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drhyde (1683)

drhyde
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http://www.cantrell.org.uk/david

Journal of drhyde (1683)

Saturday April 26, 2003
03:08 PM

zipn()

[ #11860 ]

sub zipn {
    local $[ = 0;
    [
        map {
            my $index = $_;
            map {
                defined($_[$_][$index]) ?
                    $_[$_][$index] :
                    ''
            } 0..$#_
        } 0 .. maximum(map { $#{$_[$_]} } 0..$#_)
    ]
}

sub maximum {
    local $[ = 0;
    ($#_ == 1) ?
        (
            ($_[0] >= $_[1]) ?
                $_[0] :
                $_[1]
        ) :
        maximum(maximum($_[0], $_[1]), @_[2..$#_]);
}

Call it like so:

zipn([1,2,3], [4,5,6], [7,8,9,10])

and it'll return this:

[1,4,7,2,5,8,3,6,9,'','',10]

What fun! Note that if any of the arrays are too short, it fills in the blanks with the empty string, cos that's what I needed at the time. Changing it to emit undefs would be trivial. And yes, the maximum function is hideously inefficient, but I enjoy recursion.


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  • This is unnecessary, and in fact, I'm surprised it's not a runtime error. $[ is lexical. Quoting perlvar :
    As of release 5 of Perl, assignment to $[ is treated as a compiler directive, and cannot influence the behavior of any other file.
    • If I had packaged that up in a neat little module on its own then yes, I wouldn't have bothered, but I didn't, and if someone had cut n' pasted it into some of their code where they'd been messing with $[, then I need to set it myself.

      I fail to see anything in that snippet from the man page which suggests that it is either unnecessary or an error.
      • You don't understand me : $[ acts as a pragma. It's not file-scoped, it's lexical. Using local() on it is a null operation. The perlvar phrasing is misleading, I'll see how to change it (except the part where it is said that its use is highly discouraged).
        • OK, I was wrong, it appears that $[ leaks out of some scopes at compile time, so it's not strictly equivalent to a pragma. So the following prints, undocumentedly, '001' :

          print $[;
          if (0) { local $[ = 1; }
          print $[;
          if (0) { $[ = 1; }
          print $[;
  • max() is in the module List::Util [cpan.org]. Depending on the platform, it may be in XS, or in plain perl (the fallback). Anyway, it's not necessary to reinvent that wheel.
    • If I'd wanted to do things the "right" way, I wouldn't have written "And yes, the maximum function is hideously inefficient, but I enjoy recursion.". Obviously.
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