清除低位/使2的幂倍数 [英] clearing low order bits / making a multiple of power of 2
问题描述
确保整数是给定
2的倍数的最佳方法是什么？
怎么样
int size;
...
尺寸^ =（尺寸&（4-1））; / *确保4 * /
或
size& = -4; / *确保4 * /
的倍数？
谢谢，
-
John Devereux
John Devereux< jd ****** @ THISdevereux.me。 UK>写道：确保整数是给定
2的倍数的最佳方法是什么？
i = 0;
如果那不能做你想要的，你应该更清楚你的要求
。精确地定义问题通常会让你至少获得
解决它的一半。
-
Keith Thompson（The_Other_Keith）< a href =mailto：ks *** @ mib.org> ks *** @ mib.org < http://www.ghoti.net/~kst>
圣地亚哥超级计算机中心< *> < http://users.sdsc.edu/~kst>
我们必须做点什么。这是事情。因此，我们必须这样做。
John Devereux< jd ****** @ THISdevereux.me.uk>写道：
确保整数是给定
2的倍数的最佳方法是什么？
int size;
...
尺寸^ =（尺寸&（4-1）） ; / *确保4 * /
尺寸& = -4; / *确保多个4 * /
size - = size％4;
Keith Thompson< ks***@mib.org>写道：
John Devereux< jd ****** @ THISdevereux.me.uk>写道：确保整数是给定
2的倍数的最佳方法是什么？
i = 0;
如果那不是你想要的，你应该更明确地了解你的要求。正确地定义问题通常会让你至少解决它的一半。
好吧，你剪掉的例子应该这样做！ :)但我会
尝试用文字说出来。我现在也意识到我应该使用未签名的
整数。
i是无符号整数（0,1,2,3,4,5等）
n是一个编译时无符号常量整数（或字面值），它是一个
无符号整数次幂2（1,2,4,8等）。
我需要的是将我舍入到n的最接近的倍数，或者如果已经是n的倍数则保持
不变。 （该应用程序将为低级编程生成
对齐的地址）。
我知道这是相当微不足道的，我只是想知道是什么标准
成语就是为了这个。
-
John Devereux
Hi,
What is the best way to ensure an integer is a multiple of a given
power of 2?
How about
int size;
...
size ^= (size & (4-1)); /* ensure multiple of 4 */
or
size &= -4; /* ensure multiple of 4 */
?
Thanks,
--
John Devereux
John Devereux <jd******@THISdevereux.me.uk> writes:What is the best way to ensure an integer is a multiple of a given
power of 2?
i = 0;
If that doesn''t do what you want, you should be more explicit about
your requirements. Defining the problem precisely often gets you at
least halfway to solving it.
--
Keith Thompson (The_Other_Keith) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.
John Devereux <jd******@THISdevereux.me.uk> writes:
Hi,
What is the best way to ensure an integer is a multiple of a given
power of 2?
How about
int size;
...
size ^= (size & (4-1)); /* ensure multiple of 4 */
or
size &= -4; /* ensure multiple of 4 */
size -= size % 4;
Keith Thompson <ks***@mib.org> writes:
John Devereux <jd******@THISdevereux.me.uk> writes:What is the best way to ensure an integer is a multiple of a given
power of 2?
i = 0;
If that doesn''t do what you want, you should be more explicit about
your requirements. Defining the problem precisely often gets you at
least halfway to solving it.
Well, the examples you snipped were supposed to do that! :) But I will
try to say it in words. I also now realise I should have used unsigned
ints.
i is an unsigned integer (0,1,2,3,4,5 etc)
n is a compile-time unsigned constant integer (or literal) that is an
unsigned integer power of 2 (1,2,4,8 etc).
What I need is to round i down to the nearest multiple of n, or leave
unchanged if already a multiple of n. (The application is to generate
aligned addresses for low-level programming).
I know it''s fairly trivial, I was just wondering what the "standard"
idiom was for this.
--
John Devereux
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