PHPerKaigi 2021

gmp_strval

(PHP 4 >= 4.0.4, PHP 5, PHP 7, PHP 8)

gmp_strvalGMP 数を文字列に変換する

説明

gmp_strval ( GMP|int|string $num , int $base = 10 ) : string

GMP 数を base を基数とする文字列表現に変換します。 デフォルトの基数は 10 です。

パラメータ

num

文字列の変換したい GMP 数。

GMP オブジェクト、整数、あるいは数値に変換可能な数値形式の文字列。

base

返り値の基数。デフォルトは 10 です。 基数として使用可能な値は 2 から 62 までおよび -2 から -36 までです。

返り値

数を文字列で表したものを返します。

例1 GMP 数を文字列に変換する

<?php
$a 
gmp_init("0x41682179fbf5");
printf("10 進数: %s, 36 進数: %s"gmp_strval($a), gmp_strval($a,36));
?>

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User Contributed Notes 5 notes

up
3
php at richardneill dot org
14 years ago
I think it's no longer necessary to worry about trailing nulls. This was fixed in Feb 2002 (see bug 10133)
up
0
Nitrogen
5 years ago
It may be worth noting that using gmp_strval can slow your script down a lot with large numbers.

This script for example, produces factorials in succession:
<?php

$start
= microtime(TRUE);

$fact = gmp_init(1, 10);
for(
$i=1;$i<100000;$i++) { // calculates 100,000 factorials
 
$fact = gmp_mul($fact, $i); // $fact is now the result of the $ith factorial
  // gmp_strval($fact, 10); // see below...
 
if(microtime(TRUE)-$start>1) // stop calculating after 1 second
   
break;
}
// print whatever result that took 1 second to complete
printf("\$i = %d\n\$fact = %s\n", gmp_strval($fact, 10));

?>

My server calculates around about the 69,500th factorial on average every time. Uncommenting that gmp_strval line slows this process down dramatically, and only calculates about 5,000 factorials. This is about 14 times longer than before (this is consistent with allowing it to run for any number of seconds, not just 1).

Nitrogen.
up
-1
Anonymous
1 year ago
casting to string seems to do the same (for base10) and it's much faster

(string) $num

instead of

gmp_strval($num)
up
-1
russell dot s dot harper at gmail dot com
10 years ago
A gotcha when using the value from gmp_strval as a key in an associative array. Some numbers are 2^31 are converted to signed integer format, this is usually incorrect.

<?php

$a
= gmp_init('2147483649'); // = 2^31 + 1
$b = gmp_init('3173604585'); // = 2^31 < $b < 2^32

$c = gmp_strval($a);
$d = gmp_strval($b);

$e = array($c => 'c', $d => 'd');

print
'*** Expect '."\n";
print
'Array '."\n".'('."\n".'    ['.$c.'] => c'."\n".'    ['.$d.'] => d'."\n".')'."\n\n";
print
'*** Actual '."\n";
print_r($e);

?>

Output:

*** Expect
Array
(
    [2147483649] => c
    [3173604585] => d
)

*** Actual
Array
(
    [-2147483647] => c
    [3173604585] => d
)

A workaround is to format as base 16 and append '0x' before using as a key.
up
-1
caldwell at utm dot NoSpamPlease dot edu
19 years ago
Warning: gmp_strval may return a null terminated string (e.g., gmp_strval(gmp_init(8)) has two characters. This is because gmp is built for speed, so uses estimates based on things it can get quickly, like the number of bits in N. So since 8 is a 4 bit number, it will be allocated ceil(log10(2^4))=ceil(4*0.30103....) = 2 characters. The gmp folks feel it is the caller's responsibility (in this case the PHP interface) to check whether or not all the characters were required.  PHP might have done this for us (and may in the future), but for now, you need to check (if it matters to you--it sure did in my program!).
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