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Re^2: Get the order of HTTP request headers

by ikegami (Pope)
on May 06, 2010 at 05:18 UTC ( #838641=note: print w/ replies, xml ) Need Help??


in reply to Re: Get the order of HTTP request headers
in thread Get the order of HTTP request headers

Perl's %ENV hash is randomized.

First, the order of values returned by hashes isn't random, at least not in any formal sense of the word. There's definitely no attempt to make them random as implied by saying the order is "randomized".

Second, %ENV is magical. It's not really a hash (although it might use one), so the properties of hash don't necessarily apply.

This is just a nit as your point stands without the quoted bit.


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Re^3: Get the order of HTTP request headers
by WizardOfUz (Friar) on May 06, 2010 at 10:37 UTC
    First, the order of values returned by hashes isn't random, at least not in any formal sense of the word. There's definitely no attempt to make them random as implied by saying the order is "randomized".

    I always assumed that the order is indeed "randomized". Well, I guess I misunderstood perlsec and perlrun. Or maybe not.

      The perlsec passage to which you linked says the order shouldn't be assumed to be random. The perlrun passage to which you link doesn't explicitly mention changing key orderings at all. It is not the purpose of the feature to alter the key ordering, much less making it random.

      Two problems with your assumption:

      • The feature in question only kicks in when needed.

      • Even if you could reliably trigger the feature, it's purpose is to salt the hashing algorithm (i.e. perturb bucket selection). While this has the side effect of affecting key ordering, it hasn't been shown to produce random key orderings.

      But enough theory. Let's look at a practical example:

      for (1..100) { my %h; $h{$_} = 1 for 'a'..'z'; print(join('', keys(%h)), "\n"); }
      wraxdjyukhgftienvmslcpqbzo wraxdjyukhgftienvmslpcqbzo wraxdjyukhgftienvmslpcqbzo wraxdjyukhgftienvmslpcqbzo wraxdjyukhgftienvmslpcqbzo wraxdjyukhgftienvmslpcqbzo ... wraxdjyukhgftienvmslpcqbzo wraxdjyukhgftienvmslpcqbzo

      And the same if I run the program again.

        A simple question: Are the results of your example predictable for any combination of hash keys without knowing the hash seed?

Re^3: Get the order of HTTP request headers
by Haarg (Chaplain) on May 06, 2010 at 16:20 UTC
    You are correct. I had meant it in the sense that hashes are unordered and not returned in the same order they are entered, but saying it is random isn't correct.

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