国内“为”的固有效率低下环？ [英] Inherent inefficiency in domestic "for" loop?
问题描述
C的国内用法是用于
简单增量循环效率低下？这是一个非常简单的程序，它打印出一个字节中的
位数。
#include< stdio.h>
#include< limits.h>
#include< stdlib.h>
int main（无效）
{
未签名i;
for（i = 0; i！= CHAR_BIT; ++ i）
{
printf（" Bit Index％u \ n，i）;
}
system（"暂停）;
}
（随意用i< CHAR_BIT替换i！= CHAR_BIT。）
如果我尝试使用goto复制它，我会得到以下结果：
#include< stdio.h>
#include< limits.h>
#include< stdlib.h>
int main（无效）
{
unsigned i;
Loop_Enter：
i = 0;
Loop_Condition：
if（！（i！= CHAR_BIT））goto Loop_End;
Loop_正文：
printf（比特指数％u \ n，i）;
Loop_Continue：
++ i;
转到Loop_Condition;
Loop_End：
;
system（PAUSE）;
}
但是，我们可以看到第一个条件测试是多余的 -
最好在每次迭代后测试条件，而不是之前的b $ b。类似于：
#include< stdio.h>
#include< limits.h>
#include< ; stdlib.h>
int main（无效）
{
未签名i;
Loop_Enter：
i = 0;
Loop_Body：
printf（" Bit Index％ u $ \\ n，i）;
Loop_Continue：
++ i;
Loop_Condition：
if（i！= CHAR_BIT）goto Loop_Body;
Loop_End：
;
system（PAUSE）;
}
如果我们比较两个代码片段的执行情况，我们可以看到：
片段1：测试条件9次
片段2：测试条件8次
广泛使用的C方法简单的循环增量本身就是
效率低下？乍一看，对我来说似乎是这样。
（是的，我意识到大多数编译器会展开那个循环，所以让我们假设我们正在工作有一个更复杂的循环，其数量为
迭代，直到运行时才确定。）
-
Frederick Gotham
Is the domestic usage of the C "for" loop inefficient when it comes to
simple incrementation? Here''s a very simple program that prints out the
bit-numbers in a byte.
#include <stdio.h>
#include <limits.h>
#include <stdlib.h>
int main(void)
{
unsigned i;
for( i = 0; i != CHAR_BIT; ++i )
{
printf( "Bit Index %u\n", i );
}
system("PAUSE");
}
(Feel free to substitute "i != CHAR_BIT" with "i < CHAR_BIT".)
If I try to replicate that using "goto", I get the following:
#include <stdio.h>
#include <limits.h>
#include <stdlib.h>
int main(void)
{
unsigned i;
Loop_Enter:
i = 0;
Loop_Condition:
if ( !(i != CHAR_BIT) ) goto Loop_End;
Loop_Body:
printf( "Bit Index %u\n", i );
Loop_Continue:
++i;
goto Loop_Condition;
Loop_End:
;
system("PAUSE");
}
However, we can see that the very first conditional test is redundant --
it would be better to test the condition AFTER each iteration, rather
than before. Something like:
#include <stdio.h>
#include <limits.h>
#include <stdlib.h>
int main(void)
{
unsigned i;
Loop_Enter:
i = 0;
Loop_Body:
printf( "Bit Index %u\n", i );
Loop_Continue:
++i;
Loop_Condition:
if ( i != CHAR_BIT ) goto Loop_Body;
Loop_End:
;
system("PAUSE");
}
If we compare the execution of both code snippets, we can see:
Snippet 1: Tests the condition 9 times
Snippet 2: Tests the condition 8 times
Is the widely used C method of simple loop incrementation inherently
inefficient? At first glance, it appears so to me.
(Yes, I realise that most compilers will "unroll" that loop, so lets
pretend we''re working with a more complicated loop whose amount of
iterations isn''t determined until runtime.)
--
Frederick Gotham
推荐答案
Frederick Gotham< fg ******* @ SPAM.com>写道：
Frederick Gotham <fg*******@SPAM.com> writes:
C的国内用法是用于
简单增量循环效率低吗？这是一个非常简单的程序，它打印出一个字节的
位数。
[...]
然而，我们可以看到第一个条件测试是多余的 -
测试会更好每次迭代后的条件，而不是之前的条件。类似于：
[...]
广泛使用的C方法简单的循环增量本身是否效率低？乍一看，它对我来说似乎是这样。
Is the domestic usage of the C "for" loop inefficient when it comes to
simple incrementation? Here''s a very simple program that prints out the
bit-numbers in a byte.
[...]
However, we can see that the very first conditional test is redundant --
it would be better to test the condition AFTER each iteration, rather
than before. Something like:
[...]
Is the widely used C method of simple loop incrementation inherently
inefficient? At first glance, it appears so to me.
仅当循环总是至少执行一次时。在更多
一般情况下，你有类似
的东西（i = start; i< end; i ++）
然后你做想要在第一次迭代之前进行测试。
我永远不会将循环转换为糟糕的标签形式+
你建议的那些。如果你确实想在第一次迭代中避免测试，那么使用do {...} while循环。但是这个
通常是毫无意义的微优化。
另外，我不知道你为什么把它称为国内。对于
循环。在这种情况下，这个词没有意义。
-
IMO，Perl是一个很好的语言，可以让你挣扎
--Micah Cowan
Only if the loop always executes at least once. In the more
general case where you have something like
for (i = start; i < end; i++)
then you do want the test before the first iteration.
I would never translate a loop into the awful form of labels +
gotos that you suggest. Use a do { ... } while loop if you
really want to avoid the test on the first iteration. But this
is usually a pointless micro-optimization.
Also, I have no idea why you refer to this as the "domestic" for
loop. The word does not make sense in this context.
--
"IMO, Perl is an excellent language to break your teeth on"
--Micah Cowan
文章< Xn ************************* *@194.125.133.14> ;,
Frederick Gotham< fg ******* @ SPAM.com>写道：
In article <Xn**************************@194.125.133.14>,
Frederick Gotham <fg*******@SPAM.com> wrote:
然而，我们可以看到第一个条件测试是多余的 -
However, we can see that the very first conditional test is redundant --
在这种情况下编译器也可以看到它。
- Richard
In cases like this the compiler can see it too.
-- Richard
* Ben Pfaff - >有人：
* Ben Pfaff -> Someone:
另外，我不知道为什么你把它称为国内和国内。用于
循环。这个词在这种情况下没有意义。
Also, I have no idea why you refer to this as the "domestic" for
loop. The word does not make sense in this context.
似乎一些人工智能故事产生的程序是松散的。
他们'被垃圾邮件发送者和一些Usenet海报使用，我想加上一些
人类修复。一些诗歌在[no.test]中出现的是
太棒了。
-
答：因为它弄乱了订单人们通常会阅读文字。
问：为什么这么糟糕？
A：热门帖子。
问：什么usenet和电子邮件中最烦人的事情是什么？
It seems that some AI story-generating programs are on the loose.
They''re used by spammers and some Usenet posters, I guess with some
human fixup added. Some of the "poetry" that turns up in [no.test] is
amazing.
--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?
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