gmp_init()

(PHP 4 >= 4.0.4, PHP 5, PHP 7)

Create GMP number

说明

gmp_init(mixed $number[,int $base= 0]): GMP

Creates a GMP number from an integer or string.

参数

$number

An integer or a string. The string representation can be decimal, hexadecimal or octal.

$base

The base.

The base may vary from 2 to 36. If base is 0(default value), the actual base is determined from the leading characters: if the first two characters are0xor0X, hexadecimal is assumed, otherwise if the first character is "0", octal is assumed, otherwise decimal is assumed.

返回值

GMP 数值资源.

更新日志

注释

Note:

To use the extended base introduced in PHP 5.3.2, then PHP must be compiled against GMP 4.2.0 or greater.

范例

Creating GMP number

<?php
$a = gmp_init(123456);
$b = gmp_init("0xFFFFDEBACDFEDF7200");
?>

注释

Note:

It is not necessary to call this function in order to use integers or strings in place of GMP numbers in GMP functions(such as with gmp_add()). Function arguments are automatically converted to GMP numbers, if such conversion is possible and needed, using the same rules as gmp_init().

I discovered that the gmp functions use [0-9a-f] up to base 16, but [0-9A-Za-z] (i.e. upper case first) from bases 17 to 62. This differs from most of the base-62 implementations I've found that tend to use lower case first.

Here's a way to parse a decimal (eg 3.25) into an integer and exponent:
<?
if (preg_match("/^[0-9]+\.[0-9]+$/",$input)){ 
   //Input is a base-10 decimal. Multiply as necessary to remove the decimal
   //point. Convert that to a gmp_resource, then decrement the exponent 
   //to compensate.
   $pieces=explode(".", $input);   //Split at the d.p.
   $input="$pieces[0]$pieces[1]"; //Remove the decimal point.
   $input=ltrim($input,'0');  
   //Remove any leading zeros, or gmp_init will parse the number as octal.
   if ($input==''){  //Deal with "0.0" which would otherwise be ''.
     $input=0;
   }
   $integer=gmp_init($input);  
   $ns_exponent=-strlen($pieces[1]); 
   //exponent = (-) the number of characters after the decimal point.
}
?>

Note: Leading zeros will make gmp_init parse this as octal.
Thus gmp_init(010) becomes 8. 
$a=010;       //8
$b="010" + 0;   //10
$c=gmp_strval(gmp_init(010));  //8
$d=gmp_strval(gmp_init("010")); //8
This behaviour is inconsistent: either $d should equal $b, or
$b should equal $a.

Unless the base is 16, gpm_init will fail if the string begins with "0b". 
> php -r '$v = gmp_init("b83", 17); print("$v\n");'
Resource id #4
> php -r '$v = gmp_init("0b83", 17); print("$v\n");'
[nothing prints]
In may case, where I am explicitly specifying the base, the solution is to apply ltrim first:
> php -r '$v = gmp_init(ltrim("0b83", "0"), 17); print("$v\n");'
Resource id #4

gmp_* functions don't accept strings with a leading '+':
<?php
echo gmp_strval(gmp_init('+42'));   #0
echo gmp_strval(gmp_add('42', '+42')); #42
echo bcadd('+42', '+42');       #84
?>

If you call a gmp_* function directly with an interger as parameter, this integer MUST NOT be 0:
for($i=-1;$i<=1;$i++) { echo gmp_strval(gmp_add(2,gmp_mul(1,$i))) . ' '; }
The result is 1 0 3 (wrong)
In this case you have to use gmp_init():
for($i=-1;$i<=1;$i++) { echo gmp_strval(gmp_add(2,gmp_mul(1,gmp_init($i)))) . ' '; }
The result is 1 2 3 (right)
Happy number crunching! :-)