log() - 自然对数 - php 数学函数

梵高1年前 (2023-11-21)阅读数 15#技术干货

文章标签对数

log()

(PHP 4, PHP 5, PHP 7)

自然对数

说明

log(float $arg[,float $base= M_E]):float

如果指定了可选的参数$base，log()返回 logbase$arg，否则log()返回参数$arg的自然对数。

参数

$arg

要计算对数的值

$base

The optional logarithmic base to use (defaults to 'e' and so to the natural logarithm).

返回值

log() - 自然对数 - php 数学函数

返回 logbase$arg，或者它的自然对数。

更新日志

版本	说明
4.3.0	可选参数$base可用。你可以计算任意以b为底n的对数，但其实使用的是数学等式：logb(n)= log(n)/log(b)，其中 log 是自然对数。

参见

log10() 以 10 为底的对数
exp() 计算 e 的指数
pow() 指数表达式

$val = 1000000
$val2 = floor(log($val,10)) gives a value of 5 for $val2 and not 6 as expected.
$val2 = floor(log10($val)) gives the correct value.

more general version, works fine on negative, very big ($value > 1E+18) and very small ($value In regards to the note about log in base 10 and the round() function. You need to use floor() instead of round() to find out the order of magnitude. That way, you don't have to worry about subtracting 0.5 or whatever.
For those interested. Works with older than 4.3 versions. 
If you just need to check if N is a perfect power of Base, log() is SLOW compared to a WHILE construct that will be 2x faster! 
Tested on range: 1 ... 20.000.000 => while() is 2.105 times faster 
well i been pulling my hair out trying to get log to work with big numbers and i ended up writing a bclog function so to save everyone else the stress here it is 
Seems like unit prefixes should have a standard PHP function. Maybe in the future.
I found this page while looking for a quick unit prefix function. The one by olafurw was voted down, I think because it had unchecked array indexes and /0s. So here it is fixed and readable.
-Should work down to PHP 4.
-Not meant for fractions or negatives, so anything less than 1 returns 0.
-Not very effective on really really large numbers, but it's easy to add more prefixes.
-Doesn't handle non numeric arguments. PHP 7+ can do this: function binaryprefix( int $units, $unit = '' )
// returns reduced $units with a binary prefix
// ex. ( 110974120, 'B' ) == 105.8MiB
// ex. ( 2^100, 'B' ) == 1048576.0YiB
// ex. ( 0.12314, 'B' ) == 0B
function binaryprefix( $units, $unit = '' )
{
  $prefix = array('', 'Ki', 'Mi', 'Gi', 'Ti', 'Pi', 'Ei', 'Zi', 'Yi');
  $exponent = min( floor( log( max( 1, $units ), 1024 ) ), count( $prefix ) - 1 );
  if ( $units

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