠構建諸如
do_the_opposite.rb
，這種程式的行為無法由系統預測。
為了避免這種問題，我們需要跨出自我參照系統，並使用不同且能力更強大的系統
來回答和它自己相關的問題。
2. 但若是通用程式語言，就沒有能力更強大的系統可以讓我們升級。邱奇 - 圖靈論題
告訴我們，我們為了預測程式行為所發明的任何可用的演算法，皆可由程式本身執
行，因此通用系統的功能就推給我們了。
對付不可運算性
編寫程式的所有重點就是讓電腦做些有用的事情。身為程式開發人員，我們應該如何想
像：對付會檢查程式正常工作的世界是個無法解決的問題？
拒絕或否認是個誘人的回應：
略過整個問題。如果我們可以自動驗證程式行為，那
勢必很好，但是我們做不到，所以就讓我們只希望最好而且永遠不要試著檢查程式
是否正確運作。
但這是反應過度，因為它的情況並沒有聽起來那麼糟。萊斯定理並不代表不可能程式
分析，只是我們無法寫出永遠都會停止並產生正確答案的非顯然分析器（nontrivial
analyzer）。就如第 283 頁的『建置停機核對器』所述，什麼都無法阻止我們編寫可以
為
某些
程式提供正確答案的工具，只要我們能夠容忍這樣的事實：總有
其他
程式不會
傳回任何東西並給了錯誤的答案或永遠循環。
雖然是無法決定性，但這裡還是有一些分析和預測程式行為的實用方法：
• 詢問無法決定的問題，但若找不到答案，就放棄。舉例來說，要檢查程式是否能印 ...
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I wanted to learn C and C++, but it didn't click for me until I picked up an O'Reilly book. When I went on the O’Reilly platform, I was astonished to find all the books there, plus live events and sandboxes so you could play around with the technology.Addison B.
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I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.