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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 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 {
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);
Source: Perl builtin functions Copyright: Larry Wall, et al. |
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