exp

(PHP 4, PHP 5)

expCalculates the exponent of e

Descrierea

float exp ( float $arg )

Returns e raised to the power of arg.

Notă:

'e' is the base of the natural system of logarithms, or approximately 2.718282.

Parametri

arg

The argument to process

Valorile întoarse

'e' raised to the power of arg

Exemple

Example #1 exp() example

<?php
echo exp(12) . "\n";
echo 
exp(5.7);
?>

Exemplul de mai sus va afișa:

1.6275E+005
298.87

Vedeți de asemenea

  • log() - Natural logarithm
  • pow() - Exponential expression

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User Contributed Notes 5 notes

up
1
zooly at globmi dot com
4 years ago
PHP does not have the following math function in any extensions:

frexp() - Extract Mantissa and Exponent of the Floating-Point Value

I've digged many C source codes, and found the simplest implementation as follows:

<?php

function frexp ( $float ) {

 
$exponent = ( floor(log($float, 2)) + 1 );
 
$mantissa = ( $float * pow(2, -$exponent) );

  return(
    array(
$mantissa, $exponent)
  );

}

print_r(frexp(0.0345));
print_r(frexp(21.539));

?>

Array
(
    [0] => 0.552
    [1] => -4
)
Array
(
    [0] => 0.67309375
    [1] => 5
)

I have compared the results using a lot of floats against C's frexp function - they are the same.

Note that C and PHP uses different float precisions, for example "4619.3" gives:

C: 0.56387939453125, 13
PHP: 0.563879394531, 13

/Assuming default configurations./
up
0
konrad
7 years ago
working version (checked) of below code is

<?php
 
// see bccomp for this code (signed and unsigned zero!)
 
function bccomp_zero($amount) {
    return
bccomp($amount, (@$amount{0}=="-"?'-':'').'0.0');
  }

 
// arbitrary precision function (x^n)/(n)!
 
function bcpowfact($x, $n) {
    if (
bccomp_zero($n) == 0) return '1';
    if (
bccomp($n, '1') == 0) return $x;
   
$a = $x; // 1st step: a *= x / 1
   
$i = $n;
    while (
bccomp($i, '1') == 1) {
     
// ith step: a *= x / i
     
$a = bcmul($a, bcdiv($x, $i));
     
$i = bcsub($i, '1'); // bc idiom for $i--
   
}
    return
$a;
  }

 
// arbitrary precision exp() function
 
function bcexp($x, $digits) {
   
$sum = $prev_sum = '0.0';
   
$error = '0.'.str_repeat('0', $digits-1).'1'; // 0.1*10^-k
   
$n = '0.0';
    do {
     
$prev_sum = $sum;
     
$sum = bcadd($sum, bcpowfact($x, $n));
     
$n = bcadd($n, '1'); // bc idiom for $n++
   
} while (bccomp(bcsub($sum, $prev_sum), $error) == 1);
    return
$sum;
  }
?>
up
0
boards at gmail dot com
8 years ago
Note regarding the mathematical function exp(x):

To continue accuracy of the exponential function to an infinite amount of decimal places, one would use the power series definition for exp(x).
(in LaTeX form:)
e^x = \sum_{n=0}^{\infty} \frac{x^n}{n!}

So, to do that in PHP (using BC math):

<?php
// arbitrary precision function (x^n)/(n)!
function bcpowfact($x, $n) {
  if (
bccomp($n, '0') == 0) return '1.0';
  if (
bccomp($n, '1') == 1) return $x;
 
$a = $x; // nth step: a *= x / 1
 
$i = $n;
  while (
bccomp($i, '1') == 1) {
   
// ith step: a *= x / i
   
$a = bcmul($a, bcdiv($x, $i));
   
$i = bcsub($i, '1'); // bc idiom for $i--
 
}
  return
$a;
}

// arbitrary precision exp() function
function bcexp($x, $decimal_places) {
 
$sum = $prev_sum = '0.0';
 
$error = bcdiv(bcpow('10', '-'.$decimal_places), 10); // 0.1*10^-k
 
$n = '0';
  do {
   
$prev_sum = $sum;
   
$sum = bcadd($sum, bcpowfact($x, $n));
  }
  while (
bccomp(bcsub($sum, $prev_sum), $error) == 1);
  return
$sum;
}
?>
up
-1
Mohammed AbdelRahman
3 months ago
Which is the inverse of log(float $arg, e)
up
-1
Nitrogen
5 years ago
Just a note about using the submitted codes below..
Their functions have an optional $precision parameter; however, it's not being used properly..

BCMath functions by default do not use decimal precision unless specified by BCScale($precision); or using the extra parameter in the used BC functions.

For example, a blank PHP file with their code.. executing BCExp('5.7'); returns "47" instead of the correct answer of "298.86740096706..."

So for optimum accuracy, I'd suggest setting BCScale to a healthy length before running their codes.
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