从cmath中提取PI值 [英] extracting PI value from cmath
问题描述
我正在做一些数学运算，我刚发现标准库
没有提供标准的PI值。在较老的帖子中我看到每个人都说
你必须自己定义它，但是因为一些标准函数实际上应该返回那个值，我觉得必须是一个更好的
方式。例如，这是谷歌搜索返回的内容：
浮动PI = std :: atan（1.0f）* 4.0f;
更准确吗？
谢谢
Hi,
I''m doing some maths and I''ve just found that the standard library
does not provide a standard PI value. In older post I see everyone says
you must define it by yourself, but as some of the standard functions
are in fact supposed to return that value, I feel there must be a better
way. For example, here''s what a google search returned:
float PI = std::atan(1.0f) * 4.0f;
Anything more accurate?
Thanks
推荐答案
giaro写道：
giaro wrote:
我正在做一些数学运算，我刚发现标准库
没有提供标准PI值。在较老的帖子中我看到每个人
说你必须自己定义它，但是因为一些标准的
函数实际上应该返回那个值，我觉得
必须是更好的方法。例如，这里是谷歌搜索
返回的内容：
浮动PI = std :: atan（1.0f）* 4.0f;
还有什么更准确的吗？
Hi,
I''m doing some maths and I''ve just found that the standard library
does not provide a standard PI value. In older post I see everyone
says you must define it by yourself, but as some of the standard
functions are in fact supposed to return that value, I feel there
must be a better way. For example, here''s what a google search
returned:
float PI = std::atan(1.0f) * 4.0f;
Anything more accurate?
这应该足够准确。
你可以自己定义一个（3.1415926f浮动就足够了）
然后将其与您使用atan的结果进行比较。
V
-
请在通过电子邮件回复时删除资本''A'
我没有回复最热门的回复，请不要问
This should be accurate enough.
You can define one yourself (3.1415926f for float should be enough)
and then compare this to the result you get using atan.
V
--
Please remove capital ''A''s when replying by e-mail
I do not respond to top-posted replies, please don''t ask
giaro写道：
giaro wrote:
float PI = std :: atan（1.0f）* 4.0f;
一切更准确的？
float PI = std::atan(1.0f) * 4.0f;
Anything more accurate?
std :: atan（1.0f）应该将pi / 4返回到0.5 ulp以内（尽管我不认为
标准保证这个）。如果浮点基数为2（或4），则乘以4.0仍将给出答案
精确到0.5 ulp。所以我认为
更准确无法实现。也许pi = std :: atan2（0.0f，-1.0f）将会更安全，因为它不依赖于基础。
- Ben
std::atan(1.0f) should return pi/4 to within 0.5 ulp (though I don''t think
the standard guarantees this). Multiplying by 4.0 will still give an answer
accurate to 0.5 ulp if the floating-point base is 2 (or 4). So I would think
that more accuracy is impossible. Perhaps pi = std::atan2(0.0f,-1.0f) would
be slightly safer since it doesn''t depend on the base.
-- Ben
giaro< no **** @ here.inwrote：
giaro <no****@here.inwrote:
float PI = std :: atan（1.0f）* 4.0f;
float PI = std::atan(1.0f) * 4.0f;
还有什么更准确的？
Anything more accurate?
double pi = 4 * std :: atan（1）;
这里的任何不准确都可能是由于受限制精度浮动或双倍
而不是这种计算方法。我通过获取22位数字来检查
来自 http：/ /www.piday.org/million.php 当分配给结果加倍时
与上面计算的pi完全匹配。 （gcc 4.2.1，glibc 2.6.1）
克莱门斯
double pi = 4 * std::atan(1);
Any inaccuracy here is likely due to the limited precision of float or double
rather than this method of computation. I checked by getting 22 digits or so
from http://www.piday.org/million.php and when assigned to double the result
matches exactly to pi as computed above. (gcc 4.2.1, glibc 2.6.1)
Clemens
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