unpack TEMPLATE,EXPR
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 a scalar context, it merely returns the first value
produced.) The TEMPLATE has the same format as in the pack function.
Here's a subroutine that does substring:
sub substr {
local($what,$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);
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