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bcmul

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

bcmul两个任意精度数字乘法计算

说明

bcmul(string $num1, string $num2, ?int $scale = null): string

num1 乘以 num2

参数

num1

字符串类型的左操作数。

num2

字符串类型的右操作数。

scale

此可选参数用于设置结果中小数点后的小数位数。也可通过使用 bcscale() 来设置全局默认的小数位数,用于所有函数。如果未设置,则默认为 0

返回值

返回字符串类型的结果。

更新日志

版本 说明
8.0.0 现在 scale 可以为 null。
7.3.0 现在 bcmul() 可以按想要的小数点位数返回数字。 而之前,返回的数字会忽略尾随零(trailing decimal zeroes)。

示例

示例 #1 bcmul() 示例

<?php
echo bcmul('1.34747474747', '35', 3); // 47.161
echo bcmul('2', '4'); // 8
?>

注释

注意:

Before PHP 7.3.0 bcmul() may return a result with fewer digits after the decimal point than the scale parameter would indicate. This only occurs when the result doesn't require all of the precision allowed by the scale. For example:

示例 #2 bcmul() scale example

<?php
echo bcmul('5', '2', 2); // prints "10", not "10.00"
?>

参见

  • bcdiv() - 两个任意精度的数字除法计算

add a note

User Contributed Notes 4 notes

up
10
Nitrogen
14 years ago
I made this to multiply an unlimited size of integers together (meaning no decimals)..
This could be useful for those without the BCMath extension.

<?php

function Mul($Num1='0',$Num2='0') {
// check if they're both plain numbers
if(!preg_match("/^\d+$/",$Num1)||!preg_match("/^\d+$/",$Num2)) return(0);

// remove zeroes from beginning of numbers
for($i=0;$i<strlen($Num1);$i++) if(@$Num1{$i}!='0') {$Num1=substr($Num1,$i);break;}
for(
$i=0;$i<strlen($Num2);$i++) if(@$Num2{$i}!='0') {$Num2=substr($Num2,$i);break;}

// get both number lengths
$Len1=strlen($Num1);
$Len2=strlen($Num2);

// $Rema is for storing the calculated numbers and $Rema2 is for carrying the remainders
$Rema=$Rema2=array();

// we start by making a $Len1 by $Len2 table (array)
for($y=$i=0;$y<$Len1;$y++)
for(
$x=0;$x<$Len2;$x++)
// we use the classic lattice method for calculating the multiplication..
// this will multiply each number in $Num1 with each number in $Num2 and store it accordingly
@$Rema[$i++%$Len2].=sprintf('%02d',(int)$Num1{$y}*(int)$Num2{$x});

// cycle through each stored number
for($y=0;$y<$Len2;$y++)
for(
$x=0;$x<$Len1*2;$x++)
// add up the numbers in the diagonal fashion the lattice method uses
@$Rema2[Floor(($x-1)/2)+1+$y]+=(int)$Rema[$y]{$x};

// reverse the results around
$Rema2=array_reverse($Rema2);

// cycle through all the results again
for($i=0;$i<count($Rema2);$i++) {
// reverse this item, split, keep the first digit, spread the other digits down the array
$Rema3=str_split(strrev($Rema2[$i]));
for(
$o=0;$o<count($Rema3);$o++)
if(
$o==0) @$Rema2[$i+$o]=$Rema3[$o];
else @
$Rema2[$i+$o]+=$Rema3[$o];
}
// implode $Rema2 so it's a string and reverse it, this is the result!
$Rema2=strrev(implode($Rema2));

// just to make sure, we delete the zeros from the beginning of the result and return
while(strlen($Rema2)>1&&$Rema2{0}=='0') $Rema2=substr($Rema2,1);

return(
$Rema2);
}

$A='5650175242508133742';
$B='2361030539975818701734615584174625';

printf(" Mul(%s,%s); // %s\r\n",$A,$B, Mul($A,$B));
printf("BCMul(%s,%s); // %s\r\n",$A,$B,BCMul($A,$B)); // build-in function

/*
This will print something similar to this..
Mul(5650175242508133742,2361030539975818701734615584174625);
BCMul(5650175242508133742,2361030539975818701734615584174625);

both of which should be followed by the answer:
13340236303776981390475700774516825287352418182696750
*/

?>

It was a fun experience making.. even though this took me longer than the BCAdd alternative I did..
Memory allocation might be an issue for rediculously larger numbers though.. if someone wants to benchmark the performance of my function; feel free.
Enjoy,
Nitrogen.
up
7
mgkirs
6 years ago
$float = 0.31234144143341;
$float1 = 0.00000000000000000000000000000005;
echo $float, "\n";
//0.31234144143341
echo $float1, "\n";
//5.0E-32
echo $float*$float1, "\n";
//1.5617072071671E-32

<?php
/*bcmul read float as string*/
echo bcmul($float, $float1, 32),"\n";
//0
echo bcmul($float, sprint('%.32f',$float1), 32);
//0.000000000000000000000000000000015617072071671;
?>
up
4
admin at spamhere dot sinfocol dot org
13 years ago
Well, I have a little problem implementing Blake Hash in my server because it is not a x64 server machine. I made a little function that use the powerfull of BC library to do the bitwise operation Shift.

<?php
echo 'Left Shift test<br />';
bprint('1', decbin(1));
bprint('1 << 32 (Fail)', decbin(1 << 32)); //Fail, operation not succesfull in 32-bit machine
bprint('shiftleft(1, 32) (Success)', dec2bin(shiftleft('1', '32'))); //decbin fails, so we use personalized function, success

echo '<br />';
echo
'Right Shift test<br />';
bprint('9223372036854775808', dec2bin('9223372036854775808'));
bprint('9223372036854775808 >> 63 (Fail)', decbin(9223372036854775808 >> 63));
bprint('rightshift(9223372036854775808, 63) (Success)', decbin(rightshift('9223372036854775808', '63')));

function
shiftleft($num, $bits) {
return
bcmul($num, bcpow('2', $bits));
}

function
rightshift($num, $bits) {
return
bcdiv($num, bcpow('2', $bits));
}

function
bprint($title, $content) {
echo
$title . '<br />' . str_pad($content, 64, '0', STR_PAD_LEFT) . '<br />' . PHP_EOL;
}

//http://www.php.net/manual/en/function.decbin.php#99533
function dec2bin($dec) {
// Better function for dec to bin. Support much bigger values, but doesn’t support signs
for ($b = '', $r = $dec; $r >1;) {
$n = floor($r / 2);
$b = ($r - $n * 2) . $b;
$r = $n; // $r%2 is inaccurate when using bigger values (like 11.435.168.214)!
}
return (
$r % 2) . $b;
}
?>
up
0
gar37bic at gmail dot com
11 years ago
When using printf to print the results of bcmath operations, use string format, i.e. '%s', not numeric formats such as '%d' or '%f'. For example, the output of factorial (23) will be incorrect if using %d or %f:

Result using %f:
factorial (22) = 1124000727777607680000 (correct)
factorial (23) = 25852016738884978212864 (incorrect)

Result using %s:
factorial (22) = 1124000727777607680000
factorial (23) = 25852016738884976640000

Using echo, this is not a problem - PHP will output the bcmath string type correctly.
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