PHP Unconference Europe 2015

log

(PHP 4, PHP 5)

logLogarithme naturel (népérien)

Description

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

Si le paramètre optionnel base est spécifié, log() retourne alors le logarithme en base base, sinon log() retourne le logarithme naturel (ou népérien) de arg.

Liste de paramètres

arg

La valeur pour laquelle on calcule le logarithme

base

La base logarithmique optionnelle à utiliser (par défaut, 'e' et donc, le logarithme naturel).

Valeurs de retour

Le logarithme de arg en base base, si fourni, ou le logarithme naturel.

Voir aussi

  • log10() - Logarithme en base 10
  • exp() - Calcul l'exponentielle de e
  • pow() - Expression exponentielle

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User Contributed Notes 7 notes

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2
dingus_76 at hotmail dot com
6 years ago
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

<?php
function bclog($X,$base=10,$decimalplace=12){
   
$integer_value=0;
    while(
$X < 1){
       
$integer_value = $integer_value - 1 ;
       
$X = bcmul($X , base);
    }
    while(
$X >= $base){
       
$integer_value = $integer_value + 1;
       
$X = bcdiv($X , $base );
    }
   
$decimal_fraction = 0.0;
   
$partial = 1.0;
   
# Replace X with X to the 10th power
   
$X = bcpow($X , 10);
    while(
$decimalplace > 0){
       
$partial = bcdiv($partial , 10);
       
$digit=0;
        while(
$X >= $base){
             
$digit = $digit + 1;
             
$X = bcdiv($X , $base);
        }
       
$decimal_fraction = bcadd($decimal_fraction , bcmul($digit , $partial));
       
# Replace X with X to the 10th power
       
$X = bcpow($X , 10);
       
$decimalplace = $decimalplace - 1 ;
    }
    return
$integer_value + $decimal_fraction ;
}
?>
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1
c0x at mail dot ru
10 years ago
more general version, works fine on negative, very big ($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];
}
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1
admin at worldtakeover dot tk
10 years ago
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.
up
1
ClaudiuS
1 year ago
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

<?php

$number
='fill in your number here'
$base='fill in requested base here';

//use when the power is needed
$pow=0;do { $number/=$base;$pow++; } while ($number>1);
if (
$number==1) print $base.'^'.$pow;
else print
'False';

//use when just a check is needed
do $number/=$base; while ($number>1);
if (
$number==1) print 'True';
else print
'False';
?>
up
0
Ulf Wostner
8 years ago
<?php

#--------------------------------------------------------
#     How many digits does an integer have?
#--------------------------------------------------------

function digit_count($n, $base=10) {

  if(
$n == 0) return 1;

  if(
$base == 10) {
   
# using the built-in log10(x)
    # might be more accurate than log(x)/log(10).
   
return 1 + floor(log10(abs($n)));
  }else{
   
# here  logB(x) = log(x)/log(B) will have to do.
  
return 1 + floor(log(abs($n))/ log($base));
  }
}

# Example:  How many decimal digits for 2 to the power 24?
echo digit_count(pow(2, 24));

# Example: How many bits to write 1 billion in binary, last century?

if($country_code  == 'US') echo digit_count(pow(10, 9), 2);
if(
$country_code == 'UK') echo digit_count(pow(10, 12), 2);

#--------------------------------------------------------
#     Using log to format columns.
#--------------------------------------------------------

# Suppose we have a dynamically generated list of integers,
# and want to present them as a table. The use of log10 in
# our digit_count helps calculate the proper format string.

function print_list_of_ints($ints, $line_width=40) {

 
# Apply our digit_count to the max int among ints.
 
$field_width = 2 + digit_count(max($ints));

 
# Create format string for printf.
 
$format = "%${field_width}d";

 
$ints_per_line = floor($line_width/$field_width);

 
$border = str_repeat("-", $ints_per_line * $field_width);

  echo
"\n$border\n";

  foreach(
$ints as $count => $int) {
    if(
$count and ($count % $ints_per_line == 0)) echo "\n";
   
printf($format, $int);
  }

  echo
"\n$border\n";
}

# To generate an example, here is a basic function
# returning a list of (pseudo) random numbers.

function rands($how_many) {
  for(
$i=0; $i < $how_many; $i++) $rands[] = rand();
  return
$rands;
}

# Example:  A list of random ints dynamically formatted into columns.

print_list_of_ints(rands(11));

/* Sample output. Numbers and fonts vary. Visualize monospace!

------------------------------------
  1093146637   244503173  1346204527
   638304372   140216732  1054707210
   573915416  1728677954  2038899669
   534854768    12674586
------------------------------------

*/

?>
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0
mcmeijer at yahoo dot com
9 years ago
$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.
up
-2
olafurw [at] gmail.com
5 years ago
For those interested. Works with older than 4.3 versions.

<?php
   
function byteConvert($bytes)
    {
       
$s = array('B', 'Kb', 'MB', 'GB', 'TB', 'PB');
       
$e = floor(log($bytes)/log(1024));
     
        return
sprintf('%.2f '.$s[$e], ($bytes/pow(1024, floor($e))));
    }
?>
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