log

说明

float log ( float arg [, float base] )

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

注: 参数 base 自 PHP 4.3.0 开始可用。
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：log_b(n) = log(n)/log(b)，其中 log 是自然对数。

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mcmeijer at yahoo dot com
03-Feb-2005 11:22


$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.

c0x at mail dot ru
19-Sep-2004 06:08


more general version, works fine on negative, very big ($value > 1E+18) and very small ($value < 1E-18) numbers.



function expn($value, $prec = 3, $base = 1000, $prefix = '') {

    $e = array('a', 'f', 'p', 'n', 'u', 'm', '', 'k', 'M', 'G', 'T', 'P', 'E');

    $p = min(max(floor(log(abs($value), $base)), -6), 6);

    return round((float)$value / pow($base, $p), $prec) . $prefx . $e[$p + 6];

}

admin at worldtakeover dot tk
21-Jun-2004 04:06


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.

mightye (at) mightye.org
07-Feb-2003 04:02


A minor warning:



in PHP < 4.3.0, in order to get the log base 10 of a number, you have to do:

$log10 = log($n)/log(10);



If you want a whole number (to identify the order of magnitude), and you typecast $log10 to (int), you may not get what you expect:

(int)(log(1000)/log(10)) = 2

(log(1000)/log(10)) = 3 (float with no displayed decimal places)



The mathematical error in this causes the typecast to round the result down, even though the error runs out to so many decimal places that it is not displayed, and the float value looks like a whole number.  Instead you may need to do:

round(log($n)/log(10)-0.5,0);



This will give you the order of magnitude of your number.



Presumably in PHP 4.3.0+, a similar result may occur.

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