And, just for fun, here's a very slow pure perl version that does not depend on arithmetic ops. 
#!perl

use warnings;
use strict;

# arithmetic operations on (nonegative) binary numbers
sub addbin0 { 
 my($a,$b) = @_; 
 $a || $b ?
  ( chop($a) ? 
   (chop($b) ? addbin1($a, $b) . "0" : addbin0($a, $b) . "1") : 
   (chop($b) ? addbin0($a, $b) . "1" : addbin0($a, $b) . "0") ) :
  "";
} 
sub addbin1 { 
 my($a,$b) = @_; 
 $a || $b ?
  ( chop($a) ? 
   (chop($b) ? addbin1($a, $b) . "1" : addbin1($a, $b) . "0") : 
   (chop($b) ? addbin1($a, $b) . "0" : addbin0($a, $b) . "1") ) :
  "1";
} 
sub addbin {
 addbin0(@_) || "0";
}

# multiplication algorithm to convert decimal number to binary
our @dec2bin1 = qw(0 1 10 11 100 101 110 111 1000 1001);
sub dec2bin {
        $_[0]=~/^\s*(-?)(\d+)/ or do {
                warn "dec2bin can not interpret its first argument as 
+decimal";
                return "0";
                };
        my($sgn, $dec) = ($1, $2);
        $dec=~/(\d)/g or die "internal error"; # this regexp always ma
+tches
        my $b = $dec2bin1[$1];
        while($dec=~/(\d)/g) {
                $b = addbin($b . "0", $b . "000"); # multiply by 1010b
                $b = addbin($b, $dec2bin1[$1]);
        }
        $sgn . $b;
}

# convert a number 
print dec2bin($ARGV[0]), "\n";

__END__

[download]

Update: changed sub dec2bin above by merlyn's suggestion. Well, it still doesn't check for nonnumeric junk at the end of string, sorry.

Original sub: 

# multiplication algorithm to convert decimal number to binary
our @dec2bin1 = qw(0 1 10 11 100 101 110 111 1000 1001);
sub dec2bin {
        $_[0]=~/^(-?)(\d*)/;
        my($sgn, $dec) = ($1, $2);
        $dec=~/(\d)/g;
        my $b = $dec2bin1[$1];
        while($dec=~/(\d)/g) {
                $b = addbin($b . "0", $b . "000"); # multiply by 1010b
                $b = addbin($b, $dec2bin1[$1]);
        }
        $sgn . $b;
}

[download]

In reply to Re: decimal to binary conversion need help by ambrus
in thread decimal to binary conversion need help by Anonymous Monk

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