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Programming Perl

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3.2.182 unpack


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:

$_ = <> until ($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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