bcmul

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

bcmulMultiply two arbitrary precision numbers

Description

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

Multiply the num1 by the num2.

Parameters

num1

The left operand, as a string.

num2

The right operand, as a string.

scale

This optional parameter is used to set the number of digits after the decimal place in the result. If omitted, it will default to the scale set globally with the bcscale() function, or fallback to 0 if this has not been set.

Return Values

Returns the result as a string.

Changelog

Version Description
8.0.0 scale is now nullable.
7.3.0 bcmul() now returns numbers with the requested scale. Formerly, the returned numbers may have omitted trailing decimal zeroes.

Examples

Example #1 bcmul() example

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

Notes

Note:

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:

Example #2 bcmul() scale example

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

See Also

  • bcdiv() - Divide two arbitrary precision numbers

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User Contributed Notes 4 notes

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4
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;
?>
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5
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",$A,$BMul($A,$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.
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2
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;
}
?>
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2
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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