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Mystery! Logical explanation or just Satan's work?

by BrowserUk (Pope)
on Aug 16, 2012 at 15:04 UTC ( #987794=perlquestion: print w/ replies, xml ) Need Help??
BrowserUk has asked for the wisdom of the Perl Monks concerning the following question:

In a recent post I described a method I discovered for packing hashes and arrays. As part of the description, I typed a few examples into my REPL and pasted the output in the post; 10 lines of sample code being worth a 1000 words of verbiage.

But I just realised that I made a typo. Instead of using a template of n/(n/a*)*, I accidentally omitted the final *. But the mystery is, it still worked:

$packed = pack 'n/(n/a*)', 1..10;; @array = unpack 'n/(n/a*)', $packed;; print @array;; 1 2 3 4 5 6 7 8 9 10

The question is why? Why did pack see fit to pack all 10 values rather than just the first?

Is there a logical explanation, or are the last 3 digits of the id of the above linked node somehow responsible :)


With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.

The start of some sanity?

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Re: Mystery! Logical explanation or just Satan's work?
by SuicideJunkie (Priest) on Aug 16, 2012 at 15:23 UTC
    $packed = pack 'n/(n/a*)', 1..9;; @array = unpack 'C*', $packed;; print join ', ' , @array;;
    Gives: 0, 9, 0, 1, 49, 0, 1, 50, 0, 1, 51, 0, 1, 52, 0, 1, 53, 0, 1, 54, 0, 1, 55, 0, 1, 56, 0, 1, 57

    It looks to me like the first n/ is storing the size of your list. The (n/a*) part is then used that many times (in this case, 9).

      You're right! I was misinterpreting the prefixed count. That's both good and bad news.

      1. Good news: It works!

        As in it packs/unpacks a list of data reliably.

      2. Bad news: Its not so good for length/data transmission protocols.

        Which want the prefix to be a count of the bytes in the packet; not the number of fields.

        That means an extra step is required:

        $p = pack 'N/a*', pack '(n/a*)*', 1..10, 'aaaa'..'aaaz';; %h = unpack '(n/a*)*', unpack 'N/a*', $p;; pp \%h;; { 1 => 2, 3 => 4, 5 => 6, 7 => 8, 9 => 10, aaaa => "aaab", aaac => "aaad", aaae => "aaaf", aaag => "aaah", aaai => "aaaj", aaak => "aaal", aaam => "aaan", aaao => "aaap", aaaq => "aaar", aaas => "aaat", aaau => "aaav", aaaw => "aaax", aaay => "aaaz", }

        That's cool, but important to know.


      With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
      Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
      "Science is about questioning the status quo. Questioning authority".
      In the absence of evidence, opinion is indistinguishable from prejudice.

      The start of some sanity?

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