Unpack() does the reverse of pack(): it takes a string representing a structure and expands it out into a list value, returning the array value. (In scalar context, it returns merely the first value produced.) The TEMPLATE has the same format as in the pack() function. Here's a subroutine that does substring:
sub substr {
my($what,$where,$howmuch) = @_;
unpack("x$where a$howmuch", $what);
}and then there's
sub ordinal { unpack("c",$_[0]); } # same as ord()In addition, you may prefix a field with a %<number> to indicate that you want a <number>-bit checksum of the items instead of the items themselves. Default is a 16-bit checksum. For example, the following computes the same number as the System V sum program:
while (<>) {
$checksum += unpack("%16C*", $_);
}
$checksum %= 65536;The following efficiently counts the number of set bits in a bit vector:
$setbits = unpack("%32b*", $selectmask);