bcmul

(PHP 4, PHP 5)

bcmulMultiply two arbitrary precision numbers

Description

string bcmul ( string $left_operand = "" , string $right_operand = "" [, int $scale = int ] )

Multiply the left_operand by the right_operand.

Parameters

left_operand

The left operand, as a string.

right_operand

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. You can also set the global default scale for all functions by using bcscale().

Return Values

Returns the result as a string.

Examples

Example #1 bcmul() example

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

See Also

  • bcdiv() - Divide two arbitrary precision numbers

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User Contributed Notes 4 notes

up
1
Nitrogen
5 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.
up
1
gar37bic at gmail dot com
2 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.
up
1
admin at spamhere dot sinfocol dot org
3 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
-2
ju(...)
6 years ago
Except that with xpheas method, you lose all the benefits of arbitrary precision as the * operator only works on int and float and those are restricted in length (See int ant float types for more information).
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