unpackTEMPLATE,EXPR
This function does the reverse of
pack
: it
takes a string (
EXPR
) representing a data structure
and expands it out into a list value, returning the list value. (In a scalar
context, it can be used to unpack a single value.) The
TEMPLATE
has much the same format as in the
pack
function - it specifies the order and
type of the values to be unpacked. (See
pack
for a more detailed description of
TEMPLATE
.)
Here's a subroutine that does (some of) substr , only slower:
sub substr {
my($what, $where, $howmuch) = @_;
if ($where < 0) {
$where = -$where;
return unpack "\@* X$where a$howmuch", $what;
}
else {
return unpack "x$where a$howmuch", $what;
}
}
and then there's:
sub signed_ord { unpack "c", shift }
Here's a complete uudecode program:
#!/usr/bin/perl
$_ = <> until ($mode,$file) = /^begin\s*(\d*)\s*(\S*)/;
open(OUT,"> $file") if $file ne "";
while (<>) {
last if /^end/;
next if /[a-z]/;
next unless int((((ord() - 32) & 077) + 2) / 3) ==
int(length() / 4);
print OUT unpack "u", $_;
}
chmod oct $mode, $file;
In addition, you may prefix a field with
%
number
to
indicate that you want it to return 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:
undef $/;
$checksum = unpack ("%32C*", <>) % 32767;
The following efficiently counts the number of set bits in a bit vector:
$setbits = unpack "%32b*", $selectmask;
Here's a simple MIME decoder:
while (<>) {
tr#A-Za-z0-9+/##cd; # remove non-base64 chars
tr#A-Za-z0-9+/# -_#; # convert to uuencoded format
$len = pack("c", 32 + 0.75*length); # compute length byte
print unpack("u", $len . $_); # uudecode and print
}
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