log

(PHP 4, PHP 5, PHP 7)

logLogarithme naturel (népérien)

Description

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

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

User Contributed Notes 8 notes

mcmeijer at yahoo dot com
14 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.
c0x at mail dot ru
14 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];
}
rhinopete at batpudding dot com
6 months ago
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 < 1024 )
return sprintf( '%d%s%s', max( 1, \$units + 1 ) - 1, \$prefix[\$exponent], \$unit );
else
return sprintf( '%.1f%s%s', \$units / pow(1024, \$exponent), \$prefix[\$exponent], \$unit );
}

Also more colloquially:
\$prefix = array('', 'k', 'm', 'g', 't', 'p', 'e', 'z', 'y');
14 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.
-3
ClaudiuS
6 years 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
\$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';
?>
-3
olafurw [at] gmail.com
10 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))));
}
?>
-4
dingus_76 at hotmail dot com
11 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 ;
}
?>
-14
Ulf Wostner
12 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
------------------------------------

*/

?>