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FAQ 4.53 How do I manipulate arrays of bits?

 
 
PerlFAQ Server
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      02-10-2011
This is an excerpt from the latest version perlfaq4.pod, which
comes with the standard Perl distribution. These postings aim to
reduce the number of repeated questions as well as allow the community
to review and update the answers. The latest version of the complete
perlfaq is at http://faq.perl.org .

--------------------------------------------------------------------

4.53: How do I manipulate arrays of bits?

Use "pack()" and "unpack()", or else "vec()" and the bitwise operations.

For example, you don't have to store individual bits in an array (which
would mean that you're wasting a lot of space). To convert an array of
bits to a string, use "vec()" to set the right bits. This sets $vec to
have bit N set only if $ints[N] was set:

@ints = (...); # array of bits, e.g. ( 1, 0, 0, 1, 1, 0 ... )
$vec = '';
foreach( 0 .. $#ints ) {
vec($vec,$_,1) = 1 if $ints[$_];
}

The string $vec only takes up as many bits as it needs. For instance, if
you had 16 entries in @ints, $vec only needs two bytes to store them
(not counting the scalar variable overhead).

Here's how, given a vector in $vec, you can get those bits into your
@ints array:

sub bitvec_to_list {
my $vec = shift;
my @ints;
# Find null-byte density then select best algorithm
if ($vec =~ tr/\0// / length $vec > 0.95) {
use integer;
my $i;

# This method is faster with mostly null-bytes
while($vec =~ /[^\0]/g ) {
$i = -9 + 8 * pos $vec;
push @ints, $i if vec($vec, ++$i, 1);
push @ints, $i if vec($vec, ++$i, 1);
push @ints, $i if vec($vec, ++$i, 1);
push @ints, $i if vec($vec, ++$i, 1);
push @ints, $i if vec($vec, ++$i, 1);
push @ints, $i if vec($vec, ++$i, 1);
push @ints, $i if vec($vec, ++$i, 1);
push @ints, $i if vec($vec, ++$i, 1);
}
}
else {
# This method is a fast general algorithm
use integer;
my $bits = unpack "b*", $vec;
push @ints, 0 if $bits =~ s/^(\d)// && $1;
push @ints, pos $bits while($bits =~ /1/g);
}

return \@ints;
}

This method gets faster the more sparse the bit vector is. (Courtesy of
Tim Bunce and Winfried Koenig.)

You can make the while loop a lot shorter with this suggestion from
Benjamin Goldberg:

while($vec =~ /[^\0]+/g ) {
push @ints, grep vec($vec, $_, 1), $-[0] * 8 .. $+[0] * 8;
}

Or use the CPAN module "Bit::Vector":

$vector = Bit::Vector->new($num_of_bits);
$vector->Index_List_Store(@ints);
@ints = $vector->Index_List_Read();

"Bit::Vector" provides efficient methods for bit vector, sets of small
integers and "big int" math.

Here's a more extensive illustration using vec():

# vec demo
$vector = "\xff\x0f\xef\xfe";
print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ",
unpack("N", $vector), "\n";
$is_set = vec($vector, 23, 1);
print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n";
pvec($vector);

set_vec(1,1,1);
set_vec(3,1,1);
set_vec(23,1,1);

set_vec(3,1,3);
set_vec(3,2,3);
set_vec(3,4,3);
set_vec(3,4,7);
set_vec(3,8,3);
set_vec(3,8,7);

set_vec(0,32,17);
set_vec(1,32,17);

sub set_vec {
my ($offset, $width, $value) = @_;
my $vector = '';
vec($vector, $offset, $width) = $value;
print "offset=$offset width=$width value=$value\n";
pvec($vector);
}

sub pvec {
my $vector = shift;
my $bits = unpack("b*", $vector);
my $i = 0;
my $BASE = 8;

print "vector length in bytes: ", length($vector), "\n";
@bytes = unpack("A8" x length($vector), $bits);
print "bits are: @bytes\n\n";
}



--------------------------------------------------------------------

The perlfaq-workers, a group of volunteers, maintain the perlfaq. They
are not necessarily experts in every domain where Perl might show up,
so please include as much information as possible and relevant in any
corrections. The perlfaq-workers also don't have access to every
operating system or platform, so please include relevant details for
corrections to examples that do not work on particular platforms.
Working code is greatly appreciated.

If you'd like to help maintain the perlfaq, see the details in
perlfaq.pod.
 
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