# log

(PHP 4, PHP 5, PHP 7, PHP 8)

logLogaritmo natural

### Descripción

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

Si se especifica el parámetro opcional `base`, log() devuelve logbase `arg`, y en caso contrario log() devuelve el logaritmo natural de `arg`.

### Parámetros

`arg`

El valor al que se desea calcular el logaritmo

`base`

La base opcinal del logaritmo a usar (por omisión es 'e' y por lo tanto el logaritmo natural).

### Valores devueltos

El logaritmo de `arg` en la base dada por `base`, si se indica, o en su lugar el logaritmo natural.

### Ver también

• log10() - Logaritmo en base 10
• exp() - Calcula la exponencial de e
• pow() - Expresión exponencial

### User Contributed Notes 8 notes

c0x at mail dot ru
17 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
3 years 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' ) == 0Bfunction 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 );}https://en.wikipedia.org/wiki/Binary_prefix#Adoption_by_IEC.2C_NIST_and_ISOAlso more colloquially:    \$prefix = array('', 'k', 'm', 'g', 't', 'p', 'e', 'z', 'y'); ```
-2
olafurw [at] gmail.com
13 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))));    }?> ```
-3
18 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. ```
-5
mcmeijer at yahoo dot com
17 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. ```
-6
ClaudiuS
9 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='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 neededdo \$number/=\$base; while (\$number>1);if (\$number==1) print 'True';else print 'False';?> ```
-8
dingus_76 at hotmail dot com
14 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<?phpfunction 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 ;}?> ```
-21
Ulf Wostner
15 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------------------------------------*/?> ```