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PHP pow 指数表达式
pow & 指数表达式
如果不能计算幂，将发出一条警告，pow() 将返回 FALSE。PHP 4.2.0
版开始 pow() 不要产生任何的警告。
PHP 不能处理负数的 base。
Example #1 pow() 例子
&?phpvar_dump(pow(2,&8));&//&int(256)echo&pow(-1,&20);&//&1echo&pow(0,&0);&//&1echo&pow(-1,&5.5);&//&error?&
在 PHP 4.0.6 及之前版本中，pow()
总是返回 ，并且不产生警告。
PHP pow note #1
This function returns the value of a positive base with a signed floating point exponent:
function sf_exp( $fl_x = 1, $fl_y = 0)
{& & $fl_exp = 0.0;
&&& if (0 & $fl_x)
&&& {& & // Alter this logic container to enable processing of negative bases.
&& & && $fl_exp = -1.0;
&&& } else
&&& {& & $bool_neg = (0 & $fl_y);
&& & && if ($bool_neg)
&& & && {& & $fl_y = 0 - $fl_y;
&& & && $fl_xlog = log10( $fl_x);
&& & && $fl_xylog = ( $fl_xlog * $fl_y);
&& & && $fl_exp = pow( 10, $fl_xylog);
&& & && if ($bool_neg)
&& & && {& & $fl_exp = 1/$fl_
&&& return $fl_
PHP pow note #2
You can increase the 'precision' php.ini setting a little to work with larger float numbers here, but this comes at at cost of sacrificing decimal accuracy.& The default 'precision' is 14.& 5 is about the threshhold that php can handle for decimal accuracy before at least some number corruption starts showing or it cannot output the actual number, and 16 for large number accuracy, as demonstrated by throwing this into the table below:
&&& echo "&td&".pow(10, $i) - 1)."&/td&";
See the table below for an example, and adjust your php.ini 'precision' setting according to what your OS and PHP version can handle and what number size you want to work with.& Alternatively, you can use the bc math functions for more accuracy all around, and not have to rely on the 'precision' ini setting at all, but this moves out of the realm of strictly floats and into strings.
Also, PHP just prefers to display the 'E' notation of float values where possible after about 5 decimal places rather than the actual decimal number (1.0E-5 vs 0.00005).
echo "&table&";
for($i = 0; $i & 50; $i++) {
&&& $precision = $i + 1;
&&& ini_set('precision', $precision);
&&& echo "&tr&";
&&& echo "&td&".$precision."&/td&";
&&& echo "&td&".pow(10, $i)."&/td&";
&&& echo "&td&".pow(10, (-1 * $i))."&/td&";
&&& echo "&td&".bcpow('10', (string) $i, $precision)."&/td&";
&&& echo "&td&".bcpow('10', (string) (-1 * $i), $precision)."&/td&";
&&& echo "&/tr&";
echo "&/table&";
Bottom line though is, if you're working with larger numbers or require very fine decimal precision or prefer displaying the full decimal number, use the bc math functions instead.& And, do check out PHP's considerations about the float type here:
PHP pow note #3
Calculate wind chill based on the National Weather Service formula.
$temp = 25;
$wind_speed_mph = 6;
$wind_chill = 35.74+(.6215*$temp_f)-(35.75*(pow($wind_speed_mph, 0.16)))+(.4275*$temp_f*(pow($wind_speed_mph, 0.16)));
Value only valid when the temp is 45 or below.... I used this with a weather script I wrote that reads an xml file. They don't provide wind chill.
PHP pow note #4
no integer breaking here, pow just silently switches to using floats instead of integers.
pow(2, 31) = integer value
pow(2, 32) = float value.
the manual says the limit for floats is machine dependent so i did a little loop to see how far it will go before becomming infinit. the result is 1023.
pow(2, 1023) = float
pow(2, 1024) = ifinit.
tested on php 4.4.1 under windows2000 on an AMD AthlonXP 2800+.
PHP pow note #5
Note that pow(0, 0) equals to 1 on PHP 4 (only tested it there), although mathematically this is undefined.
PHP pow note #6
Here is a function for calculating the $k-th root of $a :
function root($a,$k){return(($a&0&&$k%2&0)?-1:1)*pow(abs($a),1/$k);};
PHP pow note #7
Here's a pow() function that allows negative bases :
function npow($base, $exp)
&&& $result = pow(abs($base), $exp);
&&& if ($exp % 2 !== 0) {
&& & && $result = - ($result);
&&& return $result;
PHP pow note #8
since pow doesn't support decimal powers, you can use a different sollution,
thanks to dOt for doing the math!
a^b = e^(b log a)
which is no the 10log but the e-log (aka "ln")
so instead of: pow( $a , 0.6 ) use something like: exp( 0.6 * log($a) )
PHP pow note #9
As of PHP5beta4, pow() with negative bases appears to work correctly and without errors (from a few cursory tests):
pow(-3, 3) = -27
pow(-3, 2) = 9
pow(-5, -1) = -0.2
PHP pow note #10
A couple of points on pow():
1. One of the official examples of pow(2,8) use 1 && 8 as it's substantially faster
2. When passing variables to pow(), cast them otherwise you might get warnings on some versions of PHP
3. All the rules of algebra apply: b**(-e) is 1/(b**e), b**(p/q) is the qth root of b**p
So, e.g., sqrt($x) === pow($x, .5); but sqrt() is faster.
PHPMath - 函数C语言中怎么写一个表达式的N次方~_c语言吧_百度贴吧
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C语言中怎么写一个表达式的N次方~收藏
菜鸟，刚学C语言，请问这个式子 y=（1+r）^n要怎么写？不知道我这个写的对不，等号右边就是（1+r）的n次方。谢谢各位大大~
c语言海同强大的师资阵容,因人制定课程内容,分阶段学习.c语言就到正规IT技术培训机构-海同科技,培训IT技术面对面教学,免费重读!
有没有高手来解答下啊~
*=符号懂用吗
可以用for循环for(y=1,i=0,i&n,i++)
y*=(1+r);也可以用pow函数加头文件#include &math.h&y=pow(1+r,n);
解决了，谢谢各位的帮忙
顺便问个问题，我写程序用的是Microsoft Visual C++ ，请问为什么我按ALT+F8没办法自动排列。
不对吧，可以用math库函数
#include&math.h&颇为pow(1+r,n)返回值就是(1+r)^n
楼上的方法不错。表示支持一个。
#include&stdio.h&void main(){int i , n ,for(i=0 ; i&= i++)y*=(1+r);printf("%d" , y)}r要等于多少0哦
(1+r)**n对不对啊，我看书上说**表示次方，还没实践呢
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