循环选择

本文介绍了循环选择的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 问题描述

http：// fms。 komkon/EMUL8/HOWTO.html 我读过： "分配初始值后，我们开始主循环： for（;;） { 注意这个循环也可以实现作为 while（CPUIsRunning） { 其中CPUIsRunning是一个布尔变量。这有一定的优势， 因为你可以通过设置CPUIsRunning = 0来随时终止循环。 不幸的是，每次传递检查这个变量都需要很多 CPU时间，如果可能应该避免。另外，不要实现这个 循环作为 而（1） { 因为在这种情况下，一些编译器会生成代码检查 是否为1。你当然不希望编译器在循环的每次传递中做这个不必要的工作。 我想阅读你的意见在这些陈述上:)

In fms.komkon/EMUL8/HOWTO.html I read: "After initial values are assigned, we start the main loop: for(;;) { Note that this loop can also be implemented as while(CPUIsRunning) { where CPUIsRunning is a boolean variable. This has certain advantages, as you can terminate the loop at any moment by setting CPUIsRunning=0. Unfortunately, checking this variable on every pass takes quite a lot of CPU time, and should be avoided if possible. Also, do not implement this loop as while(1) { because in this case, some compilers will generate code checking whether 1 is true or not. You certainly don''t want the compiler to do this unnecessary work on every pass of a loop." I would like to read your opinions on these statements :)

推荐答案

Lorenzo Villari说： Lorenzo Villari said: 在 fms.komkon /EMUL8/HOWTO.html 我读过： "分配初始值后，我们启动主循环： for（;;） { 请注意，此循环也可以实现为 while（CPUIsRunning） { 其中CPUIsRunning是一个布尔变量。这有一定的优势， 因为您可以通过设置CPUIsRunning = 0随时终止循环。 In fms.komkon/EMUL8/HOWTO.html I read: "After initial values are assigned, we start the main loop: for(;;) { Note that this loop can also be implemented as while(CPUIsRunning) { where CPUIsRunning is a boolean variable. This has certain advantages, as you can terminate the loop at any moment by setting CPUIsRunning=0.

是的。但它仍然是一个糟糕的名字。 LoopIsRunning会更好。

Yes. But it''s still a lousy name. LoopIsRunning would be better.

不幸的是，在每次传递中检查这个变量需要花费很多时间，并且应该避免可能。 Unfortunately, checking this variable on every pass takes quite a lot of CPU time, and should be avoided if possible.

废话。如果循环中发生了很多事情，那么检查一个int是 epsilon努力。当你决定如何踢足球时，你是否考虑了你的队友施加的引力对路径扭曲的影响？b $ b？如果在循环中没有发生很多事情，那么 你可以节省一两个时间来测试一个int，对吗？

Nonsense. If there''s a lot happening in the loop, checking one int is epsilon effort. When you''re deciding how to kick a football, do you take into account the path-distorting effect of the gravitational force exerted by your team-mates? And if there''s *not* a lot happening in the loop, then you can spare a clock or two to test an int, right?

另外，不要实现这个 循环为 而（1） { 因为在这种情况下，一些编译器会生成代码检查 是否为1。 Also, do not implement this loop as while(1) { because in this case, some compilers will generate code checking whether 1 is true or not.

不，这不是原因。不使用它的原因是它可以产生一个完全毫无意义且令人讨厌的警告，而（;;）的等效构造避免了。如果你真的想要一个无限循环，那么（;;）就好了你想要的东西 - 如果你想让循环完成一段时间，那么你应该这样说b $ b通过插入一个条件表达式在你的循环控件中。

No, that''s not the reason. The reason not to use it is that it can generate a completely pointless and annoying warning, which the equivalent construct for(;;) avoids. If you really want an endless loop, for(;;) is exactly what you want - and if you want the loop to finish some time, you should say so in your loop control by inserting a conditional expression.

你当然不希望编译器做什么 每一个不必要的工作传递一个循环。 我想阅读你对这些陈述的看法:) You certainly don''t want the compiler to do this unnecessary work on every pass of a loop." I would like to read your opinions on these statements :)

我的意见是这些陈述毫无价值。 - Richard Heathfield< www.cpax.uk> 电子邮件：-http：// www。 + rjh @ 谷歌用户：< www.cpax.uk/prg/writings/googly.php> Usenet是一个奇怪的放置" - dmr 1999年7月29日

My opinion is that those statements are of no value. -- Richard Heathfield <www.cpax.uk> Email: -www. +rjh@ Google users: <www.cpax.uk/prg/writings/googly.php> "Usenet is a strange place" - dmr 29 July 1999

Lorenzo Villari写道： Lorenzo Villari wrote: fms.komkon/EMUL8/HOWTO.html 我读过： "分配初始值后，我们开始主循环： for（;;） { 请注意，这个循环也可以实现为 while（CPUIsRunning） { 其中CPUIsRunning是一个布尔变量。这有一定的优势， 因为你可以通过设置CPUIsRunning = 0来随时终止循环。 不幸的是，每次传递检查这个变量都需要很多 CPU时间，如果可能应该避免。另外，不要实现这个 循环作为 而（1） { 因为在这种情况下，一些编译器会生成代码检查 是否为1。你当然不希望编译器在循环的每次传递中做这个不必要的工作。 我想阅读你的意见在这些陈述:)： In fms.komkon/EMUL8/HOWTO.html I read: "After initial values are assigned, we start the main loop: for(;;) { Note that this loop can also be implemented as while(CPUIsRunning) { where CPUIsRunning is a boolean variable. This has certain advantages, as you can terminate the loop at any moment by setting CPUIsRunning=0. Unfortunately, checking this variable on every pass takes quite a lot of CPU time, and should be avoided if possible. Also, do not implement this loop as while(1) { because in this case, some compilers will generate code checking whether 1 is true or not. You certainly don''t want the compiler to do this unnecessary work on every pass of a loop." I would like to read your opinions on these statements :)

这是胡说八道。检查CPUIsRunning的开销应该可以忽略不计。检查1的开销应该与任何 不错的编译器不存在。 我不是永远循环的粉丝。我想知道的第一件事是关于循环的是如何终止的。 永久循环给出了 的印象，通常是错误的，它永远不会终止。出于这个原因， 我相信你应该总是至少移动一个条件 实际上导致循环终止进入循环条件本身; 最好是最常见的这种情况之一。它可能会使你的 代码变得非常复杂，但根据我的经验，通常情况并非如此。 我不喜欢为此目的使用布尔变量，除非每次设置 变量'的值至少读取2-3次。

It''s nonsense. The overhead of checking CPUIsRunning should be quite negligible. The overhead of checking 1 should be non-existent with any decent compiler. I''m not a fan of "forever loops". One of the first things I want to know about a loop is how it terminates. A "forever loop" gives the impression, usually falsely, that it never terminates. For that reason, I believe that you should always move at least one of the conditions that actually causes loop termination into the loop condition itself; preferably one of the most common such conditions. It might make your code prohibitively complicated to do so, but in my experience that''s usually not the case. I don''t like using boolean variables for this purpose unless the variable''s value is read at least 2-3 time for each time that it is set.

James Kuyper说： < snip> James Kuyper said: <snip> 我是不是永远循环的粉丝。我想知道的第一件事是关于循环的是如何终止的。 永久循环给出了 的印象，通常是错误的，它永远不会终止。 I''m not a fan of "forever loops". One of the first things I want to know about a loop is how it terminates. A "forever loop" gives the impression, usually falsely, that it never terminates.

如果找到有人谈论改变循环的感觉，那是多么令人耳目一新！ - Richard Heathfield< www.cpax.uk> 电子邮件：-http：// www。 + rjh @ 谷歌用户：< www.cpax.uk/prg/writings/googly.php> Usenet是一个奇怪的放置" - dmr 1999年7月29日

How refreshing to find someone talking sense about loops for a change! -- Richard Heathfield <www.cpax.uk> Email: -www. +rjh@ Google users: <www.cpax.uk/prg/writings/googly.php> "Usenet is a strange place" - dmr 29 July 1999